Plant responses- Module 5 Flashcards
what do Hormones in plants control
Anything relating to growth:
-roots and shoots
-seed germination
-leaf fall
-disease resistance
-fruit formation and ripening
-flowering time
-Bud formation
Tropism
Directional growth response
-plants grow in a certain direction in a response to a stimulus
Roots growing downwards into dark soil =
Negative phototropism
Shoots growing upwards towards light =
positive phototropism
Negative Geotropism =
basic
Shoots growing upwards against gravity
Positive geotropism =
Shoots growing downwards in direction of gravity
Auxins in the roots causes cells to _____ elongating ….
auxin causes cells to STOP elongating
and this causes positive gravitropism/geotropism
helps the plant roots to anchor the plant in place and to reach moisture underground
Hormones _____ growth in shoot
Hormones _____ growth in Roots
Stimulate
Retard
Positive Phototropism in shoot
Process
when light hits a plant shoot from one side (unilateral light), the auxin moves laterally to the shaded side of the shoot
-higher auxin conc. on shaded side
-Higher auxin conc promotes cell elongation on shaded side -> grows faster
The shoot ends up bending towards the light.
*Once the light is hitting the shoot more evenly, the amount of auxin will be even out on both sides of the shoot and the shoot will start to grow straight again
Geotropism process
In roots and shoots underground, the auxin is pulled by gravity to the lower side of the roots and shoots
The auxin makes the lower side of the shoot grow more so the shoot grows up towards the light. (negative geotropism)
In the root, the opposite happens (as higher auxin conc in roots inhibits growth), the Top grows faster, so the root grows downwards to anchor itself in the soil (positive geotropism)
Effect of auxin on Lateral shoots
-high auxin concentration inhibit lateral shoots
-produces apical dominance
-Growth in the main shoot is stimulated by the auxin in teh tip, so is rapid
-as you descend the stem, auxin concentration is lower
-Later shoot growth is more rapid
Lots of experimental evidence
Remove the apical shoot and no auxin is produced
Lateral shoots then grow faster
Auxins can be reapplied artificially
Apical dominance is reasserted
auxin functions in plants
-control cell elongation
-prevent leaf fall (Abscission)
-maintain apical dominance
-involved in tropisms
-stimulate the release of ethene
-Prevent fruit ripening
Gibberellin
-cause stem elongation (increase internode length)
-trigger mobilisation of food stores in food germination
-stimulate pollen tube growth in fertilisation (aids fertilisation)
-
ethene
-causes fruit ripening
-promotes abscission in deciduous trees
ABA (abscisic acid)
-maintains dormancy of seeds and buds
-stimulates cold protective responses e.g: antifreeze production, stimulates stomatal closing
(antagonist to gibberellins)
experimental evidence for gibberellins
-mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins these seeds do not germinate. If gibberellins are applied to the seed externally, the seed will germinate normally
-if gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins needed to break dormancy. If the inhibition is removed or gibberellins added, the seed breaks dormancy/ germinates
what are indoleacetic acid
where are they made
where can the travel
what do their effects depend upon
Auxins
growth stimulants in plants. small quantities can have powerful effects
made in cells at tips of roots and shoots, and in the meristems
can move down the stem and up the root both in transport tissue and cell to cell
the effect of the auxin depends on its concentration and any interaction it has with another hormone
process of the effects of auxin (IAA) on Cell Elongation
-auxins synthesised + released from meristem cells
-auxins diffuse down the concentration gradient, away from tip towards zone of elongation
-auxins bind to receptor sites in plant cell membrane. Opens protein channels on plasma membrane that specifically allow Hydrogen ions to enter the cell. H+ Move in.
-vacuoles form and low PH (around 5) develops
-low pH of cell walls keeps them flexible (optimum pH for enzymes maintaining plasticity of wall), allowing cells to expand as they absorb water (without bursting). Vacuoles get bigger + cell walls stretch
-Large central vacuole formed. Auxins leave and destroyed by enzymes.
-Protein channels close to prevent further entry of hydrogen ions.
-pH rises so enzymes maintaining plasticity denature. Cell wall becomes rigid. no further cell elongation is possible
what do high concentrations of auxins do to lateral shoots
explain apical dominance
suppress the growth of lateral shoots
Growth in main (apical) shoot is stimulated by auxin in tip, so is rapid.
-As you descend the stem, auxin concentration is lower
-Lateral shoot growth is more strong
what experimental evidence is there for the role apical dominance
-remove apical shoot and no auxin produced
-Lateral shoots then grow faster
-Auxins can be reapplied artificially
apical dominance is reasserted
Auxins in root growth
low to high conc
-low concentrations of auxins promote root growth
-up to a given concentration, the more auxin that reaches the root, the more they grow
-High concentrations of auxin inhibit growth
-auxin is produced by the root tips
-some makes it way from the shoot to the roots
expertimental evidence for effect of auxins on the roots
if apical shoot is removed, less auxin reaches root
Growth slows
-replacing auxins artificially at the cut apical shoot restores growth of roots
hormonal process of germination
-Seeds absorb water
-produces gibberellins
-stimulates production of enzymes
-break down food stores: starch>Maltose>glucose
-glucose used to grow plant
-Energy released for seedling growth
*evidence suggests gibberellins turn on genes for amylases and proteases
some evidence that ABA acts as an antagonist
Relative levels of both hormones determine germination
Ethene and _____ have an antagonistic relationship
Auxin
ABA and _______ have an antagonistic relationship
Gibberellins
Synergism
hormones working together
Antagonism
hormones opposing each other
Should you refer to the presence or the concentration of hormones in plants
Concentration
refers to Hormonal Balance
Generally, how do scientists find evidence of plant hormones
-Mutate genes that make the hormone
-Disrupt production pathway
-Cut shoot tip (remove auxins)
Then
-See changes
-Apply hormones externally
-Further changes?
How does apical dominance benefit a plant
Allows it to better compete for light.
e.g: Lateral shoots higher up are shorter so they don’t block the light for shoots below
Benefit of stem elongation
higher conc. of gibberellins = larger internodes
larger internodes = Taller plants
Taller plants = better compete for light
=more photosynthesis
Why would farmers want to reduce stem elongation?
- Decrease waste
-prevent crop damage by bad weather
p
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Abscission process
1) Phytochromes Detect falling light levels
2) Decreases auxin production
3) Increase Ethene levels
4)Ethene activates genes in cells in abscission zone to produce digestive enzymes
5) Enzymes digest cell wall in the Separation zone (e.g: Cellulase)
6)Vascular bundles are sealed in the protective layer.
factors such as wind will then remove the leaf
Why is Abscission important
1)Reduce energy needed to maintain leaves in winter
2)Fallen leaves insulate roots
3)Rotten leave provide minerals to plants
Plant response to lack of water Summary process
Why does this happen
1) root hair cells detect lack of water and Produce ABA
2) ABA transported up the stem to leaves and binds to receptors on guard cells
3)Causes a change in ion channels. Ions move out of guard cells. Affects WP gradient
4)Water leaves by osmosis
5)Guard cells become flaccid/plasmolysed and close stomata
occurs to reduce transpiration
Physical plant defences to herbivory
-Thorns, hairy leaves, Spikes
Chemical plant defences to herbivory
-Tannins
-Alkaloids
-Terpenoids
-Pheromones
-Volatile organic compounds
Tannins in plants
Bitter taste, toxic to insects
Alkaloids in plants
bitter taste, poisonous to animals (affect metabolism)
Terpenoids in plants
Toxic to insects and fungi (neurotoxin, repellent)
Examples of alkaloids that humans use
Caffeine - increase metabolic rate
Nicotine- Addictive, increases heart rate
Morphine- painkiller
cocaine-
use of pheromones in plant
Affect social behaviour of other members of the same species
e.g: signal to produce callose
use of Volatile organic compounds in plants
Affect social behaviour of other species
e.g: VOCs can attract predators of the pest insect
Folding response in _____ ______ does..
Mimosa pudica
Scares herbivores away
make insects slide off
Advantages of phototropism
(positive) ensure max rate of photosynthesis due to more light exposure
(Negative phttrphsm) Roots grow away from light to get water and minerals in soil
Negative phototropism
1) auxins move away from light to shaded side of root
2) Roots like LESS auxins to grow quicker (more auxins = grow less)
3)Lit up side that has less auxins grows quicker than shaded side
4) So roots bends downwards away from the light
Key points with auxins and Their interactions with light, roots, shoots
Auxins- Hate light, Obey gravity
-Promote shoot growth
-retard root growth
Scientists originally thought auxin was ____ by light but was disproved by plants growing in the dark and in unilateral light had ____ auxin levels
Destroyed
Similar
Wbu is gibberellin classed as a hormone
-chemical messenger
-produced in one part of plant, has effects in another part.
-affects activity of target cells
-Long lasting
-wide-spread effect
