Plant hormones Flashcards
1
Q
Define the term tropism
A
A directional growth response two a stimulus
2
Q
Why do plants need to respond to their environment
A
- NASTIC MOVEMENTS: Discourage herbivores eating them (e.g. mimosa podica close their leaves when touched to appear smaller and reveal their spikes)
- PHOTOPERIODISM: when temperatures get colder plants can switch on a gene to produce antifreeze chemicals (response to lack of light is caused by change in ratio of phytochrome Pr and Pfr)
- When some plants are exposed to herbivory they switch on genes that cause chemicals to be produced to protect them from herbivore (toxic or taste bad)
- TROPISMS: e.g. turning towards light (phototropism) or roots growing toward gravity (geotropism)
3
Q
Identify the main differences between nastic movements and tropisms
A
NASTIC: - Not related to direction of stimulus - Caused by changes in cell volume - Reversible - Controlled by action potentials TROPISMS: - Direction of movement depends on direction of stimulus - Caused by growth - Non-reversible/permanent - Controlled by plant hormones
4
Q
Describe what plant hormones are
A
- Produced in tiny quantities
- Often affect the tissues in which they are produced by binding to receptors on membranes or entering cytoplasm of cell
- often have different effects in different species, plant tissues, at diff stages of a plants life cycle, or when they are present at diff concs
- interact with each other
1) Antagonism - two or more substances have opposite effects so reduce each other’s effect
2) Synergism - two or more substances increase or change each others effect
5
Q
Where are auxins produced and what do they do?
A
- Produced in apical meristems of shoots and roots (also in young leaves)
- Thought to cause:
1) cell elongation
2) prevent leaf abscission (leaf drop)
3) have roles in phototropic movements and apical dominance
4) stimulate release of ethane
5) be involved in fruit ripening
6
Q
How do auxins cause cell elongation?
A
- When phototropins are hit by blue light they become phosphorylated.
- So if light is coming from above they are all phosphorylated and if light comes from a side angle, phototropins on that side are phosphorylated
- Somehow this phosphorylate causes movement or a change in activity of transporter proteins in plant cell membranes so auxin is moved to the dark side of the shoot
- the auxin then causes the cells on the dark side to elongate more than on the light side by binding to receptors on the cell membrane
- This stimulates H+ ion pumps to move H+ ions out of the cytoplasm and into cell wall
- the build up of H+ ions creates a lower pH allowing enzymes called expansins that break cross-wall linkages in cellulose molecules to be activated
- The walls become less rigid and as the cell takes on water the wall stretches
7
Q
How are auxins involved in apical dominance?
A
OVERALL: Lateral meristems are inactive as long as the apical meristem is present, so once an apical meristem is removed the lateral buds are initiated
- Auxins are produced in the apical meristem and transported down to the lateral buds where it inhibits growth
- The further the lateral buds are from the apical bud the lower the conc of auxin that reaches them so the more growth they show
8
Q
What experiments can be used to support the theory of apical dominance?
A
- If the apical bud is removed from to plants and replaced with a block of agar impregnated with IAA (auxins) on just one of them
- The one with the agar will continue to show apical dominance where lateral shoots will grow in the other
- If a growing plant is turned upside down the auxin can’t be transported from the apical bud to the lateral buds against gravity so the lateral shoots begin to grow
9
Q
How does phototropism help a plant survive?
A
- Helps ensure roots receive as much all round light as possible to allow maximum photosynthesis
- Also, if roots emerge (mainly due to rain) they will rapidly turn back into the soil
10
Q
What are the 3 conclusions of Darwins 3 experiments?
A
- The shoot will grow toward light so bending occurs behind the tip
- The tip must either detect the stimulus or produce the messenger (or both) as its removal prevents any response
- The light stimulus must be detected by the tip
11
Q
What are the 2 conclusions of Boysen-Jensens experiments
A
- Mica on the illuminated side of the shoot allows the hormone to pass only down the shaded side where it increases growth and causes bending
- As gelatin allows chemicals to pass through it but not electrical messages the bending that occurs must be due to chemicals passing from the tip
12
Q
What are the 2 conclusions from Paál and Went’s experiments?
A
- Light doesn’t cause response, it is uneven distribution of a chemical that causes the curvature
- Suggests chemicals in the tip rather than the tip itself causes the response
13
Q
How are auxins used commercially?
A
- prevent fruit abscission so fruits can be harvested all at once
- cause fruits to develop even if flowers not fertilised - seedless fruits made
- As a rooting powder for cuttings and in micropropagation
- As herbicides (synthetic auxins e.g. 2,4-D) which kill broad leaved plants but not grasses
14
Q
Where is gibberellin found and what’s it’s role?
A
- Found in growing shoots and roots and germinating seeds
- Thought to have roles in stem elongation and stimulate growth of pollen
15
Q
How does gibberellin cause stem elongation?
A
- GA affects length of internodes and is response for etiolation (when plants grow tall and skinny to find light)
- GA works by entering cells of the stem and bind to a receptor protein
- This then binds to another receptor protein resulting in the breakdown of a DELLA protein
- These DELLA proteins usually bind to transcription factors so once broken down the transcription factors are freed and transcription of the genes can begin
- This causes elongation (not currently understood)
