Drought responses Flashcards
what factors add to global environment change?
- increased air temperatures - more severe, unpredictable extreme weather events (floods, droughts, hurricanes, tsunamis).
What did Former UK Government Chief Scientist, Sir John Beddington predict by 2030
- increased energy demand by 50%
- increased water demand by 30%
- increased food demand by 50%
what did Cruzet al 2007 conclude about the state of drought in N China?
only 70% of water demand can be met from surface and groundwater sources.
drought is affecting 202,000 hectares of crops and making it difficult to supply enough drinking water to 620,000 people.
Parts of Asia - wheat, rice and maize production declined due to climate change.
how much water is lost by plants?
97% of uptaken water is lost to the atmosphere by evaporation mostly through stomata.
- one hectare field of wheat will lose 60t of water/day, replenishing this requires 6mm of rain.
- if loss of water from plant exceeds uptake from soil causes drought
what are 3 drought types?
- hydrological - reduced streamflow, inflow to reservoirs lakes etc
- meteorlogical - high temp = increased evaporation and transpiration, prec deficiency.
- agricultural - soil water def.
why does drought have many diff outcomes on crops?
outcome depends on developmental stage of plant under stress.
can be more susceptible to drought at different points in life cycle.
wheat and rice are v sensitive just before pollen production, anthesis.
what is a root response to water defeceit?
plant’s resources are preferentially allocated to the root
investment in root growth at the expense of the shoot.
what are some variations in root response?
depend on severity of drought and genotype
1. preferential primary root elongation
2. Preferential lateral root proliferation in deep soil
3. production of short lived drought induced lateral roots.
(1 and 2 better in long term drought)
also, root hair elongation
increase in no. and diameter of xylem elements
increase in hydraulic conductance due to changes in expression of aquaporins, exo and endodermis.
what is not yet known about root response to drought
how reduction in water is detected
how changes cause coordinated adaptive responses. (some involves ABA signalling)
what is requires to maintain primary root growth in a drying soil.
ABA
Evidence: application of ABA biosynthesis inhibitor (fluridone) in dry soil inhibits primary root growth
where and vvvv basically how is ABA synthesised?
in the plastid and cytoplasm
derived from Zeaxanthin, a plant pigment.
The enzyme NCED is to believed to be rate-limiting in ABA biosynthesis
when is NCED enzyme expressed?
genes induced by drought stress
precedes ABA accumulation
what controls ABA levels?
ABA levels increase during stress and decrease when stress is relieved.
controlled by the balance of ABA synthesis and inactivation
How is ABA stored and quickly released when needed?
Converted to ABA- glucosylester (ABA-GE) by ABA glucosyltransferase.
Then stored in inactive form.
ABA-GE can rapidly be activated by converting into ABA by BG1 mechanism.
what else does ABA do during drought?
induces stomatal closure
- stomatal resistance increases when water def begins and aperture decreased. shown by
Beardsell and Cohen, 1975
how does ABA induce stomatal closure?
- stimulates the efflux of K salt from guard cell (with either Cl- or malate as the counter ion). accompanied by a loss of water.
- Overall the loss of turgor causes the guard cells to shrink and the stomatal pore closes.
what is the signalling pathway of ABA induced guard cell closure?
- ABA produced.
- ABA binds to PYR causing conformational change so PYR can bind to PP2C. (otherwise, PP2C inhibits OST1 (aka SNRK), and ion channels allowing loss of water and K from guard cell {GORK, QUAC1, SLAC1})
- OST1 is active and phosphorylates QUAC1, SLAC1, and inhibits K+ ion channel, so K+not uptaken. also phosphorylates AREBs which bind to ABREs and genes expressed.
- phosphorylated QUAC1, SLAC1 allow anions out of guard cell, depolarising the membrane, activating GORK K+efflux channel. loss of turgor, pore closes.
what happens when no ABA is present in guard cells?
- ABA receptor PYR is free in cytosol as inactive dimer.
2. PP2C is free and inactivates OST1 (aka SNRK), and SLAC1, QUAC1.
why is increase in cytosolic Ca2+ required for guard cell closure?
usually BAPTA sequests ca2+, blocking ABA induced cell closure.
Ca2+ activates CPK (calcium dependent protein kinases), which activates OST1 (which in turn phosphorylates SLAC1 and QUAC1.
what do protein kinases
and phosoprotein phosphatases do?
PK - phosphorylate substrates
Phosphoprotein phosphatases - dephosphorylate substrates
how are CPK and OST1 regulated in ABA induced guard cell closure?
by the protein phosphatase PP2C and Ca2+
what is unknown about Ca2+ influx?
how the activated PYR receptor causes calcium influx channel to open
what changes occur in stomata as leaves develop?
stomatal sensitivity to ABA
- transpiration v high in youngest leaves - seen in thermal imagery.
- reduces as leaves develop.
in which mutants is ontogenetic aquisition of transpirational control reduced?
Pantin et al 2013
aba2-2 - aba biosynthesis mutant
ost1-2 - aba signalling mutant
stomata in youngest leaves of these mutants fail to respond to ABA and darkness which usually induce stomata closure.
How do leaves become sensitized to ABA as they mature?
young in the centre of arabidopsis - high 90% RH
old at edge, low RH, 70%.
if leaves are primed with a low RH, sensitizes them to ABA. (done by blowing dry air onto leaves the night before making measurement).
also, old leaved primed with 100% RH have reduced ABA sensitivity,
overall - as RH decreases, ABA biosynthesized, switching on rest of pathway. pretreatment of ABA increases exp of many genes involved in ABA signalling.
what are possible root to shoot communication methods resulting in soil drought induced stomatal aperture reductions?
Hydraulic signals
Electrical Signals
ABA
Another molecule eg strigolactone.
why are hydraulic and electrical signals unlikely to be responsible for root to shoot communication in drought
signal speed is too slow.
Also unclear how generated and perceived.
why is ABA unlikely to be responsible for root to shoot communication in drought
movement in xylem is too slow. grafting experiments have failed to support a long distance signalling role.
who suggested another molecule is responsible for root to shoot communication in droughts
Francois Tardieu, Strigolactone involved.
efficient and effective control of stomatal aperture can only result from multiple signals working together.
What are ABREs and AREBs.
involved in long term responses to water def.
Promoters of ABA regulated genes - have conserved ABA responsive element called ABRE.
ABRE recognised by ABRE-binding proteins = AREBs.
AREBs found in nucleus and act as transcriptional activators when bind to AREBs.
how are ABA responsive genes (ABREs) regulated?
regulated by same pathway as stomatal closure.
OST1 phosphorylates members of the AREB family which then bind to ABREs (ABA responsive element on genes).
