5.5 Plant responses Flashcards
Tropisms, auxin, ethene, gibberellins, apical dominance, commercial uses
What is the name for a plant response
tropism
What are the types of tropism?
Geotropism
Hydrotropism
Phototropism
Thigmotropism
Chemotropism
Explain geotropism
Positive - growing down with gravity e.g. Roots
Negative - growing up against gravity e.g. Shoots
Explain hydrotropism
Positive - growing towards water
Negative - growing away from water
Explain phototropism
Positive - growing towards light e.g. Shoots
Negative - growing away from light e.g. Roots
Explain thigmotropism
Positive - growing towards a touch / solid e.g. A creeping plant
Negative - away from solid touch
—> e.g. venus fly trap
Explain chemotropism
Positive - growing towards a chemical
Negative - growing away from a chemical
How does auxin act in plant cells?
- Auxin lowers the pH of the cell by causing H+ ions to accumulate (more acidic conditions)
- This enables enzymes that produce or stretch cellulose to function
- Once there is no auxin, the cells can no longer elongate
Auxin results in cell elongation
What is a nastic movement?
A response which is not related to the direction of the touch stimulus
—> e.g. Mimosa leaves snapping shut
How do plants respond to herbivory?
Produce chemicals:
Alkaloids
Pheromones
What are alkaloids?
Nitrogenous compounds: bitter tasting + can be toxic
–> e.g. caffeine + nicotine ( a neurotoxin)
What are pheromones?
Chemicals released by one plant which affects the behaviour/physiology of another:
—> oxides of ethene switch on genes to produce chemicals that deter insects
What are responses to abiotic stress?
- Plants can respond to drought by shutting their stomata
- producing an antifreeze chemical in their cells, that decreases the formation of ice crystals in cells
What is the function of auxin
Results in cell elongation in response to phototropism
What is the function of ethene
Results in ripening of fruits
What is the function of gibberellins?
Results in stem elongation
Which hormones contribute to leaf loss?
Ethene + auxin
What is the abscission layer
The abscission layer develops at the base of the leaf stalk
—-> a layer of parenchyma cells with thin walls, making them weak and easy to break
What causes leaf loss?
- Lack of daylight causes a drop in cytokinin level
- This causes a drop in auxin level, which starts production of ethene
- Ethene switches on genes in the abscission zone
- This causes cellulase enzymes to be produced which digest cell wall in the Abscission zone
- Vascular bundles sealed off and a waxy scar forms
- Inner layer of cells take in water - swell and the leaf breaks off
How does auxin contribute to leaf loss?
- Usually, auxins inhibit leaf loss as they are antagonistic to ABA (which encourages aging) and are produced in young leaves, making the leaf stalks insensitive to ethene
- The concentration of auxins in leaves decreases as they age until leaf loss can once again occur in response to ethene
Why do plants close their stomata?
Times of water stress,
—> Certain environmental conditions can cause water stress, such as very high temperatures or reduced water supplies
Which hormones contribute to stomatal closure?
Abscisic acid
What steps lead to stomatal closure?
- Lack of water in roots causes ABA to be produced and transported to the leaves
- ABA binds to receptors on guard cells causing removal of ions from the guard cells
- Water potential increase in the guard cell, so water moves out by osmosis
- this leads to a loss of turgor in the guard cell
- stomata close as guard cell collapses
How does ABA cause stomatal closure?
- Guard cells have ABA receptors on their cell surface membranes
- ABA binds with receptors, inhibiting proton pumps and stops active transport of H+ ions out of the guard cells
- ABA causes Ca2+ ions to move into cytoplasm of guard cells through cell surface membranes
- Ca2+ ions act as second messengers:
—> cause channel proteins to open, allows negatively charged ions to leave the guard cells
—> stimulates opening of channel proteins that allow K+ ions to leave guard cells
—> Ca2+ ions stimulate closing of channel proteins that allow K+ ions to enter the guard cells
—> loss of ions increases WP of guard cells
—> Water leaves the guard cells by osmosis
The guard cells become flaccid, causing the stomata to close
Which hormones are involved in seed germination and ending dormancy ?
gibberellins
What does a seed contain? and how is it specialised
When dropped from parent plant it is in a state of dormancy - to survive harsh weather/winter
Contains: embryo, endosperm, aleurone layer
What are the steps to seed germination?
- seed starts to absorb water
- stimulates embryo to produce gibberellins
- Gibberellin molecules diffuse into aleurone layer, stimulate cells there to synthesise amylase
- gibberellin stimulates production of amylase
- amylase hydrolyses starch molecules in endosperm, producing soluble maltose molecules
- maltose is converted to glucose and transported to the embryo
- glucose can be respired by embryo, breaking dormancy and providing the embryo with the energy needed for it to grow
What is the start of germination determined by?
Levels of abscisic acid acid and gibberellins as abscisic acid has the opposite effect to gibberellins, maintaining dormancy by inhibiting amylase production
What is apical dominance?
When the stems of a plant grows upwards and lateral buds are not produced
What is the apex?
The tip of the plant - where auxins are produced
What happens if the apex is removed?
lateral buds grow from the top of the plant, as the source of auxins has been removed and there is no longer apical dominance
What can be inferred if when the apex is cut off, lateral buds start forming?
Auxins from the apical bud prevents lateral buds from growing
How can apical dominance be shown experimentally?
- Apical bud of first test plant is removed
—> This allows the lateral buds to grow - A second - genetically identical test plant is decapitated but this time the cut tip is immediately replaced with an agar block containing auxin
—> This restores the inhibition of lateral bud growth and no lateral buds grow
How can auxin paste and auxin transport inhibitor show the role of auxins?
Plant 1: Apex cut off and auxin paste IAA applied to cut shoot
—> no lateral bud growth
Plant 2: Auxin transport inhibitor applied below apex of shoot
—> Lateral bud growth
Conclusion: low auxin levels promotes growth of lateral buds, but normal to high levels of auxins inhibits growth
How does auxin level affect abscisic acid?
High levels of auxins keep high levels of ABA
When auxin levels drop, ABA levels drop and buds begin to grow
What is some experimental evidence that gibberellins stimulates germination?
- Seeds of mutant varieties of the Arabidopsis plant that do not produce gibberellins can be induced to germinate if gibberellins are applied
- Seeds certain lettuce varieties that require light to germinate can be made to germinate in the dark if gibberellins are applied
What is some experimental evidence that gibberellins stimulates stem elongation
- Dwarf plant varieties have very low levels of gibberellins
—->This is often due to a mutation in a gene involved in the synthesis of gibberellin
—> Gibberellins can be applied and they grow to the same height as usual plants of the same species
How do gibberellins affect fruit production?
Promotes cell elongation - stalks can elongate so can hold larger fruit
How can auxins be used as weed killer?
Promotes rapid shoot growth - the stem cannot support this, buckles and dies
