key processes in plant hormones Flashcards
how do auxins elongate cells?
- IAA redistributed to the shaded side of the cell creating a gradient of the hormone across the stem.
- IAA activates the H+ proton pump (ATPase) which releases H+ + lowers the pH to 5
-pH 5 optimal for enzymes (expansins) to break H bonds in the cell wall –> making it more loose. - loosening of the cell wall allows water to enter the cell -> expands as vacuole absorb more water.
This causes the cell wall to stretch. - Turgor pressure will increase so that the cells on the shaded side are elongated causing shoot tip to bend towards the light.
As the cells mature, auxin is destroyed + plant pH rises which makes the expansinsinactive —> cell wall more rigid + fixed. Cells no longer grow + expand.
how does ABA cause stomatal closure
- ABA binds to the ABA receptor in a leaf.
- binding stimulates the release of Ca2+ ions from the vacuole into the cytosol.
- high conc of Ca2+ in cytosol stimulates ion channels on the tonoplast + guard cell membrane to open and so lots of ions diffuse out of the cell.
The remains with a high water potential and thus it moves by osmosis down water pot grad causing the cell to be more flaccid + thus stomata will close.
how does ethene stimulate leaf abscission.
produced by ageing leaves.
- as plants get older, a layer of cells called the abscission layer will develop at the bottom of the leaf stalk
- ethene stimulates the abscission layer to expand, putting pressure on the cell
- as a result the leaf will fall off as cell walls break due to high pa
- suberin is formed where the leaf fell off to prevent pathogens entering to plant
How do giberellins lead to seed germination.
they stimulate the breakdown of starch to glucose in seed germination so that the embryo plant can use glucose for respiration + it can release NRG as it grows.
1) Gib is released from a plant embryo in seed germ.
2) the seed absorbs water by osmosis + thus embryo releases gib.
3) gib enable production of amylase which hydrolyses starch to glucose.
4) glucose enters resp + provides ATP that drives met reactions + growth of the seed.
how can you investigate gravitropism?
method 1 ) get a klinostat + grow a plant on the slowly rotating klinostat (4 rotations/hr). The gravitational stim is applied ALL sides and thus the root + shoot grow straight.
method 2) line 3 petri dishes w/ moist cotton wool(use same vol of water to moisten cotton).
- space out 10 cress seeds on the surface of cotton + press into surface
- tape lid onto the dish + wrap in foil (to remove influence of light)
- prop one horizontally, one at a 45 deg angle and one at 90 deg angle.
- leave the seeds for 4 days + observe shoot + root growth
- u should find that whatever the angle of petri dish, the roots grow DOWN( +ve) and shoots UP (-ve gravitropism)
To get quantitative values –> measure the amount grow by the roots + shoots + angle of growth
describe the experimental evidence for the role of auxins in the control of apical dominance.
1) plac 30 plants that are similar age, weight + height in plant pots.
2) count + record d the no of side shoots each one has + record
3) for 10 , cut of the apex tip of shoot + apply paste w/ aux, for another 10 do the same but add paste w/ auxins, for the last 10 do nothing ==> control
4) control needed for comparison to see if auxin is only factor affecting dependant variable.
5) let each group of plants grow for 6 days in same conditions (same light intensity, water, humidity etc) —> make sure that the other variables are controlled ad thus experiment is valid.
results
- removing tips+ add paste w/ aux -> no side shoots + lateral buds
- removing tips but no aux paste –> lateral bud devel + so do side shoots.
suggest auxins are involved in lateral bud growth.
describe the experimental evidence of giberellins in stem elongation?
1) plant 40 plants (eg dwarf pea plants) that are similar age, height, + weight.
2) leave 20 as they are to grow, water with normal ater + keep in same conditions
3) leave the other 20 to grow in same conditions BUT water w/ dilute solution of gibs.
4) let plants grow for 28 days + measure the length of stems each week
5) results probs show that stems grow MORE with dilute soln of giberellins
what is the relationship between auxin + calcium ions
Ca2+ are 2nd messengers for auxins.
describe the experimental evidence for giberellins in plants
- two mutant varieties of plants both lack giberellins
- one is v v short and lacks the precursor that is converted to giberellins by an enzyme
- the other one lacks the enzyme needed to convert the precursor (ent - kaurene) to giberellins.
- when both varieties were grafted together, they produced plants that grew to a normal height as the enzyme needed the converdt the precursor ent kaurene to Ga12 was present and thus Ga12 could be converted to GA1
what is the definition of seed germination
the development of a plant from a period of dormancy
how can you investigate phototropism.
- germinate plants in 3 diff conditions, dark, one sided light, unilateral light.
- or take 9 wheat shoots + place in individual pots in same conditions. cover 3 w/ foil on tip , cover 3 with foil on bottom half, leave 3 as they are
- u should see after placing in front of light –> exposed shoot tips grew towards light(explain mech) and the one with foil cap grew straight due to lack of light stimulus triggering unequal distribution of auxins.
exam q :outline the process of leaf abscission including the role of the plant hormones
- leaf abscission will occur when conc of auxins are low and conc of ethene is high
- ethene will switch on genes in the abscission zone
- the genes code for enzymes that will digest walls of the cells in the abscission layer
- ethene also stem cell in AB zone to expand, putting gpa on cell walls causing them to weaken + thus cells in abscission layer break off those i the separation layer of the stalk.
- the leaf will then break off.
