Animal And Plant Responses - Plant Hormones Flashcards
What stimuli do plants respond to?
Abiotic stress - non living
Biotic stress - Herbivory
Tropisms
Examples of abiotic stress for plants and e.g how they respond to these?
Freezing
Drought
Increased soil water salinity
Presence of heavy metals
RESPONSE TO DROUGHT: shut stomata/lose leaves - reduce water loss by transpiration
RESPONSES TO LOW TEMPS: produce antifreeze chemical in cells - decrease formation of ice crystals
Chemical defences to herbivory (being eaten by herbivores)?
Produce chemicals:
ALKALOIDS (caffeine/nicotine) - bitter/toxic - deters or kills herbivores
PHEROMONES - signal to nearby plants of same species that they are under attack from herbivores - triggers other defences
- signal can attract herbivorous insect’s predator
Additional response to herbivory?
Responds to touch:
Touch sensitive leaves FOLD rapidly when touched
- can protect against herbivorous insects/reduce transpiration
Most likely caused by bioelectrical signals
What are tropisms?
Growth response of a part of a plant in response to a directional stimulus
- can be towards stimulus (+) or away from stimulus (-)
Types of tropisms and their stimulus type?
PHOTOTROPISM: response to light - ABIOTIC - maximise light access for max photosynthesis
GEOTROPISM : response to gravity - ABIOTIC
- ensure shoots/roots grown in right directions
HYDROTROPISM: response to moisture - ABIOTIC
- root tips grow to damper areas - increase water access
THIGMOTROPISM : response to touch - ABIOTIC/BIOTIC - good for climbing plants - help get access to light/detect a support and curl around it
CHEMOTROPISM: response to chemicals - ABIOTIC/BIOTIC
Darwin’s experiment to investigate phototropism?
Removed tip of coleoptile - stopped phototropic response to unidirectional light source (light from one side) from occurring
- to ensure it was not due to wound in of plant - covered tip of coleoptile with opaque cover/cap to block out light - also stopped phototropic response from occurring (shows tip is responsible for detecting light)
Boysen-Jensen’s Experiment to investigate phototropism?
Found that phototropic response restored when he replaced the cut tip back on to coleoptile and a gelatin block in between - showed the stimulus for growth was a chemical (hormone) which travelled through gelatin block
- then inserted mica barrier (impermeable to chemicals) halfway through coleoptile on lit side and shaded side :
On lit side : phototropic response occurred
Shaded side : response didn’t occur
Confirmed the stimulus for growth was a hormone (chemical) produced at tip and travelled down coleoptile on the opposite side of stimulus - shaded side - causes growth on shaded side
Paál’s experiment to investigate phototropism?
Paál cut off tip of coleoptile +replaced it off centre in the dark
- the side that the tip was placed on grew more than other side - cause coleoptile to curve
Showed that, in light, phototropic response was caused by hormone diffusing through plant tissue /stimulates growth of tissue
Went’s experiment to investigate phototropism ?
Went placed cut tip of coleoptile on gelatin block, allowing the hormones from the tip to diffuse into block
- block was placed on coleoptile, off-centre and in the dark
- the side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve
The greater the concentration of hormone present in the block, the more the coleoptile curved
How does auxin cause cell elongation?
Indole-3-acetic acid (IAA) - an auxin , made in growing tips of roots/shoots (meristems)
- IAA molecules activate proteins in cell wall - EXPANSINS
- they loosen bonds between cellulose microfibrils - cell wall more flexible
- cells can elongate
How does phototropism occur?
- affects shoots/top of stems
In shoots - IAA moves from illuminated side of shoot to shaded side
higher conc of IAA on shaded side causes greater rate of cell elongation
Causes shoot to bend towards light
What happens when conc of IAA is not uniform on either side of root/shoot?
Uneven growth occurs
How does geotropism occur in shoots and roots?
SHOOTS - negative geotropism
- gravity modifies distribution of IAA so accumulates on lower side on shoot
- IAA causes increase in rate of growth —> shoot grows upwards
ROOTS - positive geotropism
- in roots, higher conc of IAA = lower rate of cell elongation
IAA that accumilates on lower side of root inhibits cell elongation - lower side grows at slower rate than upper side of root
Causes root to bend downwards
Role of hormones in leaf loss?
Layer of cells - ABSCISSION LAYER - develops at base of leaf stalk
- a layer of parenchyma cells with thin walls - makes them weak/easy to break
ETHENE stimulate breakdown of cell walls in abscission layer - leaf drops off
AUXIN can inhibit leaf loss (produced in young leaves) - less sensitive to ethene , but conc of AUXIN decreases with age - leaf loss occurs
Role of hormones in stomatal closure?
Hormone ABSCISIC ACID (ABA) produced during times of water stress - stimulate closing of stomata
Guard cells have ABA receptors on their cell surface membranes
ABA binds to these receptors - INHIBITING PROTON PUMPS , therefore stopping active transport of H+ ions OUT guard cells
ABA also causes Ca2+ ions to move into cytoplasm of guard cells
- calcium ions act as second messengers:
They cause channel proteins to open- allow negatively ions to leave guard cells
This stimulates the opening of further channel proteins that allow (K+) ions to leave guard cells
Ca2+ ions also stimulate the closing of channel proteins that allow (K+) ions to enter the guard cells
- loss of ions increases the water potential of the guard cells
Water leaves the guard cells by osmosis
The guard cells become flaccid, causing the stomata to close
Role of hormones in seed germination?
Gibberellins involved in controlling seed germination/ stem elongation
- in good conditions - seed starts to absorb water to begin germination
- stimulates embryo in seed to PRODUCE GIBBERELLINS
- gibberellin molecules diffuse into aleurone layer + stimulate cells there to synthesis AMYLASE
- amylase hydrolyses STARCH molecules in endosperm - produces soluble maltose molecules
- maltose converted to glucose +trasnprted to embryo
- glucose respired by embryo, breaking dormancy and proving energy for growth
Why do deciduous plants lose leaves?
In hot/dry conditions - reduce water loss
Winter - absorption of water is difficult due to frozen soils /photosynthesis is limited by low temps and sunlight
How does ABA have opposite effect to gibberellins?
Maintains dormancy by inhibiting amylase production
What is apical dominance?
Auxins produced at growing tip at the APEX (TOP) of plant stem causes STEM TO GROW UPWARDS and STOPS lateral buds from growing
What happens if growing tip at apex of plant is removed?
Lateral buds grow from top of plant (auxin source removed/no apical dominance)
- lateral shoots that grow from lateral buds curl up towards light - plant continues to grow upwards
Experimental evidence for role of gibberellins in stem elongation?
Dwarf plant varieties shown to have very low levels of gibberellins - due to mutation
Under experimental conditions , treating the dwarf varieties with gibberellins - they grow to same height as normal varieties
Experimental evidence for role of gibberellin in seed germination?
Seeds of mutant varieties of the Arabidopsis plant that don’t produce gibbeellins can be induced to germinate if gibberellins applied
Seeds of certain lettuce varieties that need light to germinate can be made to germinate in dark if gibberellins are applied
How can response to the removal of source of auxins be shown experimentally?
- apical bud of the 1st test plant is removed (decapitated)
- allows the lateral buds to grow
A second (genetically identical) test plant is decapitated but 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 are plant hormones used commercially?
- selective weed killers
-rooting powders
-control ripening
How are plant hormones used in selective weed killers?
High conc of auxins cause very rapid growth that plant tissues become distorted +damaged —> pathogens can enter
- use synthetic auxins (100x greater in conc than natural hormones in plants) - effective against weeds (grasses are less sensitive to these killers - don’t die)
How are plant hormones used in rooting powders?
Low doses of auxin use to stimulate cuttings to grow new roots
- these auxins sold as rooting powders
Lower end of cutting dipped in powder, then planted —> good environment allows roots to grow
How are plant hormones used to control ripening?
Ethene stimulates fruit to ripen
- delicate/soft fruits harvested when unripe , transported and ripened artificially using ethene during transport
Extra use of auxins and gibberellins?
- used to make unpollinated flowers develop fruit (normally only occur when flower is pollinated/fertilised)
- used to produce seedless fruit
Known as PARTHENOCARPIC FRUITS - fruits formed without fertilisation
Auxin used to stop trees dropping fruit before its harvested (cant be used once dropped)
PRACTICAL: investigating the role of auxins in apical dominance
- Put 30 similar plant in pots containing same soil
- Count and record no. Side shoots growing from main stem of each plant
- For 10 plants, remove tip of shoot + apply a paste containing auxins to top of stem
- For another 10 plants, remove tip of shoot + apply paste with NO AUXINS to top of stem
- Leave final 10 plants as they are - controls
- Leave all plants to grow for 6 days - keep control variables constant (light,water,temp)
- After 6 days, count no. Side shoots growing from main stem of each plant+ record in table
Plants with auxin should prevent extra side shoots from growing
