Plant responses

the main plant hormones

auxins, gibberellins, cytokinins, ethene, abscisic acid (ABA)

summary of the role of auxins

• control cell elongation

• prevent leaf fall

• maintain apical dominance

• involved in coordinating directional growth in response to stimuli (tropisms)

• stimulate the release of ethene

• involved in fruit ripening

summary of the role of gibberellins

• cause stem elongation

• trigger mobilisation of food stores in a seed at germination

• stimulate pollen tube growth in fertilisation

summary of the role of cytokinins

promote cell division

summary of the role of abscisic acid

• maintains dormancy of seeds and buds

• stimulates cold protective responses

• stimulates stomatal closing

what is the site of release for plant hormones

cells in a variety of tissues

what is the method of moving around the organism of plant hormones

active transport, diffusion or mass flow in phloem or xylem

what is the molecular size of plant hormones

small molecules only

where are the target cell/tissues of plant hormones

often local to the site of production

what is the range of effects of each plant hormone

varied particularly in interaction with other plant hormones

what is the regulation of the production of plant hormones

decentralised- various tissues detect stimuli and release plant hormones independently

what is the number of different types of plant hormones

many

why do some cell respond to plant hormones and other don’t

presence or absence of receptors for the hormone

how are hormones detected by target cells

by binding to receptors on the plasma membrane of cells

what is the effect of hormones on target cells

alter processes occurring within the cell through second messenger systems to produce a response by the cell

what are the sections of the triggering of specific events

• times events through life e.g. fruit ripening, seed germination (including responding to darkness as an abiotic stress), leaf loss

• protecting against biotic and abiotic factors e.g. responding to the onset of freezing conditions, stomatal closure, producing defences against herbivory and moving in response to touch

which hormone causes fruit ripening

ethene

what is the name of the type of fruits that continue to ripen after they have been picked

climacteric

what fruits don’t require ethene for ripening

non-climacteric

why is picking fruit unripe and then artificially controlling ripening as desired useful

• hard, unripe fruit damage less in transport

• transportation is not as time sensitive

• all fruit in a bunch can be ripened at the same time

which hormone is involved in germination

gibberellin

what can trigger germination

abiotic factors

what evidence led to the discovery of the role of gibberellin in seed germination

a variety of experiments;

• finding mutant varieties that don’t germinate and discovering that the mutation is in a gene that prevents making gibberellin

• applying gibberellin to the mutant varieties and triggering germination

• applying inhibitors to seeds that prevent gibberellin production and seeing that germination is prevented

• removing the inhibition above or adding gibberellin and seeing seeds germinate

when do many seeds germinate

in the dark

why do many seeds germinate in the dark

indicated they are buried and likely to have good access to water and mineral ions, safe from herbivores

what is etiolation

where plants put most resource into stem growth and less into leaves and roots or chlorophyll production in germination, stem’s cells have thin cell walls so carbohydrate reserves go further, maximises plants chances of reaching surface

what is it called when a plant puts most resources into stem growth at germination

etiolation

what happens to the plant when it reaches the surface

greening- plant produces chlorophyll and invests more in leaves, makes thicker, sturdier cell walls

what is greening

occurs after etiolation- when plant reaches surface, produces chlorophyll, invests more in leaves, makes thicker cell walls

what is etiolation controlled by

gibberellin

what are deciduous plants

they lose their leaves in the autumn

what plants lose their leaves in the autumn

deciduous plants

why are the benefits of keeping leaves over winter low

productivity from photosynthesis is relatively low ddue to low light intensities, low temperatures, decreased access to liquid water

what advantages are there to losing leaves over winter

reduced chance of storm damage, minimise surface area to minimise water loss, leaves could be lost anyway due to frost damage- in controlled leaf loss the plant can reabsorb nutrients used for spring growth and to prevent freezing in parts over winter

how many stages are there in controlled leaf loss

2

what are the stages in controlled leaf loss

senescence and abscission

what is the senescence stage in controlled leaf loss

the first stage, process of reabsorbing nutrients from the leaf, leaf changes from green to brown/orange

what is the abscission stage in controlled leaf loss

second stage, process of sealing off the leaf from rest of the plant so it falls off

where is the leaf sealed in the abscission stage in controlled leaf drop

abscission zone

how are leaves sealed in abscission stage in controlled leaf drop

cells on the plant side of the abscission zone become suberized- cell walls filled with suberin- to seal leaf from plant, cells on leaf side of abscission zone secrete cellulase to break down cell walls, allows leaf to separate from rest of plant

what triggers senescence and abscission

a series of different plant hormones working together to coordinate the process

how do cytokinins impact controlled leaf drop

in summer presence of high levels of cytokinins inhibit senescence

what inhibits senescence in summer

high levels of cytokinins

how do auxins impact controlled leaf drop

in summer high levels of auxins inhibit abscission

what impact abscission in controlled leaf drop

high levels of auxins

what lowers concentration of cytokinins

changes in day length

what does changes in day length change

lowers concentration of cytokinins

what does lower concentration of cytokinins cause in controlled leaf drop

triggers senescence, reduces auxin production, reduces concentration of auxin, makes cells in abscission zone sensitive to ethene, ethene initiates cellulase production, digests cell walls of cells in abscission zone, abscission occurs

what is the impact of ethene on controlled leaf drop

when auxin is low in concentration cells in abscission zone are sensitive to ethene, initiates cellulase production which digests cell walls of cells in abscission zone, abscission occurs

what causes abscission to occur when auxin concentration drops

ethene

what terms describe the way hormones interact with each other

synergism and antagonism

what is synergism

interaction of two factors where the outcome is greater than the sum of the individual effects of either of the

what is antagonism

interaction of two factors that are driving the outcome in opposite directions

why is unmanaged freezing bad for cells

ice crystals grow big and pierce cells, destroying them

how do some plants combat freezing conditions

• evolved lifecycles to avoid freezing conditions, being seeds or only existing underground during coldest times of year

• evolved processes to protect themselves, triggered as freezing conditions approach

what happens in some plant as day length get shorter and temperatures get cooler

certain genes are suppressed, other activated to prepare cells for frosty conditions

what is the key factor in protection from freezing

more solutes dissolved in water, lower its freezing point, less likely cells are to freeze

what do sugars, polysaccharides, amino acids or proteins do in preventing freezing

act as an antifreeze in cytoplasm and vacuole, lower chance of freezing, protect cells from damage if they do freeze by reducing size ice crystals can grow to

what substances can act as antifreeze

sugars, polysaccharides, amino acids, proteins

what happens to the substances that act as antifreeze when warmer temperatures occur

reverses the process and resources can be used to support growth

what reverses the process of substances acting as antifreeze

warmer temperatures and longer days

why do plants close their stomata

abiotic stressor- drought, or when detect pathogenic spores

what triggers stomatal closure

abscisic acid

when is abscisic acid released in stomatal closure

• leaf releases it at times of water stress

• roots release it when soil water levels are low so plants close stomata before they become water stressed

• darkness, no point having open stomata to allow CO2 in if no photosynthesis can occur

how does ABA control stomatal closure

binds to receptors on guard cells, triggers events that lead to potassium and chloride ions leaving the cells, water follows by osmosis, guard cells become flaccid, stomata close

what defences are there against herbivory

• physical

• chemical

• moving in response to touch

what are the physical defences against herbivory

thorns, barbs, spikes, spiny leaves, fibrous inedible tissue, hairy leaves, stings

what are larger physical defences for

vertebrate herbivory

what are smaller physical defences for

insect herbivory

what counts as a physical response to herbivory

when plants invest more resources in defences if they detect they are exposed to a lot of herbivory, not plants that have evolved to produce these defences as a normal part of growth and doesn’t vary

what is the conceptual framework relating costs and benefits of plasticity to information content for plant defences

balancing: herbivores reduce plant fitness and plant defences make the reduction less but employing the defences is costly

what are the chemical plants produce against herbivory

tannins, alkaloids, pheromones

what are tannins

chemicals in response to herbivory, part of a phenols, make up to 50% of dry weight of leaves, bitter taste to deter herbivores, toxic to insects

what are alkaloids

chemicals in response to herbivory, bitter, nitrogenous compounds found in many plants, many affect metabolism of animals, toxic, e.g. caffeine- toxic to fungi and insects

what is an example of an alkaloid

caffeine

what is the chemical caffeine

an alkaloid

what are pheromones

airborne molecules, trigger a response in another organism of the same species

what triggers the production of chemical defences

plant detecting herbivory in different parts of plants or plants nearby, produce volatile organic compounds

what do volatile organic compounds do

trigger production of chemical defences in other plants or other areas of the plant, they can attract a predator or parasite that kills the herbivore

how do volatile organic compounds work

evaporate from plant area producing them, travel through the air, detected by receptors on other parts of the plant, other individuals of same species or different species

what plant folds and drops its leaves when touched

mimosa pudica

what does a mimosa pudica do when touched

folds and lowers its leaves

how does moving in response to touch protect a plant from herbivory

stops insects laying eggs on leaves, won’t eat leaves

what allows a leaf to move

pulvinus- joint

what is the pulvinus

the joint that allows leaves to move

what keeps leaves in a normal, extended position

swollen turgid cells called motor cells

what do motor cells do

when turgid they hold leaf in the normal, extended position

what happens to the cells when leaves detect touch

electrical impulse sent to motor cells causing them to lose water, become flaccid, leaflets fold together and leaf drops

what are tropisms

directional growth response, direction of response is determined by the direction of an external stimulus

examples of tropisms

phototropism, geotropism, chemotropism, thigmotropism

what is phototropism

response to direction of light

what is geotropism

response to direction of gravity

what is chemotropism

response to chemical gradients

what is thigmotropism

response to direction of touch

where are changes that cause tropisms to occur

at the meristems

what are the locations of meristems in plants

• apical meristems- tips of roots and shoots

• lateral bud meristems- give rise to side shoots

• lateral meristems- in cambium, responsible for roots and shoots getting wider

• intercalary meristems- between nodes where leaves and buds branch off stem, cause shoots to get longer

where are the processes involved in phototropism and geotropism

apical meristems

what control phototropism and geotropism

auxins

what is an example auxin control phototropism and geotropism

IAA (indole-3-acetic acid)

what are the two ways organisms can grow

mitosis- make more cells, makes cells bigger