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What genes in a plant determine whether a stem is erect or climbing?

What genes in a plant determine whether a stem is erect or climbing?


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I was randomly reading this Wiki article on Jasmine and this question crossed my mind after reading the following lines:

Jasmine can be either deciduous (leaves falling in autumn) or evergreen (green all year round), and can be erect, spreading, or climbing shrubs and vines.

How can a plant be both erect and climbing at the same time?

I have looked for genetic mechanisms related to this, correlating environmental or other stimulus with this phenomenon, but have not found a satisfactory explanation.

So how does this occur?


Genetic diversity in developmental responses to light spectral quality in barley (Hordeum vulgare L.)

Plants use light wavelength, intensity, direction and duration to predict imminent seasonal changes and to determine when to initiate physiological and developmental processes. Among them, crop responses to light are not fully understood. Here, we study how light quality affects barley development, using two broad-spectrum light sources, metal halide (M) and fluorescent (F) lamps. Eleven varieties with known allelic variants for the major flowering time genes were evaluated under controlled conditions (long days, same light intensity). Two experiments were carried out with fully-vernalized plants: 1) control treatments (M, F) 2) shifting chambers 10 days after the start of the experiment (MF, FM).

Results

In general, varieties developed faster under longer exposure to M conditions. The greatest differences were due to a delay promoted by F light bulbs, especially in the time to first node appearance and until the onset of stem elongation. Yield related-traits as the number of seeds were also affected by the conditions experienced. However, not each variety responded equally, and they could be classified in insensitive and sensitive to light quality. Expression levels of flowering time genes HvVRN1, HvFT1 and PPD-H1 were high in M, while HvFT3 and HvVRN2 were higher under F conditions. The expression under shift treatments revealed also a high correlation between HvVRN1 and PPD-H1 transcript levels.

Conclusions

The characterization of light quality effects has highlighted the important influence of the spectrum on early developmental stages, affecting the moment of onset of stem elongation, and further consequences on the morphology of the plant and yield components. We suggest that light spectra control the vernalization and photoperiod genes probably through the regulation of upstream elements of signalling pathways. The players behind the different responses to light spectra found deserve further research, which could help to optimize breeding strategies.


The genetic and molecular basis of crop height based on a rice model

This review presents genetic and molecular basis of crop height using a rice crop model. Height is controlled by multiple genes with potential to be manipulated through breeding strategies to improve productivity.

Height is an important factor affecting crop architecture, apical dominance, biomass, resistance to lodging, tolerance to crowding and mechanical harvesting. The impressive increase in wheat and rice yield during the ‘green revolution’ benefited from a combination of breeding for high-yielding dwarf varieties together with advances in agricultural mechanization, irrigation and agrochemical/fertilizer use. To maximize yield under irrigation and high fertilizer use, semi-dwarfing is optimal, whereas extreme dwarfing leads to decreased yield. Rice plant height is controlled by genes that lie in a complex regulatory network, mainly involved in the biosynthesis or signal transduction of phytohormones such as gibberellins, brassinosteroids and strigolactones. Additional dwarfing genes have been discovered that are involved in other pathways, some of which are uncharacterized. This review discusses our current understanding of the regulation of plant height using rice as a well-characterized model and highlights some of the most promising research that could lead to the development of new, high-yielding varieties. This knowledge underpins future work towards the genetic improvement of plant height in rice and other crops.

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Plant architecture and grain yield traits in Triticeae

Barley and wheat belong to the Triticeae species possessing similar plant architecture with an unbranched inflorescence known as the spike. The architecture as well as their climatic and agronomic requirements differ from the model species rice. Spikes of wheat and barley most likely evolved from an ancestral compound inflorescence, producing branches (Endress 2010 Kellogg et al. 2013 Remizowa et al. 2013). Archetypal inflorescences and spikes bear three single-flowered spikelets per rachis node. The spikelets are arranged in two opposite rows along the main axis. In barley, if the two outer lateral spikelets at each node are sterile, the spikes are two-rowed when all three are fertile, the spikes are six-rowed (Ramsay et al. 2011). Final grain yield depends on grain number and grain weight (reviewed by Sreenivasulu and Schnurbusch 2012 Kesavan et al. 2013 Distelfeld et al. 2014). The yield components influencing grain number include number of tillers bearing fertile spikes, extension of vegetative as well as reproductive growth and differentiation phase, inflorescence architecture, culm hardiness, spike initiation, elongation and branching as well as spikelet formation. Grain weight is affected by grain cell number (Brocklehurst 1977) and sink capacity (Millet and Pinthus 1984).


Care for Indeterminate Types

Also called vine, pole, or climbing tomatoes, indeterminate cultivars just keep growing and growing until something stops them &ndash usually your first fall frost.

Gardeners with long growing seasons can take advantage of this trait, receiving small but frequent harvests from each plant after it reaches maturity and throughout the growing season.

Remember how our determinate varieties grow flowers on the ends of their shoots? Indeterminate types grow flower clusters along the sides of their stems instead, which allows the shoot to keep growing.

The plant&rsquos energy keeps going into growing more foliage along with producing fruit, which results in smaller quantities of fruit over an extended period of time.

If your first fall frost tends to come early, like it does in my short-season garden, you may not have time to get much of a harvest from the later-maturing indeterminate varieties &ndash so make sure you pick one that is appropriate for the length of your growing season.

If given the time though, indeterminate tomato plants often grow to be 10-12 feet in height or more.

Because of this extensive growth, these cultivars aren&rsquot usually the best choice for growing in containers.

They can become too top-heavy for container gardening &ndash and if the plant grows quite tall, its roots will grow extensively as well, requiring more water and nutrients than may be manageable in a container setup.

On the other hand, they are wonderful candidates for vertical gardening, since it&rsquos easy to train them to climb upwards.

Whereas some determinate varieties can get by without support, these varieties need to have their rampantly growing vines managed. This is usually done with pruning and staking.

Climbing types are typically pruned to just one or two leader stems, and trained to climb a string or trellis.

Pruning and training these plants in this way also helps with airflow, an important tactic for keeping disease at bay in more humid climates.

With indeterminate varieties, pruning and removing suckers allows the plant to focus energy on fruit production for the remaining stems &ndash putting less energy into foliage growth.

In fact, unpruned, overgrown plants may produce fruits that fail to turn red on the vine. Read our article on this topic to learn more about how to make sure this summer crop turns red in your garden.

Speaking of red fruit &ndash you&rsquoll notice that indeterminate cultivars will often have flowers, immature fruit, and ripe fruit on the plant all at the same time.

Instead of producing all of their fruit more or less at once, as determinate varieties tend to do, indeterminate ones gradually produce smaller but more frequent harvests over the growing season.

These varieties are great if you want to have a steady harvest throughout your growing season &ndash just make sure to grow several plants if you want tomatoes on your menu every single day.

That doesn&rsquot mean you can&rsquot also use indeterminate varieties for canning, though.

There are many delicious paste varieties that are indeterminate, such as &lsquoSan Marzano.&rsquo

To can these cultivars, the trick is to grow several plants at once, so that you have enough fruit ripening at the same time for canning.

Most heirloom and large-fruited tomatoes are indeterminate. But not all indeterminates are large or heirlooms! There are many small-fruited varieties that have this growth habit as well.

One of my favorite cherry tomatoes, &lsquoChadwick Cherry,&rsquo is an indeterminate cultivar. The fruits weigh about 1 ounce each, a bit larger than the average cherry tomato.

This late-maturing variety will reach a height of about five feet when staked, so it remains fairly manageable, and it&rsquos suitable even for smaller spaces like a patio.

It&rsquos also disease resistant, and produces heavy yields about 85 days from transplanting.

You&rsquoll find &lsquoChadwick Cherry&rsquo seeds for purchase in a selection of package sizes from Mountain Valley Seed Company via True Leaf Market.


Biology Questions and Answers Form 4 - Biology Form Four Notes

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KCSE Revision Questions and Answers

Biology Form 4 Notes - Biology Form Four Notes

a) i) Define the term genetics

ii) List some characteristics which are inherited

iii)State the importance of genetics

b) i) Explain the meaning of the following terms

ii) List the types of chromosomes

c) i) What is variation?

ii) State the causes of variation in organisms

iii) Name the types of variation

iv) Explain the following terms

Acquired characteristics

Dominant gene (character)

d) i) Explain Mendels first law of inheritance

ii) Give an example of this law

iii) What is monohybrid inheritance?

i) What is complete dominance?

ii) Give an example of co dominance

In a certain plant species, some individual plants may have only white, red or pink flowers. In an experiment a plant with white flowers was crossed with a parent with red flowers. Show results of Fl generation. Use letter R for red gene and W for white gene.

If the plants form F1 were selfed, work out the phenotype ratio for the F2 generation Phenotypic ratio 1 red:2 pink: 1 white

f) i) What is a test cross?

ii) State the importance of a test cross in genetics

iii) What are multiple alleles?

example is blood group which can be determined by any two of three alleles i.e. A,B and O

iv) Explain the inheritance of ABO blood groups

ii) Explain the inheritance of Rhesus factor (Rh) in human beings

people who have Rh antigen are Rh(+ve) while those without Rh antigen in their blood are Rh(-ve)

recessive the result is as shown below

Let the gene for dominant Rh factor be R while gene for recessive be r

iii) How is sex determined in human beings .

g) i) What does the term linkage mean?

- These are genes which occur together on a chromosome and are passed to offspring without being separated ii) Define the term sex-linked genes

iii) What is meant by the term sex linkage?

iv) Name the sex-linked traits in humans

v) Give an example of a sex linked trait in humans on:

vi) In humans red-green colour blindness is caused by a recessive gene C, which is sex- linked. A normal man married to a carrier Woman transmits the trait to his children. Show the possible genotypes of the children.

Let C represent the gene for normal colour vision (dominant)

Let c represent the gene for colour blindness

Parental phenotype Norman man x carrier woman

iv) State the importance of sex linkage

possible to determine sex of day old chicks

v) Haemophilia is due to a recessive gene. The gene is sex-linked and located on the x chromosome. The figure below shows sworn offspring from phenotypically normal parents

What are the parental genotypes?

Work out the genotypes of the offspring

ii) Describe how mutations arise

iii)State the factors that may cause mutation

X-rays gene/chromosome alteration

Ultra violet rays structural distortion of DNA

colchicines prevents spindle formation

Cyclamate chromosome aberrations

Mustard gas chromosomes aberrations

Nitrous acid adenine in DNA is deaminated so behaves like guanine

Acridone orange addition and removal of bases of DNA

iv) State the characteristics of mutations

v) Explain chromosomal mutation

- Change in nature, structure or number of chromosomes

vi) Explain how the following types of chromosomal mutations occur

vii) What are gene mutations?

i) Explain how the following occur during gene mutation

I. State the practical applications of genetics

i. Breeding programmes (research)

ii. Genetic engineering

- legal questions of paternity knowledge of blood groups or blood transfusion

iv) Genetic counseling

Understanding human evolution and origin of other species.

2. a) i) Explain the meaning of evolution

ii) Differentiate organic evolution from chemical evolution as theories of origin of life

iii) What is special creation?

b) Discuss the various kinds of evidence for evolution

ii) Comparative anatomy

iii) Comparative embryology

iii) Comparative serology/physiology

iv) Geographical distribution

as a result of continental drift isolation of organisms occurred bring about different patterns of evolution

vi) Cell biology (cytology)

c) i) State the evolutionary characteristics that adopt human beings to the environment

- Upright posture/bipedal locomotion

ii) State the ways in which Homo sapiens differs from Homo habilis

d) i) Explain Larmarck’s theory of evolution

- Inheritance of acquired characteristics/environment induces production of a favorable trait which is then inherited

ii) Explain why Lamarck’s theory of evolution is not accepted by biologists today

- evidence does not support Lamarck’s theory

- acquired characteristics are not inherited/inherited characteristics are found in reproductive cells only

iii) Explain Darwin’s theory of evolution

- inheritance of genetically acquired characteristics

- a character happens to appear spontaneously which gives advantage to an organism therefore adapted then inherited through natural selection

e) i) What is natural selection?

- Organisms with certain characteristics are favoured by the environment

Such organisms tend to survive and produce viable offspring

Others not favored are eliminated from subsequent generations

ii) With examples, explain how natural selection takes place

- organism with certain characteristics are favored by their environment

- such organisms tend to survive and produce viable offspring

- others not favored are eliminated from subsequent generations

- as the environmental conditions change the survival value of a character may alter with time so that characteristics which were favored may no longer have advantage and other characters may then become favorable

- if a favorable character is inherited, then offspring produce generations which are better adapted to survive in a population

- more offspring are produced than can survive which results in struggle for survival - the fittest survive

iii) State the advantages of natural selection to organisms

- assist to eliminate disadvantageous characteristics/perpetuates advantageous characteristics

- allows better adapted organisms to survive adverse changes in the environment/less adapted organisms are eliminated

iv) State the ways in which sexual reproduction is important in the evolution of plants and animals

- brings about useful variations/desirable characters

- variations make offspring better adapted for survival/more resistant to diseases

- may lead to origin of new species

v) Explain the significance of mutation in evolution

- Mutation bring about variation which can be inherited

- Some of these variations are advantageous to the organism

- Others are disadvantageous

- The advantageous variations favour the organism to compete better in the struggle for survival

- This results into a more adapted organism to its environment or new species/varieties

- Those with disadvantageous characters will be discriminated against therefore eliminated from the population/death/perish

vi) Plain why it is only mutations in genes of gametes that influence evolution

- gametes form the new offspring

vii) How would you prove that evolution is still taking place?

- resistance of organism to antibiotics, pesticides and drugs

- new varieties of bacteria are resistant to certain antibiotics such as penicillin

- houseflies and mosquitoes are resistant to DDT

vii) Explain why some bacteria develop resistance to a drug after they have bee subjected to it for some time

- bacteria mutates/develops a new strain/chemical composition is altered hence is able to produce enzymes/chemicals which degrade the drug rendering it non-susceptible to the drug

- the new strain is favoured by selection pressure natural selection

f) How has industrial melaninism i.e. peppered moth contributed towards the mechanism of evolution

- This is an example of natural selection

- The peppered moth exists in two distinct forms, the speckled white form (normal form) and a melanic form (the black/dark)

- They usually rest on leaves and barks of trees that offer camouflage for protection

- Originally the “speckled white” form predominated the unpolluted area of England

- This colouration offered protection against predatory birds

- Due to industrial pollution tree barks have blackened with soot

- The white form underwent mutation

- A black variety/mutant emerged suddenly by mutation

- It had selective advantage over the white forms that were predated upon in the industrial areas

- The speckled white form is abundant in areas without soot/smoke

3. a) i) Define irritability, stimulus and response irritability

- Responsiveness to change in environment

A change in the environment of organism which causes change in organism’s activity

- change in activity of an organism caused by a stimulus

ii) State importance of irritability to living organisms

- Adjusting to environmental conditions. Sensitive/defect/responding

iii) List the examples of external stimuli to organisms

- chemical concentration (chemo)

b) i) What are tactic responses?

- response in which whole organism or its motile parts move e. g. gamete

ii) What causes tactic responses?

- caused by unidirectional stimulus

- usually doesn’t involve growth

- response is either positive or negative

- named according to source of stimulus

- e.g phototaxis, aerotaxis, chemotaxis

iii) State the importance of tactic response to:

Members of kingdom protista

- move towards favorable environment/move away from unfavorable environment

- move towards their prey/food

- escape injurious stimuli/seek favorable habitats

iv) Name the type of response exhibited by:

Euglena when they swim towards the source of light

- sperms when they swim towards the ovum

v) State the advantages of tactic responses to organisms

- to avoid unfavorable environment/injurious stimuli

- to seek favorable environment

c) i) Define the term tropism

- growth movement of plants in response to external unilateral/unidirectional stimuli

ii) Explain the various types of tropism in plants

- growth movements of plant shoots in response to unilateral sources of light

- the tip of the shoots produce auxins down the shoot

- light causes auxins to migrate to outer side/darker side causing growth on the side away from light hence growth curvature towards source of light roots are negatively phototrophic

- response of roots/pans of a plant to the direction of force of gravity

- auxins grow towards the direction of force of gravity causing positive geotropism in roots while shoot grows away from force of gravity (negatively geotrophic)

Thimotropism/Haptotropism

- growth response of plant when in contact with an object

- contact with support causes migration of auxins to outer side causing faster growth on the side away from contact surface

- this causes tendrils/stem to twin around a support

- growth movement of roots in response to unilateral source of water/moisture

- the root grows towards the source of water/ positively hydrotropic while leaves are negatively hydrotropic

- growth movement of parts of plant to unilateral source of chemicals

- the chemicals form a gradient between two regions e.g. pollen tube growing towards the ovary through the style

iii) State the ways in which tropisms are important to plants

- expose leaves/shoots in positions for maximum absorption of sunlight for photosynthesis

- enables roots of plants to seek/look/search for water

- enables plant stems/tendrils to obtain mechanical support especially those that lack woody stems.

- enables roots to grow deep into the soil for anchorage

- enables pollen tube grow to embryo sac to facilitate fertilization

iv) Explain the differences between tropic and tactic responses

-growth curvature in response

d) The diagram below represents growing seedlings which were subjected to unilateral light at the beginning of an experiment

i) State the results of P, Q and R after S days

- P will bend/grow towards light

- Q will remain straight/have little or no growth

- R will remain/grow straight/grow upwards

ii) Account for your results in (i) above

P- Growth substance/growth hormone/IAA/auxin are produced by the stem tip

- they move (downwards and get distributed) to the side away from light where they cause rapid/more growth/cell division/elongation that results in bending

Q- Source of auxin has been removed

R- The auxins cannot be affected by light because the tip has been covered

iii) If the tin foil were removed from the tip of seedling R, what results would be observed after two days

- it will bend/grow towards light

iv) State the expected results after 3 day is if the box were removed

- all seedlings will grow straight/upwards

e) In an experiment to investigate a certain aspect of plant response, a seedling was placed horizontally as shown in diagram I below. After seven days the appearance of the seedling was as shown in diagram 2

Account for the curvature of the shoot and root after the seven days

- auxins accumulate on the lower side of the seedling due to gravity

- high concentration of auxins in shoot stimulates faster growth causing more elongation on the lower side than the upper side hence curvature occurs upwards

- the high concentration of auxins inhibits growth hence the upper side with less auxins grows faster than the lower side therefore the curvature occurs downwards

- phenomenon exhibited by plants when grown in darkness

- such plants are pale yellow due to absence of chlorophyll, have small leaves, long stems/hypocotyle and slender stems

- plants exhibit etiolation to reach light/obtain light

- this is a survival response

4. a) i) What is coordination in animals

- The linking together of all physiological activities that occur in the body so that they take place at the night time and in the correct place

ii) Name the main systems for coordination in animals

- Nervous system/sensory system

- Endocrine (hormonal system)

iii) List the components of the mammalian sensory system

- Central nervous system (CNS), brain & spinal cord

- Peripheral nervous system (PNS) cranial and spinal nerves

- Autonomic nervous system (ANS) nerve fibers and ganglia

iv).Explain the terms receptors, conductors and effectors

- Receptors are structures that detect stimuli i.e. sense organs

- Conductors transmit impulses from receptors to effectors e. g. neurons

- Effectors are the responding parts e.g. muscles, glands

v) What are the functions of the central nervous system?

- provides a fast means of communication between receptors and effectors

- coordinates the activities of the body

vi) State the differences between somatic and autonomic systems of peripheral nervous system

- Somatic is concerned with controlling the conscious or voluntary actions of the body i.e. skin, bones, joints and skeletal muscles

- the autonomic (automatic) nervous system controls involuntary actions of internal organs, digestive system, blood vessels, cardiac muscles and glandular products.

b) i) What is a neurone?

ii) Name the parts of a typical neurone and state the functions of each part

i) Describe the structure and function of a motor neurone

ii) Describe the structure and of sensory neurone

iii) State structural differences between motor and sensory neurons

iv) Describe the structure and function of a relay neurone

c) State the function of the major parts of the human brain

a) i) What is reflex action?

ii) Describe a reflex action that will lead to the Withdrawal of a hand from a hot object

iii) Explain how an impulse is transmitted across the synapse (gap)

ii) Briefly describe the transmission of a nervous impulse across a neuro-muscular junction

iii) What are the functions of a synapse?

b) i) What is a conditioned reflex?

ii) Explain a conditioned reflex

iii) Compare a simple reflex action with a conditioned reflex

c) i) What are endocrine glands?

ii) State the functions of hormones in animals

iii) Name the main endocrine glands, their secretions and functions in the human body

increases the rate of metabolism

regulates calcium and phosphate levels

regulates growth of the body

gonadotrophic hormone

stimulates the growth of male and female organs

lactogenic hormone (prolactine)

stimulates secretion of milk after child birth

thyrotropic hormone( TSH)

proper functioning of thyroid glands/thyroxine production

adrenocorthicotropic hormone (ACTH)

stimulate release of adrenal cortex hormone

stimulates smooth muscles

stimulates contraction of uterus during child birth

aids flow of milk from mammary glands

follicle stimulating hormone (FSH)

causes maturition of egg in females

stimulates sperm production in male

Vasopressin (ADH) antidiuretic hormone

regulates water balance by kidney

adrenaline (epinephrine hormone)

prepares body to cope up with stress

maintain balance of salt and water in blood

break down the stored proteins to amino acids

aids in the break down of adipose tissue

regulates sugar levels in the blood

supplements sex hormones produced by gonads

promotes development of sexual characteristics

regulates levels of sugar in blood

enables liver to store sugars

regulates levels of sugar in blood

oestrogen Function:

causes secondary sexual characteristics in female

prepares the uterus for pregnancy

progesterone Function:

growth of mucus lining of uterus

androgen testosterone

causes secondary sexual characteristics in male

stimulates release of gastric juice

stimulates secretion of pancreatic juice

iv) Give the differences between nervous and endocrine (hormonal) communication

v) State the effects of over secretion and under secretion of adrenaline and thyroxine in humans

g) i) Define the following terms

ii) State the types of drugs, examples and side effects

iii) State the general effects of drug abuse on human health

h) i) List the special sense organs in mammals and the major function of each

- Ear for hearing and balance

- Skin for touch, temperature detection, pain detection

iii) How is the human eye adapted to its function?

iii) What is accommodation of the eye?

iv) Explain how an eye viewing a near object adjusts to viewing a far object

v) What changes occur in the eye if it changes from observing an object at a distance to one at a closer range?

- Tension in suspensory ligaments reduces/relaxes slackens

- Lens bulges/thickens/increases curvature

- Size of pupil becomes large to allow in more light.

viii) State the changes which would take place in the eye if a person in a dark room had lights switched on

ix) Explain how the eye forms an image

x) Name the defects of the eye and state how they can be corrected

Long sight (Hypermetropia)

near image is formed behind the retina but a distant one is correctly focused on the retina

xi) State the advantages of having two eyes in human beings

i) What are the functions of the human ear?

iv) How are the structures of the human ear suited to perform the function of hearing?

iii) Explain how the structure of the human ear performs the function of balancing

sensory impulses are generated

iv) State what would happen if the auditory nerve was completely damaged

5. a) i) What is support?

ii) What is locomotion?

iii) State the importance of support systems in living organisms

iv) State the importance of locomotion in animals

b) i) Name the tissues in higher plants that provide mechanical support

ii) State the importance of support in plants

iii) Name the types of plant stems

iv) Name the tissues in plants that are strengthened with lignin

v) What makes young herbaceous plants remain upright?

vi) State the ways by which plants compensate for lack of ability to move from one place to another

c) i) Explain the Ways in which erect posture is maintained in a Weak herbaceous stem

- This is the function of turgidity and presence of collencyma

Cells take in water and become turgid

ii) Explain how support in plants is achieved

d) i) Give the reasons why support is necessary in animals

ii) Why is movement necessary in animals?

e) i) Name the organ used for support by animals

ii) Name the different types of skeletons in animals, giving an example of an animal for each type of skeleton named

iii) State the difference between exoskeleton and endoskeleton

iv) State the advantages of having an exoskeleton

v) Explain the importance of having an endoskeleton

f) i) Explain how a fish is adapted to living in Water

ii) Explain how a finned fish is adapted, to locomotion in Water

g) i) Name the main parts of the vertebral column giving the types of bones found in each part

Appendicular skeleton

hind limbs are connected to the pelvic girdle (hips)

ii) What are the vertebrae?

iii) State the functions of the vertebral column

iv) State the general characteristics of vertebrae

v) Name the bones of the vertebral column

vi) Describe how the various vertebrae are adapted to their functions

cervical region Atlas (first cervical)

cervical (others) Structure:

vii) Describe the bones that form the appendicular skeleton

pectoral girdle (scapular shoulder bone)

ii) State the functions of joints

iii) Name the main types of joints

iv) Give the features of movable joints

b) Describe the synovial joints

c) i) What is synovial fluid?

ii) State the functions of synovial fluid

d) Explain the following terms

ii) State the functions of muscles

f) Describe the structure and function of various types of muscles

ii) Involuntary muscles

organs, bladder, uterus, urinary tract, reproductive system, respiratory tract, ciliary body iris

g) Explain how muscles cause movement of the human arm

h) i) State the structural differences between skeletal muscles e.g. biceps and smooth muscles e.g. gut muscle

ii) Name the cartilage found between the bones of the vertebral column

iv) What are the functions of the cartilage named in (d) ii) above


Natural variation at the DEP1 locus enhances grain yield in rice

Grain yield is controlled by quantitative trait loci (QTLs) derived from natural variations in many crop plants. Here we report the molecular characterization of a major rice grain yield QTL that acts through the determination of panicle architecture. The dominant allele at the DEP1 locus is a gain-of-function mutation causing truncation of a phosphatidylethanolamine-binding protein-like domain protein. The effect of this allele is to enhance meristematic activity, resulting in a reduced length of the inflorescence internode, an increased number of grains per panicle and a consequent increase in grain yield. This allele is common to many Chinese high-yielding rice varieties and likely represents a relatively recent introduction into the cultivated rice gene pool. We also show that a functionally equivalent allele is present in the temperate cereals and seems to have arisen before the divergence of the wheat and barley lineages.


II. Hormonal control

Plant hormones are major regulators of growth and development, and have dramatic effects on stature, form, and physiology. Early experiments with exogenous applications pointed to roles in regulation of elongation growth, flowering, apical dominance, lateral/adventitious root formation, and vascular differentiation ( Davies, 1995 ). Some of these applications have been commercialized and provided important improvements in crop propagation and management ( Woodward & Bartel, 2005 ). More recently, genetic dissection has allowed new insights into the molecular mechanism of hormone biosynthesis and signal transduction pathways, and has provided new options for crop improvement ( Sakamoto, 2006 ). We review the effect of auxin, gibberellins, brassinosteriods, and a novel hormone in regulation of plant form and size, but ignore the roles of cytokinins (CKs) and ethylene. They can also modulate plant growth responses, but because their effects are frequently less specific, they are more prone to have undesirable pleoiotropic effects, limiting their value for manipulation of plant stature and form.

1. Auxin – the master switch

There is probably not a single aspect of the growth and development of a plant that is not affected by auxin ( Davies, 1995 Leyser & Berleth, 1999 ). The multiplicity of auxin responses reflects the central role that this hormone plays in coordinating growth and developmental effects in plants, and thus it is not surprising that genes involved in auxin biosynthesis and signal transduction can be strong modifiers of plant size and form. Auxin metabolism and signaling have been the subjects of extensive genetic, genomic, and biochemical dissection (reviewed in Abel & Theologis, 1996 Friml & Palme, 2002 Leyser, 2002 Liscum & Reed, 2002 Woodward & Bartel, 2005 Tanaka et al., 2006 Teale et al., 2006 Berleth et al., 2007 DeSmet & Jurgens, 2007 Kerr & Bennett, 2007 ). Below we summarize the genes that are strong modifiers of plant form and shape.

Polar auxin transport Because auxin acts in a concentration-dependent manner and auxin gradients serve as positional signals, plants have developed an intricate system of auxin carriers that regulate hormone distribution ( Friml & Palme, 2002 ). Efflux/influx carriers are transmembrane proteins that mediate the passage and residence time of auxin in cells ( Liu et al., 1993 Galweiler et al., 1998 Muller et al., 1998 Marchant et al., 2002 ). In Arabidopsis, auxin influx is carried out by Auxin Permease1 (AUX1) and three LAX (like AUX1) proteins ( Bennett et al., 1996 Parry et al., 2001 ). The efflux is mediated by PIN-FORMED (PIN) proteins, encoded by a gene family of eight members ( Galweiler et al., 1998 Muller et al., 1998 Friml & Palme, 2002 Friml et al., 2002 , 2003 ).

Recently, auxin carriers have been implicated in controlling phyllotaxis, a major determinant of plant architecture ( Reinhardt et al., 2003 Jonsson et al., 2006 ). Phyllotaxis is the periodic arrangement of leaves and branches along the stem that is characterized by Fibonacci numbers ( Roberts, 1978 ). Through elegant expression analyses and micro-scale auxin applications, Reinhardt et al. (2003 ) provided evidence for the involvement of auxin carriers in the control of phyllotaxis. According to the model proposed, acropetal flux toward the apex is mediated by PIN1 expression and intracellular redistribution, and creates regional sinks and high-concentration pockets of auxin that determine the periodicity of leaf emergence along the stem.

In addition to the AUX1 and PIN1 proteins, auxin transport is mediated by a group of ATP binding cassette (ABC) transporter proteins that show a high level of similarity to mammalian multi-drug-resistant genes, which are a subset of the P-glycoprotein (PGP) gene family ( Noh et al., 2001 , 2003 ). To date, three PGP proteins in Arabidopsis have been found to mediate auxin transport (e.g. AtPGP1, 4, and 19) and loss-of-function mutations produce highly pleitropic auxin-related morphological abnormalities in roots and shoots ( Noh et al., 2001 , 2003 Geisler et al., 2004 Terasaka et al., 2005 Bouchard et al., 2006 ). The lesion in a gene encoding a similar PGP protein in sorghum (Sorghum spp.) and maize (Zea mays) seemed to condition more moderate phenotypes ( Multani et al., 2003 Salamini, 2003 ). Plants were characterized by compact lower stalk internodes, and no other plant organ was affected in size or growth. The mutant phenotype provided major agronomic benefits that, although unexploited in maize, are widely used in sorghum breeding ( Multani et al., 2003 ). Using transposon tagging, the gene was found to encode a PGP protein with high similarity to Arabidopsis AtPGP1 ( Multani et al., 2003 ).

Polar auxin transport is motivated by gradients in cytoplasmic and apoplastic pH ( Grebe, 2005 ) that are set up by proton pump-transporter proteins. The acid theory of growth postulates that auxin-stimulated excretion of protons into the cell wall that causes acidification and loosening of the cell wall allows expansion. However, until lately there had been little molecular genetic evidence to support this theory. The recently cloned H + pyrophosphatase (PPase) Arabidopsis vacuolar pyrophosphatase1 (AVP1) provides supporting evidence for this hypothesis ( Grebe, 2005 Li et al., 2005b ). In addition to maintaining vacuolar pH, AVP1 was found to be localized in the plasma membrane, and its overexpression increased auxin transport, and loss of function decreased transport. Particularly interesting were the overexpression phenotypes, where ectopic expression of the protein increased the number of rosette leaves and leaf size, mainly via increased cell numbers (Fig. 1a). Overexpression was accompanied by a similar increase in root size.

(a) Arabidopsis Arabidopsis vacuolar pyrophosphatase1 (AVP1) overexpression phenotypes, with the wild type (WT) shown on the left and two independent AVP1-overexpressing lines on the right (reproduced from Li et al., 2005b with permission from The American Association for Advancement in Science (AAAS)). Bar, 1 cm. (b) Tomato (Lycopersicon esculentum) phenotypes produced by down-regulation of the gene encoding an AUX/IAA transcription factor IAA9 the wild type is shown on the left, an antisense (AS) mutant in the middle, and a monogenic spontaneous entire putative iaa9 mutant on the right (AC, Ailsa Craig reproduced with permission from Wang et al., 2005 , ©American Society of Plant Biologists). Bar, 100 mm. (c) Dwarf field phenotype (foreground) of an activation tagged mutant in poplar resulting from hyperexpression of the catabolic gibberellin oxidase gene PtaGA2ox1 ( Busov et al., 2003 ). Poplars (Populus tremula×alba) showing wild-type growth after two growing seasons are in the background (approx. 5 m in height). (d) Arabidopsis ARGOS (auxin-regulated gene controlling organ size) mutant phenotypes (reproduced with permission from Hu et al., 2003 , ©American Society of Plant Biologists). Antisense knock-down (left), vector control (middle), and 35S overexpression (right) mutants are shown. Bar, 5 mm. (e) Upper row, branching morphologies of maize (Zea mays) (left) vs its ancestor teosinte (right) lower row, segregation of form among recombinant inbred progeny derived from maize–teosinte hybridization that are homozygous for maize (left) or teosinte (right) chromosomal segments containing the major quantitative trait loci (QTL) for branching with the TEOSINTE BRANCHED1 (TB1) gene. (Provided by, and used with permission from, J. Doebley.) (f) Arabidopsis branched1 (brc1) and branched2 (brc2) mutant phenotypes (reproduced with permission from Aguilar-Martinez et al., 2007 , ©American Society of Plant Biologists). (g) Arabidopsis jaw miRNA mutant phenotypes the wild type is shown on the left, and the mutant on the right (reproduced with permission from Macmillan Publishers Ltd, from Palatnik et al., 2003 ). (h) Tomato mutants for the SELF-PRUNING gene the wild-type indeterminate form is shown on the left, and the homozygous determinate mutant on the right (AS, axillary shoot TI, terminal inflorescence from Pnueli et al., 1998 reproduced with permission of the Company of Biologists).

Aux/IAA gene family One of the hallmarks of auxin response in plants is the strong and rapid induction of auxin/indole acetic acid (Aux/IAA) genes ( Abel & Theologis, 1996 ). They are primary auxin-response genes, meaning that their activation does not need de novo protein synthesis. Arabidopsis has 29 Aux/IAA genes, and mutant screens have identified mutations with distinct phenotypes in 10 of them ( Liscum & Reed, 2002 ). The isolated mutations are predominantly gain-of-function lesions in the conserved domain II that is present in all gene family members the mutations seem to render the protein resistant to degradation, and several size and form characteristics were found to be modified in the mutant plants. For example, iaa3/short hypocotyl (shy)2-2 ( Tian & Reed, 1999 Tian et al., 2002 ), iaa6/shy1-1 ( Kim et al., 1996 ), iaa7/auxin resistant (axr)2-1 ( Timpte et al., 1994 Nagpal et al., 2000 ), and iaa17/axr3-1 ( Rouse et al., 1998 ) seem to condition shorter hypocotyls. Conversely, the iaa18 mutant was found to have longer hypocotyls ( Reed, 2001 ). The effect of these mutations on mature plant size and form (e.g. stem elongation and branching) remains unclear.

Aux/IAA genes also have a strong effect on apical dominance in inflorescence stems. For example, iaa17/axr3-1 mutants have increased apical dominance ( Leyser et al., 1996 ) with fewer side branches while iaa28-1 ( Rogg et al., 2001 ) has decreased apical dominance with more inflorescence branching. Lateral root branching is also affected in several mutants –iaa3/shy2-2 ( Tian & Reed, 1999 Tian et al., 2002 ), iaa14/slender (slr)-1 ( Fukaki et al., 2002 ) and iaa28-1 ( Rogg et al., 2001 ) produce fewer root branches while iaa17/axr3-1 ( Leyser et al., 1996 Rouse et al., 1998 ) plants display more lateral roots than wild-type plants. Loss-of-function mutations in Aux/IAA genes seem to condition very subtle phenotypes likely because of redundancy and/or a feedback mechanism. However, antisense suppression of IAA9 in tomato (Lycopersicon esculentum) produced numerous growth and form alterations ( Wang et al., 2005 Fig. 1b). Wild-type compound leaves were transformed into simple leaves, stem/hypocotyl elongation was enhanced, and apical dominance was reduced.

Auxin response factor (ARF) genes The founding member of the ARF gene family, ARF1, was discovered using the yeast one hybrid system because of its property to bind to the auxin response cis-element (AuxRE) found in the promoters of many auxin-regulated genes ( Ulmasov et al., 1997 ). ARFs can be activators or repressors of transcription, depending on the nature of a central protein domain ( Ulmasov et al., 1999 Tiwari et al., 2003 ). They bind to DNA to regulate transcription as homo- or heterodimers with other ARFs or Aux/IAA proteins ( Liscum & Reed, 2002 ). Loss-of-function mutations in a few ARFs have strong and specific phenotypes, including effects on stature, leaf morphology, and root architecture ( Okushima et al., 2005b ). Three Arabidopsis null ARF2 alleles, generated by T-DNA insertions and identified using a reverse genetics approach, produce plants that display longer, thicker inflorescence stems, and larger, darker green leaves compared with wild-type plants ( Okushima et al., 2005a ). In addition to stem and leaf enlargement, arf2 seeds were also larger than wild-type seeds. ARF2 overexpression and RNAi suppression resulted in transgenic plants that phenocopied the arf2 mutant, a result of cosuppression and RNAi downregulation, respectively.

Although many of the ARF single loss-of-function mutants do not show growth and developmental defects, presumably because of functional redundancy among the 23 members, some of the double mutations have a significant effect on stature and form ( Okushima et al., 2005b ). For example, arf19 is phenotypically indistinguishable from wild-type plants. ARF7 is the putative paralog of ARF19 and the double arf7/arf19 mutant displays thin and short florescence stems, enhanced apical dominance, and reduced and delayed lateral root formation ( Okushima et al., 2005b ). Overexpression of AFR19 in transgenic plants produces a distinctive dwarf phenotype, decreased apical dominance, and narrow, elongated leaves ( Okushima et al., 2005b ).

Ubiquitin-mediated regulatory degradation Regulated protein degradation plays an essential role in auxin signaling ( Dharmasiri & Estelle, 2004 Leyser, 2002 ). The central role of this mechanism in auxin signaling is exemplified by the discovery that the auxin receptor is part of the ubiquitination pathway that leads to protein degradation ( Dharmasiri et al., 2005 Kepinski & Leyser, 2005 ). Several mutants affected in components of the pathway can display strong modifications in stature and form. For example, the axr1-12 mutant of Arabidopsis harbors a loss-of-function mutation in a gene encoding the amino-terminal part of a ubiquitin-activating enzyme ( Leyser et al., 1993 Stirnberg et al., 1999 ). Recently, two ubiquitin C-terminal hydrolases (UCHs) (i.e. UCH1 and UCH2) that are involved in de-ubiquitination and reversing the effect of ubiquitin conjugation were also found to be involved in auxin signaling through increasing or decreasing AUX/IAA protein stability, respectively, in overexpressing and loss-of-function mutants ( Yang et al., 2007 ). Overexpressing plants increased, while double mutants suppressed, the outgrowth of cauline lateral branches.

2. Gibberellin – the ‘Green Revolution’ hormone

Gibberellins (GAs) are a complex family of tetracyclic diterpenoid growth regulators that play a critical role in many plant growth and developmental processes (reviewed in Hooley, 1994 Davies, 1995 ). Advances in molecular genetics have allowed identification of many of the genes involved in the metabolism and signaling pathway, and dissection of their role in regulation of plant stature and form (reviewed in Hedden & Phillips, 2000a Sun, 2000 Olszewski et al., 2002 ). The results of these studies were eloquently summarized in a recent review article as ‘a tale of the tall and the short’ ( Thomas & Sun, 2004 ). Typically, mutants with a deficiency in GA concentrations or response are dwarf or semi-dwarf in stature, while elevated GA concentrations or increased signaling result in taller plants. GA metabolic and response genes have provided the basis of the ‘Green Revolution’ varieties of rice and wheat ( David & Otsuka, 1994 ) and have been a logical focus for improving crop performance via both conventional breeding and genetic engineering ( Sakamoto et al., 2003 ). The Reduced height1 (Rht1) allele in wheat is a dominant gain-of-function mutation in the coding sequence of a DELLA protein (discussed below in ‘Negatively acting components’ Peng et al., 1999 ), while the semidwarf1 (sd1) ‘Green Revolution’ allele in rice is a recessive loss-of-function mutation in one of the major GA biosynthetic genes – GA20-oxidase (GA20ox) ( Monna et al., 2002 Sasaki et al., 2002 Spielmeyer et al., 2002 ).

Several properties of GA metabolic and response genes make them particularly attractive targets for manipulation. First, many of the genes act in a dose-dependent manner, allowing generation of a gradient of phenotypic responses ( Cowling et al., 1998 ). Secondly, similar phenotypic effects can be readily achieved in heterologous species ( Hynes et al., 2003 Busov et al., 2006 ). Finally, in contrast to most other plant hormone modifications, the pleiotropic effects are usually positive with respect to crop performance – including increased nitrogen assimilation ( Nagel & Lambers, 2002 ), photosynthesis ( Biemelt et al., 2004 ), and lateral root production ( Busov et al., 2006 ).

Metabolic genes GAs are synthesized in three successive steps localized in separate intracellular compartments, with the first stage in chloroplasts, the second in the endoplasmic reticulum, and the third in the cytoplasm ( Hedden & Phillips, 2000a ). The flux of bioactive GAs is controlled by the enzymes in the third compartment, such as GA20ox, GA3-oxidase (GA3ox), and GA2-oxidase (GA2ox) ( Hedden & Phillips, 2000b ). GA20ox and GA3ox are biosynthethic enzymes that catalyze the last two steps in the biosynthetic pathway. Until recently, GA2ox was the only known GA-inactivating enzyme ( Olszewski et al., 2002 ) a new deactivation reaction that is catalyzed by a P450 enzyme was recently described in rice ( Zhu et al., 2006 ). Each of these enzymes is encoded by a small family or subfamily of genes. Loss-of-function mutations in the GA20ox and GA3ox genes or overexpression of the GA2ox genes has a dwarfing effect and has been observed in numerous plant species, including Arabidopsis ( Sun & Kamiya, 1994 Helliwell et al., 1998 Yamaguchi et al., 1998 ), rice ( Sakamoto et al., 2001 ), potato (Solanum tuberosum Carrera et al., 2000 ), and poplar (Populus tremula×alba Busov et al., 2003 Fig. 1c) (reviewed in Hedden & Phillips, 2000b ). By contrast, GA-overproducing mutants, with hyperactivated GA biosynthetic activity, or reduced activity of the catabolic genes, often show extreme shoot elongation ( Martin et al., 1999 Carrera et al., 2000 ).

Signal transduction mutants Genes involved in the GA signal transduction pathway have been identified through GA response mutants. These mutants are either GA-insensitive dwarfs or constitutive GA response mutants (reviewed in Sun, 2000 ). GA-insensitive mutants show symptoms of GA deficiency, but unlike GA metabolic mutants cannot be rescued by GA treatment. The signaling components identified through such mutations can be broadly classified into positively and negatively acting groups.

Positively acting components. These represent a diverse group of genes encoding heteromeric G proteins, transcription regulators, chromatin-remodeling factors, and enzymes. Loss-of-function mutations in some of these genes cause distinct dwarf or semi-dwarf phenotypes. DWARF1 (D1) in rice is the only gene that encodes an α subunit of the heteromeric G protein ( Ashikari et al., 1999 ). Knockouts of the gene cause reduced stature and dark-green leaves, similar to GA-deficient rice plants. The first leaf in d1 plants is GA insensitive but the second leaf shows a normal GA response. Knockout mutations in the Arabidopsis ortholog, although this is also a single-copy gene, do not result in the dwarf phenotypes observed in rice ( Ullah et al., 2001 ). The normal GA sensitivity of the second leaf in rice and the lack of dwarf phenotype in Arabidopsis suggest that D1 may not be directly involved in GA signaling, and that its importance in GA signal transduction varies among species.

PHOTOPERIOD RESPONSIVE1 (PHOR1) was identified in potato in a screen for mRNAs that accumulate during short day (SD) inductive treatment ( Amador et al., 2001 ). Antisense knockouts of the gene cause a semi-dwarf phenotype similar to that of GA metabolic mutants, and overexpression results in enhanced growth. Sequence predictions and PHOR1::GFP fusion experiments suggest that PHOR1 is a transcription factor that is regulated through modification of a Cys-Pro-Ile (CPI) domain, resulting in differential accumulation in the nucleus under GA signaling, and sequestration in the cytosol in the absence of GA signaling.

SLEEPY is a gene that was initially identified as a suppressor of the Arabidopsis abscisic acid insensitive mutant abi1-1, and its loss-of-function causes dwarf phenotypes and dark-green foliage typical of mutants associated with GA signaling or metabolism ( Steber et al., 1998 ). The corresponding gene was subsequently cloned and found to encode an F-box subunit of an Supressor of kinetochore protein1/Cullin/F-box protein complex (SCF) E3 ubiquitin ligase that participates in ubiquitination of proteins targeted for degradation, with the putative targets being DELLA proteins.

Negatively acting components. These include_mutations in GA INSENSITIVE (GAI), REPRESSOR OF GA1 (RGA), and RGA-LIKE1 (RGL1). These genes have been identified in many plant species, including Arabidopsis ( Peng et al., 1997 Silverstone et al., 1997 Lee et al., 2002 Wen & Chang, 2002 ), rice ( Ikeda et al., 2001 ), wheat ( Peng et al., 1999 ), maize ( Peng et al., 1999 ) and grapevine (Vitis vinfera Boss & Thomas, 2002 ). Gain-of-function mutations in these genes cause a semi-dominant dwarf phenotype, while loss-of-function mutations are recessive and result in increased growth. Mutant analyses of these proteins suggest that they are negative regulators of the GA signal transduction pathway.

GAI, RGA and RGL belong to the larger GAI, RGA and SCARECROW (GRAS) family of transcription factors and are also known as DELLA proteins because of a conserved N-terminus DELLA domain that is absent in the other family members ( Pysh et al., 1999 ). Complete deletion or nonsynonymous substitutions in this domain produce strong gain-of-function, dominant mutations that result in constitutive inhibition of one or several GA responses ( Peng et al., 1997 ). Such mutations result in dwarf or semi-dwarf plants and similar effects of transgenic expression of the mutant forms can be observed in heterologous species ( Fu et al., 2001 Busov et al., 2006 ). As discussed above, natural mutations identified in rice and wheat through traditional breeding became the basis for the development of the ‘Green Revolution’ varieties ( Silverstone & Sun, 2000 ).

SPINDLY (SPY) is also believed to be a negative regulator of the GA response in plants ( Jacobsen et al., 1996 Thornton et al., 1999a ). Constitutive overexpression of the Arabidopsis SPY gene in Arabidopsis ( Swain et al., 2001 ) and petunia (Petunia hybrida Izhaki et al., 2002 ) causes dwarfing. SPY shows protein sequence similarity to UDP-GlcNAc protein transferases (OGTs) in animals ( Thornton et al., 1999b ), and has been demonstrated to possess OGT activity ( Thornton et al., 1999a ). OGT protein modification regulates protein activity, and the extent of this modification depends on metabolic hormonal and developmental signals ( Corner & Hart, 2000 ). SHORT INTERNODES (SHI) is part of a nine-member gene family that have RING finger-class zinc finger motifs, which have been suggested to play roles in protein–protein interactions during proteolysis or transcription activation ( Fridborg et al., 1999 ). Overexpression of the gene results in decreased shoot elongation, suggesting that SHI is also a negative regulator of GA responses ( Fridborg et al., 2001 ).

3. Brassinosteroids

Brassinosteroids (BRs) are a class of more than 40 sterol derivatives in plants that have profound effects on plant size and architecture. Biosynthesis and signal transduction have been subjects of intense genetic dissection (reviewed in Fujioka & Yokota, 2003 Vert et al., 2005 Haubrick & Assmann, 2006 ). Several mutations found recently hold promise for modification of stature and form relevant to crop improvement ( Bishop, 2003 ), and are summarized below.

The rate-limiting biosynthetic and catabolic steps in BR metabolism have been identified in BR-deficient mutants. The classic BR-deficient phenotype is characterized by short robust stems, and small, round, dark-green leaves. The C6- and C22α-oxidation steps are rate limiting in synthesis of brassinolide – the most bioactive BR found to date ( Choe et al., 2001 ). The tomato DWARF gene D was isolated via transposon tagging and found to show homology to two P450s (CYP90A and CYP90B) and was classified as CYP85 ( Bishop et al., 1999 ). Mutant plants showed classic BR phenotypes but, unlike in Arabidopsis, did not display reduced apical dominance. Overexpression of the gene under the 35S promoter fully complemented the dwarf allele, and the lines were larger than wild type however, a limited number of lines were screened, precluding general conclusions on its growth-enhancing effects. More conclusive results with respect to the growth-promoting effects of these genes were obtained in a study of the DWARF4 (DWF4) gene in Arabidopsis ( Choe et al., 2001 ). DWARF4 was found to encode a P450 enzyme with highest homology to the Arabidopsis CONSTITUTIVE PHOTOMORPHOGENESIS DWARFISM (CPD) protein (CYP90A1). Loss-of-function produced a dwarf phenotype, while overexpression caused strong growth-promoting effects in both Arabidopsis and tobacco (Nicotiana tabacum) that was similar to that of exogenously applied bioactive BR. The height of transgenic Arabidopsis plants was 40% greater than that of wild-type plants and resulted primarily from continued growth beyond 35 d after germination, when wild-type plants had ceased elongation. Height was similarly but more modestly (14%) increased in tobacco. In both Arabidopsis and tobacco, DWF4 increased petiole and leaf blade length and increased lateral branching.

In cereals, BR deficiency is associated with an increase in leaf erectness, which is an important crop trait because it increases photosynthesis in lower leaves, yet allows normal growth under dense planting conditions on farms ( Feldmann, 2006 ). In contrast to Arabidopsis, DWARF4 in rice is encoded by two genes, OsDWARF4L1 and OsDWARF4 ( Sakamoto et al., 2006 ). The two genes encode enzymes of redundant biochemical functions but of very different developmental roles, likely because of their different expression patterns. For example, OsDWARF4L1 loss-of-function results in semi-dwarf phenotypes with small seeds, while knockout mutations in OsDWARF4 cause more modest dwarfing, do not affect seed size, and increase leaf erectness. A small field trial experiment with plants carrying the osdwarf4-1 mutant allele with two planting densities and three levels of nitrogen revealed that, under highest density and nitrogen, the osdwarf4-1 plants produced 40% more biomass. The osdwarf4-1 plants displayed increased grain yields (17–20%) compared with wild-type plants at all nitrogen levels under a dense planting environment. Differences in both biomass and grain yield were less dramatic under normal planting density.

4. A novel hormonal pathway controls branching

Plant form is largely determined by the activity of axillary meristems, whose growth is regulated by auxin and CK. In many plant species, shoot apices grow predominantly and repress axillary bud growth, a process termed apical dominance (e.g. Cline, 2000 ). Shoot tips produce the majority of auxin, and thus removal (decapitation) typically induces outgrowth of axillary buds application of auxin to the cut tip prevents outgrowth. Moreover, application of auxin transport inhibitors to the stems of intact plants can reduce apical dominance, further supporting the hypothesis that apically derived auxin is transported basipetally and inhibits outgrowth of axillary buds. By contrast, application of CK to axillary buds or to roots often promotes outgrowth. In addition to modification of the form of annual plants, genes that affect auxin signals are likely to affect apical dominance in trees, which is an important determinant of wood quality, fruit yield, and biomass production (e.g. Bradshaw & Strauss 2001 ).

Studies in Arabidopsis and other annual plants have identified a novel hormonal pathway regulating the outgrowth of axillary meristems (reviewed in McSteen & Leyser 2005 Bennett & Leyser, 2006 Dun et al., 2006 ). Reciprocal grafting studies between mutant and wild-type pea (Pisum sativum) showed that regulation of bud outgrowth involves long-distance signaling that does not involve auxin and CK ( Dun et al., 2006 ). In Arabidopsis, four genes, MORE AXILLARY BRANCHING (MAX)1–4, have been identified that are involved in this signaling pathway they act to repress lateral outgrowth. MAX3 and MAX4 are required for the production of a yet unidentified graft transmissible branching signal, and belong to the carotenoid cleavage dioxygenase (CCD) family, suggesting that this signal might be a carotenoid derivative ( Sorefan et al., 2003 Booker et al., 2004 ). MAX1 acts downstream of MAX3/4 in the synthesis of the branching signal and encodes a member of the cytochrome P450 family ( Booker et al., 2005 ). MAX2 encodes an F-box protein, which is typically involved in ubiquitin-mediated protein degradation – a common strategy employed in plants for signal perception and transduction ( Stirnberg et al., 2002 ). MAX2 interacts with the core components of SCF-type E3 ubiquitin ligases and acts locally at the node ( Stirnberg et al., 2002 ).

Mutant and reciprocal grafting analyses have also revealed a similar inhibitory pathway in pea and petunia, controlled respectively by the RAMOSUS (RMS) and DECREASED APICAL DOMINANCE (DAD) genes (e.g. Dun et al. 2006 Simons et al., 2007 ). RMS5, RMS1 and RMS4 have been cloned and are orthologs of MAX3, MAX4 and MAX2, while DAD1 is orthologous to MAX4/RMS1 ( Sorefan et al., 2003 Snowden et al., 2005 Johnson et al., 2006 ). Recently, rice orthologs of MAX2, MAX3 and MAX4 have been shown to repress tiller bud outgrowth ( Ishikawa et al., 2005 Zou et al., 2006 Arite et al., 2007 ). Although this pathway is conserved among diverse angiosperms, differences are also apparent. For example, RMS1 expression is altered in different rms mutant backgrounds and expression in the stem is affected by auxin concentrations ( Foo et al., 2005 ), but such regulation was not observed for the orthologous MAX4 ( Sorefan et al. 2003 Bainbridge et al., 2005 ). Whether these and other differences reflect major differences in this pathway between species, variation in the importance of various pathway components or differences in experimental techniques remains to be determined ( Dun et al., 2006 Ongaro & Leyser, 2007 ). Studies by Bennett et al. (2006 ) suggested that the MAX pathway acts by controlling auxin transport capacity in the stem. With the exception of rms2, pea rms mutants as well as Arabidopsis max mutants show reduced xylem sap CK but have near wild-type concentrations of shoot CK ( Foo et al., 2007 ). Detailed studies in pea have provided strong evidence for a basipetally moving feedback signal involving RMS2 that reduces xylem CK and promotes the expression of RMS1 and RMS5. Although the identity of this feedback signal and many other details are still unknown, the MAX/RMS pathway clearly involves cross-talk with auxin and cytokinin.


Materials and methods

Plant material

The maternal inbred line 541 was obtained in West Pomeranian University of Technology, Szczecin. This self-fertile line has a tall phenotype and has been reproduced by self-pollination for more than 20 years. The detailed characteristics of the line were described by Milczarski et al. [28]. Bashirskaja karlikovaja, a Russian source of a recessive dwarfing gene, was kindly provided by the N.I. Vavilov All-Russian Scientific Research Institute of Plant Industry, St. Petersburg, Russia. The paternal BK-1 line was derived in the University of Life Sciences in Lublin by two cycles of self-pollination. The BK-1 line is characterised by at least 2 times higher tillering capacity than cultivated varieties or lines and flexed, semi-erect stem with high number of short internodes. This specific, not upright growth form results from the fact that from a given node successive internodes grow at an angle to the previous one. The nodes are significantly thicker than the diameter of the stem. The spikes are short and flat, filled with very small grains of low weight. In addition, one stem may have supernumerary ears growing from a lower node. The leaves of the BK-1 line are proportionally shorter and narrower in comparison to other rye varieties and lines.

The two parental lines were hybridized in 2012, and in the next season, 2 tall F1 plants (code: BK2 and BK3) were self-pollinated. In 2013, F2 populations were sown in Czesławice in the Experimental Farm of the University of Life Sciences in Lublin. The height of 331 plants of these populations was measured to verify the expected 3:1 ratio of segregation. The distribution of dwarfism in the F2 population was confirmed by observation of the F3 progeny (7–20 plants per line). If the number of plants assessed in generation F3 was less than 7, such a genotype was not classified into any of the groups (dwarf, tall, segregating), unless both tall and dwarf plants were visible. The hypothesis of the monogenic inheritance of plant height in F2 and F3 progenies was verified using the χ 2 test.

The parents and F2 population were assessed for their morphological traits: plant height, length, and thickness of the second internode from the bottom, main spike length, and number and weight of kernels in the main spike. The measurements of the parental plants were performed in 2014–2016. The BK-1 line was also crossed with Dańkowskie Amber (Danko Ltd.) to test the gene effect and introduce the dw9 to a modern population variety. A basic characterization of these newly developed plant materials was conducted to assess six morphological traits, including thousand-kernel weight (TKW). The statistical analyses of the mean values, standard deviation, correlation coefficients, and significant differences were conducted using Statistica 13.3 software (TIBCO Software Inc.).

Genetic mapping

Genomic DNA was extracted from young, healthy leaves of F2 plants and parental individuals using a GenElute ™ Plant Genomic DNA Miniprep Kit (Sigma). DNA from 94 randomly chosen F2 plants and parents was used for DArTSeq genotyping (Diversity Arrays Technique Pty Ltd). The results obtained from phenotype segregations and the DArTSeq scores were used to construct a genetic map of the rye chromosome carrying the new recessive dwarfing gene. DNA from the F2 mapping population and parents was used for DArTseq genotyping (Diversity Arrays Technique Pty Ltd). Linkage and localization of the identified markers with the dwarfing gene were performed with Kosambi mapping function using JoinMap 5.0 software [29].

In the final analysis, only a linkage group with dwarfing gene segregation was constructed with an LOD ratio of 17. The linkage groups were thereafter compared to the RIL-S (population 541 × 2020) consensus genetic map of rye [30]. An original map showing the relationships to the reference map was generated using MapChart 2.0 software [31]. The chromosome location was verified using data from the reference sequence of the Secale cereale Lo7_v2 database [32]. For this purpose, scaffolds with a description of the chromosome 6R were extracted from the Sc_Lo7_v2 library. DArTSeq sequences were mapped to reference scaffolds (Sc_Lo7_v2_6R) and filtered out, and the chromosome position has been verified. All scaffolds to which DArTSeq markers have been mapped were annotated as well using a custom "nt" library. The DArTseq sequence mapping to reference scaffolds and an annotation analysis of selected scaffolds were performed using CLC Genomic Workbench 11.0 (QIAGEN Ltd.).

GA3-seedling test

The GA3-seedling test was performed according to Worland [33] with minor modifications. Seeds of tall parental line 541 and the semi-dwarf BK-1 line were germinated on moistened filter paper. The seeds were kept at 4˚C for 48 h to synchronize germination and then kept at 20˚C for 3 days. Seedlings of both lines were then transferred into plastic boxes containing moistened perlite, and a standardized nutrient solution supplemented with 5 ppm of GA3 was applied every day until the seedlings reached the 2–3 leaf stage. The second group of plants of both lines was treated with a nutrient solution without GA3 (control group).

Plants were planted in three replications. At 14 days after the initiation of germination, the length of the seedlings was measured from the base to the top. Statistical analyses were performed to calculate significant differences between the means of groups using a t-Student’s test in Statistica 13.3 software (TIBCO Software Inc.).


What genes in a plant determine whether a stem is erect or climbing? - Biology

74 notecards = 19 pages ( 4 cards per page)

Campbell Biology Chapter 29

1) The structural integrity of bacteria is to peptidoglycan as the structural integrity of plant spores is to

2) All of the following are common to both charophytes and land plants except

3) In animal cells and in the meristem cells of land plants, the nuclear envelope disintegrates during mitosis. This disintegration does not occur in the cells of most protists and fungi. According to our current knowledge of plant evolution, which group of organisms should feature mitosis most similar to that of land plants?

A) unicellular green algae

E) multicellular green algae

4) On a field trip, a student in a marine biology class collects an organism that has differentiated organs, cell walls of cellulose, and chloroplasts with chlorophyll a. Based on this description, the organism could be a brown alga, a red alga, a green alga, a charophyte recently washed into the ocean from a freshwater or brackish water source, or a land plant washed into the ocean. The presence of which of the following features would definitively identify this organism as a land plant?

A) alternation of generations

C) rings of cellulose-synthesizing complexes

5) Some green algae exhibit alternation of generations. All land plants exhibit alternation of generations. No charophytes exhibit alternation of generations. Keeping in mind the recent evidence from molecular systematics, the correct interpretation of these observations is that

A) charophytes are not related to either green algae or land plants.

B) plants evolved alternation of generations independently of green algae.

C) alternation of generations cannot be beneficial to charophytes.

D) land plants evolved directly from the green algae that perform alternation of generations.

E) scientists have no evidence to indicate whether or not land plants evolved from any kind of alga.

6) Which taxon is essentially equivalent to the "embryophytes"?

7) A student encounters a pondweed which, judging from its appearance, seems to be a charophyte. She brings a sample back to her biology lab. Using only a compound light microscope to study the sample, which of the following features should help her to determine whether the sample comes from a charophyte or from some other type of green alga?

  1. molecular structure of enzymes inside peroxisomes
  2. structure of sperm cells
  3. presence of phragmoplasts
  4. rings of cellulose-synthesizing complexes

8) Given its composition and location, the phragmoplast should be directly involved in the

A) segregation of daughter chromosomes during anaphase.

B) poleward migration of centrosomes during prophase.

C) synthesis of sporopollenin during G1 and G2 phases.

D) construction of the cell plate during cytokinesis.

E) reinforcement of the nuclear envelope during S phase.

9) Structurally, phragmoplasts should be most similar to

B) the myofilaments of muscle cells.

C) the internal support structures of microvilli.

D) the cytoskeletal elements that produce cytoplasmic streaming and amoeboid motion.

10) What is true of charophytes?

A) They are the ancestors of green algae.

B) They are examples of seedless vascular plants.

C) They are the closest living algal relatives of land plants.

D) They share some features in common with land plants, namely spores surrounded by sporopollenin and alternation of generations.

11) The functional role of sporopollenin is primarily to

A) comprise spore surface structures that catch the wind and assist in spore dispersal.

C) make spores less dense and able to disperse more readily.

E) provide nutrients to spores

12) If the kingdom Plantae is someday expanded to include the charophytes, then the shared derived characteristics of the kingdom will include

  1. rings of cellulose-synthesizing complexes.
  2. chlorophylls a and b.
  3. alternation of generations.
  4. cell walls of cellulose.
  5. ability to synthesize sporopollenin.

13) Which of the following were probably factors that permitted early plants to successfully colonize land?

  1. the relative number of potential predators (herbivores)
  2. the relative number of competitors
  3. the relative availability of symbiotic partners
  4. air's relative lack of support, compared to water's support

14) Which of the following was not a challenge for survival of the first land plants?

15) The following are all adaptations to life on land except

A) rings of cellulose-synthesizing complexes.

D) reduced gametophyte generation.

16) Mitotic activity by the apical meristem of a root makes which of the following more possible?

A) increase of the aboveground stem.

B) decreased absorption of mineral nutrients.

C) increased absorption of CO2

D) increased number of chloroplasts in roots.

E) effective lateral growth of the stem.

17) Which event during the evolution of land plants probably made the synthesis of secondary compounds most beneficial?

A) the greenhouse effect present throughout the Devonian period

B) the reverse-greenhouse effect during the Carboniferous period

C) the association of the roots of land plants with fungi

E) the rise of wind pollination

18) Which of the following statements is true of archegonia?

A) They are the sites where male gametes are produced.

B) They may temporarily contain sporophyte embryos.

C) They are the same as sporangia.

D) They are the ancestral versions of animal gonads.

E) They are asexual reproductive structures.

19) Which of the following is a true statement about plant reproduction?

A) Embryophytes are small because they are in an early developmental stage.

B) Both male and female bryophytes produce gametangia.

C) Gametangia protect gametes from excess water.

D) Eggs and sperm of bryophytes swim toward one another.

E) Bryophytes are limited to asexual reproduction.

20) Assuming that they all belong to the same plant, arrange the following structures from largest to smallest.

21) The leaflike appendages of moss gametophytes may be one to two cell layers thick. Consequently, which of the following is least likely to be found associated with such appendages?

B) rings of cellulose-synthesizing complexes

22) Considering that the mature sporophytes of true mosses get their nutrition from the gametophytes on which they grow, and considering these generations as individual plants, what is true of the relationship between true moss sporophytes and gametophytes?

A) Sporophytes are endosymbionts of gametophytes.

B) Sporophytes are mutualists of gametophytes.

C) Sporophytes are commensalists of gametophytes.

D) Sporophytes are parasites of gametophytes.

23) As is true of the gametophytes of all land plants, the gametophytes of true mosses lack stomata. Yet, the feather moss Pleurozium harbors nitrogen-fixing cyanobacteria. Which of the following is a feature of moss gametophytes that is most important for the continued survival of these cyanobacteria in the tissues of the feather moss gametophyte?

B) lack of vascular tissues

C) lack of true leaves or roots

D) lack of an independent sporophyte

E) lack of multiple cell layers in "leaves" of "buds"

24) Which of the following is true of the life cycle of mosses?

A) The haploid generation grows on the sporophyte generation.

B) Spores are primarily distributed by water currents.

C) Antheridia and archegonia are produced by gametophytes.

D) The sporophyte generation is dominant.

E) The growing embryo gives rise to the gametophyte.

25) Beginning with the germination of a moss spore, what is the sequence of structures that develop after germination?

26) At some time during their existence, bryophytes may feature

27) Two small, poorly drained lakes lie close to each other in a northern forest. The basins of both lakes are composed of the same geologic substratum. One lake is surrounded by a dense Sphagnum mat the other is not. Compared to the pond with Sphagnum, the pond lacking the moss mat should have

A) lower numbers of bacteria.

B) reduced rates of decomposition.

28) If you are looking for structures that transfer water and nutrients from a bryophyte gametophyte to a bryophyte sporophyte, then on which part of the sporophyte should you focus your attention?

29) In which of the following taxa does the mature sporophyte depend completely on the gametophyte for nutrition?

D) Pterophyta, Bryophyta, and horsetail (Equisetum)

E) Pterophyta and Bryophyta

30) You are hiking in a forest and happen upon a plant featuring a central stemlike structure from which sprout many, tiny, leaflike structures. Which of the following would be the most certain means of distinguishing whether it was a true moss or a club moss?

D) if conducting tissues are present

E) the appearance of its spore-producing structures

31) Which of the following characteristics helped seedless plants better adapt to life on land?

E) an unbranched sporophyte

32) A botanist discovers a new species of plant in a tropical rain forest. After observing its anatomy and life cycle, he notes the following characteristics: flagellated sperm, xylem with tracheids, separate gametophyte and sporophyte generations with the sporophyte dominant, and no seeds. This plant is probably most closely related to

33) You are hiking in a forest and come upon a mysterious plant, which you determine is either a lycophyte sporophyte or a pterophyte sporophyte. Which of the following would be most helpful in determining the correct classification of the plant?

A) whether or not it has true leaves

B) whether it has microphylls or megaphylls

C) whether or not it has seeds

E) whether or not it has chlorophyll a

34) Sporophylls can be found in which of the following?

35) If a fern gametophyte is a hermaphrodite (that is, has both male and female gametangia on the same plant), then it

A) belongs to a species that is homosporous.

C) has lost the need for a sporophyte generation.

D) has antheridia and archegonia combined into a single sex organ.

E) is actually not a fern, because fern gametophytes are always either male or female.

36) Assuming that they all belong to the same plant, arrange the following structures from largest to smallest (or from most inclusive to least inclusive).

37) If humans had been present to build log structures during the Carboniferous period (they weren't), which plant types would have been suitable sources of logs?

A) whisk ferns and epiphytes

B) horsetails and bryophytes

C) lycophytes and bryophytes

D) ferns, horsetails, and lycophytes

E) charophytes, bryophytes, and gymnosperms

38) Which of the following is true of seedless vascular plants?

A) Extant seedless vascular plants are larger than the extinct varieties.

B) Whole forests were dominated by large, seedless vascular plants during the Carboniferous period.

C) They produce many spores, which are really the same as seeds.

D) The gametophyte is the dominant generation.

E) Sphagnum is an economically and ecologically important example.

39) Which of the following are land plants that use the same means of getting sperm to egg that algae use?

A) true mosses, hornworts, and liverworts

B) ferns, whisk ferns, and horsetails

D) Two of the responses above are correct.

40) Arrange the following terms from most inclusive to least inclusive.

  1. embryophytes
  2. green plants
  3. seedless vascular plants
  4. ferns
  5. tracheophytes

41) Evidence indicates that plants increase the number of stomata in their leaves as atmospheric CO2‚‚ levels decline. Increasing the number of stomata per unit surface area should have the effect of doing which of the following?

  1. increasing dehydration of leaf tissues
  2. decreasing dehydration of leaf tissues
  3. countering the effect of declining CO2‚‚ on photosynthesis
  4. reinforcing the effect of declining CO2‚‚ on photosynthesis
  5. decreasing the O2‚‚ content of air next to the leaves lower than it would otherwise be
  6. increasing the O2‚‚ content of air next to the leaves higher than it would otherwise be

42) Increasing the number of stomata per unit surface area of a leaf when atmospheric CO2‚‚ levels decline is most analogous to a human

A) breathing faster as atmospheric CO2‚‚ levels increase.

B) putting more red blood corpuscles (RBCs) into circulation when atmospheric O2‚‚ levels decline.

C) removing RBCs from circulation when atmospheric O2‚‚ levels increase.

D) breathing more slowly as atmospheric O2‚‚ levels increase.

43) Which of the following should have had gene sequences most similar to the charophyte that was the common ancestor of the land plants?

44) Of the following list, flagellated (swimming) sperm are generally present in which groups?

45) If intelligent extraterrestrials visited Earth 475 million years ago, and then again 300 million years ago (at the close of the Carboniferous period), what trends would they have noticed in Earth's terrestrial vegetation over this period?

  1. a trend from dominant gametophytes to dominant sporophytes
  2. a trend from sporangia borne on modified leaves (sporophylls) to sporangia borne on stalks (seta)
  3. a trend from no true leaves, to microphylls, to megaphylls
  4. a trend from soil-surface-hugging plants to "overtopping" plants
  5. a trend toward increased lignification of conducting systems

46) If you were faced with the choice of eliminating all mutualistic symbioses involving plants and other organisms (besides humans), with the goal being to preserve the most plant biomass, which of the following would you save from elimination?

A) the dispersal of seeds in or on animals

B) the dispersal of male gametophytes by animals

C) plants harboring nitrogen-fixing bacteria

D) associations between soil fungi and roots or rhizoids

47) During glacial periods in the early evolution of land plants, which of the following would have been a beneficial adaptation regarding the number of stomata per unit surface area, and what accounts for it?

A) increased numbers of stomata, to maximize absorption of increasing levels of atmospheric CO2‚‚

B) increased numbers of stomata, to maximize ability to absorb low levels of atmospheric CO2‚‚

C) decreased numbers of stomata, to retain CO2‚‚ produced by the chloroplasts

D) decreased numbers of stomata, to maximize absorption of low levels of atmospheric CO2‚‚

48) What is thought to be the correct sequence of the following events during the Carboniferous period?

  1. vascular plants become more prevalent
  2. megaphylls with large surface areas become more prevalent
  3. atmospheric CO2‚‚ levels decline by a factor of five
  4. global cooling occurs, leading to widespread glaciations

49) Researchers tested nitrogen loss from soil where the moss Polytrichum was growing, and compared it to soil from which Polytrichum had been removed. The data are presented below.

Loss of soil nitrogen via "gaseous emission" was found to be negligible. Rather, most loss of soil nitrogen was due to water erosion of the soil. Which of these hypotheses is least likely to account for the observed results?

A) If rhizoids had helped stabilize the soil, then less erosion and less loss of nitrogen would occur.

B) If protonemata had absorbed, and stored, nitrogen from the soil, then they would have reduced loss of nitrogen by erosion.

C) If the overlying mat of gametophores had slowed the entry of water into the soil, then it would have reduced water's ability to erode the soil and carry away its nitrogen.

D) If sporophyte stomata had absorbed nitrogen from the soil, then they would have reduced loss of nitrogen by erosion.

Researchers decided to test the hypothesis that if the 2-m tall Polytrichum gametophyte-sporophyte plants had acted as a physical buffer, then they would have reduced water's ability to erode the soil and carry away its nitrogen. They began with four equal-sized areas where Polytrichum mosses grew to a height of 2 m above the soil surface. One of the four areas was not modified. In the second area, the mosses were trimmed to a height of 1 m above the soil surface. In the third area, the mosses were trimmed to a height of 0.5 m above the soil surface. In the fourth area, the mosses were trimmed all the way to the ground, leaving only the rhizoids. Water, simulating rainfall, was then added in a controlled fashion to all plots over the course of one year. Figure 29.2 presents four graphs that depict potential results of this experiment.

50) Which graph of soil nitrogen loss over time in Figure 29.2 most strongly supports the hypothesis that if the 2-m tall Polytrichum gametophyte-sporophyte plants had acted as a physical buffer, then they would have reduced water's ability to erode the soil and carry away its nitrogen?

51) If the actual results most closely resembled those in Figure 29.2(A), then a further question arising from these data is: "Do the Polytrichum rhizoids have to be alive in order to reduce soil nitrogen loss, or do dead rhizoids have the same effect?" Arrange the following steps in the correct sequence to test this hypothesis.

  1. Add metabolic poison to the soil of the experimental plot of mosses.
  2. Apply water equally to the experimental and control plots.
  3. Measure initial soil nitrogen contents of control and experimental plots.
  4. Determine nitrogen loss from soil of control and experimental plots.
  5. Establish two identical plots of Polytrichum mosses one as a control, the other as the experimental treatment.

52) Which of these potential results of applying a metabolic poison to the rhizoids of Polytrichum should interfere the least with the ability to draw valid conclusions from this experiment?

A) If, upon dying, the rhizoids leak nitrogenous compounds into the soil before final nitrogen content is measured.

B) If, upon dying, decomposition of the rhizoids introduces nitrogenous compounds to the soil before final nitrogen content is measured.

C) If the metabolic poison is hydrogen cyanide (HCN) or sodium azide (NaN3), and much of the poison remains in the soil.

D) If the metabolic poison acts against the mitochondria of the rhizoid cells.

E) If the metabolic poison absorbs nitrogen and strongly adheres to soil particles, acting as a sort of glue.

53) Why should we expect the soil's nitrogen not to be contained solely within the rhizoids of the Polytrichum mosses?

A) Rhizoids are associated with fungi that inhibit mineral transfer from soil to rhizoids.

B) Rhizoids are not absorptive structures.

C) Rhizoids consist of single, tubular cells or of filaments of cells.

D) Rhizoids lack direct attachment to the moss sporophytes.

54) The 2-m height attainable by Polytrichum moss is at the upper end of the size range reached by mosses. What accounts for the relative tallness of Polytrichum?

A) the cuticle that is found along the ridges of "leaves"

B) "leaves" that are more than one cell layer thick

C) high humidity of surrounding air which provides support against gravity

D) reduced size, mass, and persistence of the sporophytes which allows gametophores to grow taller


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