What is tropism vs nasty?
Tropism- directional growth response towards stimulus caused by asymmetiric growth of plant axis eg. gravi-, photo-, thigmo-
Nasty- Does not involve growth and response is non-directional relative to stimulus
eg. photonasty, nyctinasty (associated with changes in circadian rhythm), thigmonasty
What are tropic responses due to and who first studied them?
Darwin studied oat coleoptiles, 50 years later hypothesis proved that it is due to asymmetric auxin distribution
How is gravity sensed in the root? (vague outline)
By central cells of the root cap which contain large dense amyloplasts (starch-containing plastids), which sediment in the cellular lumen according to gravity
What is name of gravity sensing cells and their amyloplasts?
How does the response to gravity work
On change in gravity statocytes change position, induces auxin efflux carrier PIN3 to relocalise membrane at new bottom side, thus auxin fluxes are redirected to lower side of root tip
From here it is transported to root elongation zone by PIN 2 and AUX1
Here auxin inhibits cell elongation and the root bends down
How does geotropism in the shoot work?
Statoliths also present in endodermal stem layer, gravitstimulation makes PIN3 re localise new basal membrane of endodermal cell, changes flux of auxin to outer cell layers
Auxin stimulates cell elongation and you get upward bending
What are phototropic responses in arabidopsis mediated by?
blue light phototropins PHOT1 and PHOT2
Mutation in phototropin-encoding genes results in…
non-phototropic hypcotyl mutants
What happens if you treat a WT plant with 3 minutes of unilateral blue light?
Temporary stop of auxin delivery to elongation zone, PIN3 resumes AUXIN transport preferentially on the shaded side
15 mins: auxin has accumulated asymmetrically
2 hr: you get bending
How do plant responses to salt stress work?
Rapid movement of ion fluxes root to shoot
Arabidopsis can be made to be transgenic so it is fluorescent when ca2+ binds
Local salt stresses at root tip lead to rapid distribution of calcium waves through cortex and endodermal cell layers and induced molecular responses (400 micrometers a second)
How does thigmonasty in Mimosa Pudica work?
AP stimulated upon mechanical stimulation
Signal goes from base of stimulus to pulvinus (base of stalk) at 25 mms^-1
AP transmitted laterally via plasmodesmata into cells of motor cortex, results in pulvinar action potential initiating K+ and Cl- transport
Change in cells’ turgour pressure
How does Venus Fly Trap work?
If 2 AP fired within 20s trap shuts within 0.5 s
There is Ca2+ stimulated Cl- efflux resulting in depolarisation
lobe tissue switches convex –> concave on closure
Trap accumulates elastic potential energy until lobe buckles inside out
What is the annulus in ferns?
Layer of ~12 cells with asymmetrically thick walls
How does fern spore ejection work?
Towards the end of spore maturation the tissue dries so the cells of the annulus lose water and shrink
Lower volume converted to bending energy in lateral walls (because there is negative pressure)
Then at -20MPa the cells suck in air and cavitate, and rapid cell expansion occurs, making the sporangium spring back to its original shape and launch spores into the air