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(P) Water Stress - Coggle Diagram
(P) Water Stress
Adaptation and Acclimation Responses
1) Growth and development
(longer term acclimation)
a, Limiting water transpiration
Limitation of leaf surface area
reduced leaf expansion
growth (cell expansion)
GR = m (⍦p -Y)
m :arrow_down:🧼
more rigid cell walls
Y :arrow_up: 🧵
function of more complex cell wall modifications
Early biophysical effect
reduction of turgor pressure -> limitation of turgor- dependent leaf expansion
reduced number of leaves produced (limited stem growth)
active leaf abscission
when the overall leaf surface is large enough for basic energy supply :banana: :snowman_without_snow:
increasing the deposition of cuticular waxes
reduces water loss through epidermis (transcuticular transpiration ≠ transstomatal)
typically marginal (5-10% of transpiration)
only significant in
extreme
stress conditions or upon physical damage (e.g. wind-sand)
Alternatives to transpiration (for leaf cooling)!
reduced leaf surface area / size
xerophytic plants :cactus:
less direct heat transfer
leaf shape can be adjusted
thinner boundary layer
better evaporative cooling
reflective structures
e.g.
wax depositions, hairs (trichomes), salt crystals
reduce energy absorption
leaf positioning
active
para-heliotropism // :sunny:
passive
wilting
leaf curling
due to loss of turgor P
can significantly reduce heating of larger leaves
b. Securing water uptake
Increased root growth :scales:
initially, increased root/shoot ratio
ABA involved
Root ‘hydro-tropism’
:droplet:
dependent on ABA signaling (SnRK2 kinase activity)
Root system architecture
🧬
genetically determined
steeper root growth (= larger root growth angles, RGAs)
Limiting cavitation
🫧
2) Fast responses
Closing of stomata - ABA
. compromise between limiting transpiration / sufficient CO2 uptake
ABA synthesis
Mevalonate (cytoplasm) & MEP (plastid) pathways
C5 isoprene subunits
start from
tetraterpenes (C40)
e.g.
1 more item...
Water stress = increase stomatal resistance (less diffusion)
need to increase ABA
increase biosynthesis, decrease degradation & inactivation, & use pre-stored ABA (compartmentalisation)
ABA compartmentalisation
weak acid (anion ABA- in basic env.)
cannot cross the membrane once in an alkaline environment
1 more item...
Transport
mainly via phloem, also via xylem in water stress conditions (glucose-esters)
water stress (H+ pump decreased function)
alkalinization of the apoplast + acidification of the cytosol
dissociation of ABAH into ABA- & H+
ABA- doesn't get taken up by mesophyll cells (anion impermeable)
1 more item...
ABA function in guard cells:
key 2nd messenger:
Ca2
+
there are others
ABA signaling does 2 things
activates anion channels and inhibits PM H+-ATPase
resulting in membrane depolarization and
activation of K+ efflux channels
inhibits K+ influx channels
net result
K+ leaving ccs, lowering water potential outside and drawing water osmotically out of ccs
decreases stomatal opening size
BUT!
auxin can also trigger stomatal
opening
via increases in Ca2+
specific
Ca2+ “signature”
determines the cellular response!
Photosynthesis: induce (facultative) CAM metabolism
water deficit eventually also limits photosynthesis
but translocation of photosynthate is maintained
through accumulation of enzymes such as:
. Phosphoenolpyruvate (PEP) carboxylase (PEPC)
. Pyruvate-orthophosphate dikinase (PPDK)(regeneration PEP)
Osmotic adjustment
active
accumulation of solutes, far exceeding the passive concentration by dehydration
. Protonated (-onium) compounds
Proline
, dimethylsulfoniopropionate,
Glycine betaine
, b-Alanine betaine, Proline betaine, Choline-O-sulfate
. Sugar-alcohols
Pinitol,
mannitol
. Non-reducing sugars
sucrose,
trehalose
No free hemi-acetal/hemi-ketal groups
non-reducing (not reacting with e.g. amino acids)
to lower osmotic potential (WITHOUT interfering w/cellular metabolism)
inc. biosynthesis
upregulate P5C synthetase (≠PRODH) (Proline)
upregulate CMO and BADH (Glycine betaine)
inhibit sucrose synthesis & mannitol dehydrogenase (Mannitol)
some of these also function as osmoprotectants :shield:
(PS) glycine betaine, (O) sorbitol, mannitol, myo-inositol, proline, (:ghost:) trehalose
Protective
proteins
LEA
Late Embryogenesis Abundant :droplet: ➿ :banana:
extremely
hydrophilic
, randomly
coiled
,
cytoplasmic
proteins rich in
GRE
A
TK
with low amts of
CW
residues :warning:
mostly unknown funct.
but their transgenic over-expression increased water-related stress-tolerance
different groups defined by sequence motifs
Osmotin
multifunctional
:mushroom:, basic (
ca+ionic
) protein (vacuolar, cytoplasmic :hole:, and tonoplast- and plasma membrane-associated :frame_with_picture:)
expression is induced by at least 10 different stress-signals
osmolyte function + triggers the induction of proline biosynthesis (via an as-yet unknown signaling pathway) + antifungal activity
Inducing gene expression (ABA-dependent
ABRE
and ABA-independent
DRE
)
cell senses osmotic stress (elusive)
ABA
bZIP (TF)
binds to ABA responsive element (ABRE ; promotor)
altered gene expression
Osmotic stress tolerance
Protein synthesis (MYC/MYB)
no ABA
DRE-binding protein/CBF (TF)
binds to DRE (dehydration responsive element)
altered gene expression
MAPK cascade
Water status of the plant
2 parameters describe plant water status (to which degree the tissue/plant is hydrated)
water potential
⍦w
negative value – maximal water potential = 0 (pure water)
expressed in units of pressure: MPa (Mega Pascal) = 10 bar
movement: high -> low ⍦w (high solute conc.)
⍦w = ( ⍦π / s + ⍦p ) + ⍦m + ⍦g
describes well the overall osmotic water transport
but: physiological and metabolic changes in water-stressed plants do not always correlate with ⍦w, more complex (not practical)
RWC
(fresh weight – dry weight) X 100 –––––––––––––––––––––––––––––––––––––– (fully turgescent weight – dry weight)
optimal conditions: 85-95 % (of max)
How do plants respond to water deficit?