Growth regulators

Question 1

Plant responses to environmental signals

Answer 1

Plant evolved mechanisms that allow them to respond to environmental signals in a way that allows them to maximise their growth and survival

Question 2

Phases in plant growth and development

Answer 2

Seed dormancy
Germination
Seedling growth, differentiation, maturation
Flowering
Seed and fruit production
Clonal growth
Seasonal growth – perennials in temperate regions

Question 3

Auxins

Answer 3

e.g IAA = enlargement and differentiation of cells (growth)
- Main effects = cell enlargement and tissue differentiation
- Transportation = polar transport from apex to base (basipetal) in parenchyma surrounding vascular bundles, approx. 1 cm per hour (faster than diffusion), specific auxin transport proteins, requires energy and carrier proteins, use of Arabidopsis mutans very important in elucidation
- Effects = apical dominance, tropisms, tissue differentiation

Question 4

Gibberellins

Answer 4

growth, development (germination, dormancy and flowering)
- Main effects = cell division and enlargement, seed germination, environmental effects (vernalisation, photoperiodism), maturation, flowering initiation, fruit formation
- Transportation = not polar, bidirectional in phloem
- Stimulate internode elongation with striking effects on dwarf mutants and rosette plants
- Dwarf mutants can result from mutations in GA biosynthesis pathway genes, mutations in genes that control response to GA

Question 5

Cytokinins

Answer 5

cell division in roots and shoots
- Main effects = cell division, germination, retard senescence, root growth and differentiation, overcomes bud dormancy
- Transportation = long distance in xylem
- Interact w other plant growth regulators = cytokine and auxin interaction in tissue culture differentiation. High cytokine ratio = shoots, high auxin ratio = roots
- Promotes cell expansion in dicots (IAA and GA do not do this in cotyledons)
- Promotes chloroplast development
- Delays leaf senescence
- Promotes movements of nutrients

Question 6

Abscisic acid

Answer 6

many processes, bud dormancy
- Main effects = growth inhibitor, seed and bud dormancy, embryo development, senescence
- Stress responses = stomatal closure
- Transportation = root to shoot in xylem, shoot to root in phloem
- Promotes efflux of k+ out of guard cells

Question 7

Ethylene

Answer 7

many processes, ripening of fruit
- Main effects = fruit ripening, leaf and flower senescence, abscission, root hair development, stress responses
- Inhibition of cell expansion = triple response
- Transportation = diffusion – ethylene is a gas

Question 8

Brassinosteroids

Answer 8

many processes, pollen tube growth (works w auxin)

Question 9

Important properties of plant growth regulators

Answer 9

Growth promotion and//or growth inhibitions – e.g auxin effects
Often act in combination or antagonistically
Transported in different ways
Plant growth regulators rather than hormones

Question 10

Signal transduction pathway

Answer 10

Hormones bind to proteins associated w membranes of the cells they will affect – e.g an extracellular signalling molecule activates a membrane receptor, that in turn alters intracellular molecules (second messengers) = creates a response
In the absence of auxin, the auxin response gene is repressed by AUX/IAA protein
Auxin receptor is a ubiquitin E3 ligase

Question 11

Cytokinin signal transduction pathway

Answer 11

Cytokinin receptors are trans-membrane protein kinases (chemically modify proteins via addition of phosphate groups)
Binding of cytokinin to the receptor protein initiates a phosphorylation cascade
Ultimately this leads to the transcription of genes
Protein kinases and phosphatases in signal transduction pathways
Nearly all biological signalling systems involve protein phosphorylation (by kinase enzymes) and de-phosphorylation (by phosphatase enzymes) in cascade systems
Signal transduction pathways in plants are still incompletely understood
Experiments with Arabidopsis mutants has advanced our understanding of these processes enormously

Question 12

Apical dominance

Answer 12

Axillary buds dormant until apical tip removed
Buds remain dormant if auxin is applied to cut tip
Auxin inhibits axillary bud growth

Question 13

Tropisms

Answer 13

Growth responses =- e.g phototropism, gravitropism) may be pos or neg
Roots are pos gravitropic, stems are neg gravitropic
Stems of many plants are usually pos phototropic
With pos phototropism, blue light is detected by protein-flavin complex and auxin accumulates on shaded side

Question 14

Phototropism

Answer 14

Light has to be perceived - at the tip
Growth response has to be elicited – in elongation zone
Phototropin = blue light receptor
Phototropin is a flavoprotein = e.g a nucleic acid containing a nucleic acid derivative of riboflavin (vitamin b2)
Curvature response = auxins cause acidification of cell wall – loosening of wall matrix (expanisns) turgor pressure results in cell elongation

Question 15

Auxin promotes tissue differentiation

Answer 15

Auxin transported by polar transportation mechanism to area of wound
Cells in pith differentiate into new vascular tissue and connect with vascular tissue above and below wound

Question 16

Gibberellins and germination

Answer 16

Many seeds require light or cold period to break dormancy – GA breaks dormancy
GA promotes alpha-amylase activity in barley seeds
a) Inhibited by repressor
b) Binding to receptor
c) Transcription activated
Repressor has two domains – one binds to GA-receptor complex (DELLA domain), other binds to response gene (GRAS domain)
Type of regulation = de-repression
Mutation in DELLA domain results in no response to GA
Mutation in GRAS domain results in growth response in the absence of added GA

Question 17

Dormancy

Answer 17

ABA levels increase during early seed development = promotes production of seed storage proteins, prevents premature germination
Breaking dormancy corresponds with decrease in ABA in seeds
Bud dormancy also under ABA control
ABA effects overcome by GA

Question 18

Drought stress response

Answer 18

Stressed plants contain much higher amounts of ABA than non-stressed plants
Roots response to drought stress by increased synthesis of ABA
ABA transported from root to shoot in xylem
ABA reaches leaves and induces stomatal closure

Question 19

Ethylene triple response

Answer 19

Exposure of seedlings to ethylene in dark results in – short hypocotyl, thickened hypocotyl, horizontal hypocotyl
Seedlings grown in the dark w atmosphere containing ethylene

Question 20

Ethylene and leaf abscission

Answer 20

Leaf maintenance phase - high auxin from leaf reduces ethylene sensitivity of abscission zone and prevents leaf shedding
Shedding induction phase - a reduction in auxin from the leaf increases ethylene production and ethylene sensitivity in the abscission which triggers the shedding phase
Shedding phase - synthesis of enzymes that hydrolyse the cell wall polysaccharides, resulting in cell separation and leaf abscission

Question 21

Vernalisation and stratification

Answer 21

Exposure of seedlings to ethylene in dark results in – short hypocotyl, thickened hypocotyl, horizontal hypocotyl
Seedlings grown in the dark w atmosphere containing ethylene

Question 22

Seed germination

Answer 22

Seed contains a desiccated, dormant embryo
Must absorb water and obtain oxygen
Impervious seed coat may impose dormancy
Physiological dormancy imposed by hormones (e.g. ABA versus GA)
In temperate regions, a cold period may be required (stratification)
In deserts, seeds remain dormant during the dry season and may require inhibitors to be leached out by rain (rain gauge strategy)
Some seeds require light to stimulate germination (see Canvas)
A cold (chilling) period is often required to break bud dormancy or promote flowering (vernalisation)