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(P) Heat Stress - Coggle Diagram
(P) Heat Stress
Decreased absorption of solar radiation
:partly_sunny:
reflective structures :dark_sunglasses:
(hairs, crystals, waxes)
leaf positioning :four_leaf_clover:
paraheliotropy, curling
small, narrow or strongly dissected leaves to
limit the leaf boundary layer > <
heat loss by
convection and conduction ~
dimorphic leaves :green_heart: š¤
(seasonal dimorphism) in desert plants
Effects on membrane stability and acclimations
Effects
high temperatures cause
excessive
fluidity of membrane lipids
can even lead to ion leakage at some pt
good readout of excessive membrane fluidity
K+ leakage
acclimations
higher degree of fatty acid saturation in membranes
increased rigidity
photosynthesis
cold-adapted plts more senstiive
electron transport in PSII more sensitive
enzymes such as Rubisco, GAPDH and PEPCarboxylase (membrane-assoc.) are less stable
however: no protein denaturation yet
first effects of heat stress are due to membrane stability problems & the resulting uncoupling of the energy transfer mechanism
Other, HSP-independent mechanisms: hormones and Ca2+ signaling
Hormones
ABA signaling
SA (salicylic acid) signaling
Ethylene signaling
pre-treatment increases heat stress tolerance
Ca2+-signaling
role in transcription regulation
production of GABA, a non-proteinaceous aa
maintains
pH
heat stress leads to
acidification of the cytosol because of inhibition of the membrane H+-pumps
[Ca2+] in the cytosol increases by direct (H+ gradient) or indirect (ATP) effects on channels, H+-antiporters, Ca2+-ATPases
Ca2+-CaM activates glutamate decarboxylase (GAD)
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redox homeostasis
a.k.a
the GABA āshuntā
heat stress
affects redox status of the cell
NADH and NAD+ imbalance
GABA gets metabolized in the mitochondria
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Drought stress resistance š§
by limiting transpiration + sufficient uptake and retention of water, for example by osmotic adaptation
Effect on metabolism
:fries:
PS
sensitive to heat stress
activity declines faster than
that of respiration
unbalance and net loss of carbon!
temperature compensation point:
point where CO2 fixation = CO2 release
lower in shadow leaves than in sun-exposed leaves
more problematic for C3 plants (increased dark respiration at higher T)
Production of āheat shock proteinsā (HSPs)
upon sudden increases in temperature of 5-10Ā°C:
discovered in fruit flies; widespread in animals, plants, fungi, other micro- organisms; highly
conserved
induced upon heat stress but also!
some types occur in non-stressed cells = housekeeping activities (folding)
some are induced by other stress conditions
. contribute significantly to cross-protection
function as molecular chaperones:
. preventing mis-folding and aggregation of proteins
assisting in correct (re-) folding
5 classes, based on molecular weight, w/ different subcellular locations
smHSPs more abundant in plants (6 subclasses,1 in other eukaryotes)
bind to aggregating proteins, making them accessible for refolding
by HSP100 and HSP70
. induction of HSP synthesis by sub-lethal heat stress
acclimation
mechanism?
HSF (heat shock factor) transcription factor trimer binds HSE (heat shock element)
HSE:
5bp repeats (n
GAA
n) in alternating orientation (5-7)
HSF cycle
Inactive monomers in the cytosol (associated with HSP70 proteins)
Heat stress: HSF release and trimerisation
Binding to HSE
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Link with ROS (reactive oxygen species) signaling
heat stress leads to ROS production
oxidative damage
H2O2 = signal molecule
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Many plants have MODIFICATIONS of leaves against excessive heating. What are they?