Plant responses to internal + external stimuli Flashcards
Briefly describe how plant cells respond to stimuli
- hormone or enviro stimulus binds to receptor
- relay proteins + 2nd messengers in cascade of transduction events
- activation of cellular responses
How were plant growth regulators discovered?
Coleoptile bent towards the light
Phototropic response only when tip is illuminated
Phototropic response only when tip separated by permeable barrier
Describe the conclusions formed when auxin was discovered
Derived from coleoptile tip
Diffusible
Present at higher conc on dark side of shoot
What is required for plants to respond to hormones?
Different hormones must interact in the right ratios and balance
What does IAA stand for?
What is it, briefly?
Indole acetic acid
A common auxin
How is auxin transported?
Tip to base polar transport
What are auxins?
Any substance that promotes elongation of coleoptiles
What does auxin do?
Stimulates cell elongation in young developing shoots
by binding to auxin receptors + transporters at the plasma membrane
-> permits fluxes in auxin conc in target cells
What can high auxin conc do?
Inhibits growth by inducing the production of other hormones
e.g. ethylene which inhibits growth
What is the acid growth hypothesis?
Ability of plant cells + cell walls to elongate or expand quickly at low pH.
This form of growth does not involve an increase in cell no.
Describe the acid growth hypothesis
1. auxin increases proton pump activity = pumps H+ into cell wall from cytoplasm 2. cell wall becomes more acidic 3. low pH activates expansins which separate microfibrils from cross-linking polysaccharides 4. cleaving allows microfibrils to slide 5. cell can elongate
What does the polar transport of auxin play a role in?
Pattern formation of developing plant
- reducing auxin flow from shoot of a branch stimulates growth in lower branches (=apical dominance)
Leaf venation pattern
Phyllotaxy
What is IBA?
What does it do?
Indolbutyric acid
= an auxin
Stimulates adventitious root formation
What are synthetic auxins used for?
> preventing leaf abscission
preventing fruit drop
promoting flowering + fruiting
controlling weeds
What is 2,4-D?
What is it used for?
A synthetic auxin
Overdose can kill plants
= herbicide
What do cytokinins do?
Stimulate cytokinesis
cell division
What are all cytokinins similar to?
Adenine
Where is cytokinin synthesised?
Mainly in roots
also embryos + fruit
How is cytokinin transported?
Non-polar fashion
through xylem, phloem + parenchyma cells
Which hormone to cytokinins work with?
Auxin
- to regulate cell division _ differentiation
- ratio is important*
What do high concs of auxin alone promote?
What about high concs of cytokinin alone?
Cell expansion
Cell division
What is apical dominance?
A terminal bud’s ability to suppress development of axillary buds
How are auxins and cytokinins involved in apical dominance?
Auxins - maintaining dominance of apical bud
Cytokinins - stimulate axillary buds
Cytokinin acts antagonistically to auxin in apical dominance
What is the direct inhibition hypothesis?
Auxin from apical buds travels down shoots to inhibit axiliary bud growth
–> promotes shoot growth, + restricts lateral branching
Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth
How do cytokinins delay leaf senescence?
> inhabit protein degradation
stimulate RNA + protein synthesis
mobilise nutrients from surrounding tissues
What are the roles of gibberellins (GAs)?
Enhance cell elongation + cell division
Can induce dwarfing mutants to grow tall
Which hormone is responsible for ‘bolting’ during the switch from vegetative to reproductive growth?
Gibberellins
Which 2 hormones must be present in most plants for fruit to develop?
Auxins + Gibberellins
Which hormone is sprayed onto Thompson seedless grapes?
Why?
Gibberellins
No seeds to produce the hormone
How are gibberellins involved in germination?
After taking on water…
- GAs send signal to aleurone
- Aleurone secretes alpha-amylase + other enzymes
- Sugars + other nutrients are consumed
What does ABA stand for?
Abscisic acid
Where is ABA produced + how is it transported?
Most cells
Non-polar fashion
What is the overall role of ABA?
Slows growth - acts as antagonist to other hormones
What are the 2 effects of ABA?
Seed dormancy
- stops germination at inappropriate times of year etc
Drought tolerance
How is seed dormancy broken?
When ABA is removed by heavy rain, light or prolonged cold
What determines whether a seed remains dormant or germinates?
Ratio of GA to ABA
How is ABA involved in drought tolerance?
ABA accumulation causes stomata to close rapidly
due to massive efflux of K+ ions
What are the effects of ethylene?
> response to stresses
senescence
leaf abscission
fruit ripening
What is ethylene synthesis inhibited by?
CO2
What can high auxin levels stimulate the synthesis of?
Ethylene
What is the triple response to mechanical stress?
Slowing stem elongation , thickening the stem + horizontal growth in order to avoid obstacles
Which hormone induces the triple repose to mechanical stress?
Ethylene
What is a burst in ethylene production associated with?
Apoptosis
What does a change in the auxin-ethylene balance lead to?
Leaf abscission
What is the ripening of fruit?
Enzymatic breakdown of cell wall components + conversion of starches + acids to sugars
Which hormone leads to fruit ripening?
Ethylene
What is the effect of light on morphology called?
Photomorphogenesis
What are the 2 main classes of photoreceptor?
Blue-light
Red/far-red (phytochromes)
What do blue-light photoreceptor control?
Hypocotyl elongation
Stomatal opening
Phototropism
What do phytochrome control?
Seed germination Shade avoidance Photoperiodism Seasonal responses Flowering
How does phytochrome work?
Red light causes conformational change in protein from Pr to Pfr
-> responses: seed germination, flowering control etc
What is phytochrome reversed by?
Far-red light
What does phytochrome conversion mark?
Sunrise + sunset
= thus provides biological clock w/ environmental cues
What is photoperiodism?
A physiological response to photoperiod
= environmental stimulus that enables plants to sense time of yr
What is crucial for the control of flowering?
The length of the dark period
What are the other internal + external stresses that plants respond to?
Gravitropism
Thigmotropism
Abiotic stresses - drought, flooding, salt, heat, cold
Biotic - herbivores, pathogens
