heat/cold tolerance Flashcards
when does chilling stress and injury occur?
what happens
>0 degrees. reduced growth rate and ph, increased respiration membrane injury and loss of permeability, electrolyte leakage, less turgor and surface lesions meristem damage.
when does freezing stress and injury occur?
what happens
<0 degrees
following cold snaps in freezing sensitive species.
ice formation, begins in apoplast (due to lower solute content than intracellular fluid).
90% of osmotically active water is drawn into extracellular fluid, = cellular dehydration
- severe membrane damage, leakage of electrolytes, damage to macromolecules, ROS production, organelle damage, cell death.
7 adaptations to cold
- short stature - protective in strong winds
- shallow roots
- small waxy leaves retain moisture
- hairs and buds trap warm air
- hibernating buds
- photoperiodism - produce flowers quickly in short growing seasons
- heliotropism - position flowers and leaves towards sun
how do plants sense the cold
maybe through membrane fluidity increases with temp. although not much info known.
there is likely to be multiple mechanisms
what are 3 strategies to survive the cold
freezing tolerance
freezing avoidance
cold acclimation
2 mechanisms in freezing tolerance
- thermal ice barriers, eg in alpine cushion plants. Gradients form, so flower coldest and veg cushion warmest, freezing is restricted to flowers. increases a single flowers chance of surviving.
- promotion of extracellular freezing. dehydrin proteins involved in moving water to extracellular spaces to prevent cell freezing
2 mechanisms in freezing avoidance
- accumulate compatible solutes so cells dont freeze. lowers freezing point of cell contents
- super cooling. water <0 degrees doesnt freeze. anti ice nucleators bind to ice crystals and inhibit growth.
eg flavenol glycosides in parenchyma cells of Cercidiphyllum japonicum supercools to -40 degrees.
what is cold acclimation
process where plants increase freezing tolerance in response to low non freezing temps.
requires prolonged period of cold.
upregulates genes for antifreeze.
freezing snap damage due to inability to cold acclimate.
how is cold acclimation regulated?
why is this beneficial
regulated by photoperiod.
plants grown in long days observed more electrolyte leakage than those in short days, less freezing tolerance.
Expression of CBF reduced in long days. CBF transcript shows c rhythm, peaks greater in short days.
Less CBF in long days - reduced COR expression.
Prevents resource use in cold acclimation during summer months.
what increase is there upon cold exposure?
increase in cytosolic Ca2+
influx is v rapid, primary response.
chemicals blocking Ca2+ reduce expression of cold inducible genes.
Ca2+ binds to calmodulin protein. complex enters nucleus and binds to CAMTA transcription factors. CAMTA then binds to ICE 1 gene
what is CBF
how do they help protect against cold?
C-repeat binding factor
TF, switch on COR genes.
in cold, ICE1 gene is activated, by sumoylation of SIZI, which activated CBF 1, 2, 3.
CBF bind to C repeat of COR genes, upregulating COR stabilises the membrane and increase production of cryoprotectants.
what is ICE1
gene which is activated by sumoylation of SIZI, to upregulate CBF TFs.
ICE1 is degraded by HOS1, but HOS1 is inhibited by cold.
what does OE of CBF do?
constitutive freezing tolerance, short stature.
what does CBF1 do?
upregulates CA2ox1
increases CA catabolism, reducing plant growth.
what is a phenotype of CAMTA mutants?
CAMTA 1/3 double mutants have impaired cold acclimation and freezing tolerance
why is high temp damaging?
reduced: growth, ph, crop quality, yield
increased ROS, water loss,
improper development, altered phenology, inhibited seed production.
desert adaptations
- To conserve water: spikes, waxy cuticle
- Stem modifications
- root adaptations - short, branched, water doesnt penetrate deep into soil. alt, may have deep tap root to reach ground water.
- physiology - quick dev and seed set.
- CAM photosynthesis
avoidance and tolerance of heat stress
Avoidance: transpirational cooling roll leaves alter leaf orientation early maturation. Tolerance: upregulate antioxidants, osmoprotectants, stress proteins, signalling cascades and transcriptional control. pre exposure to high temp will induce heat tolerance.
3 heat stress proteins required for thermal tolerance
- chaperones/heat shock proteins
- osmolytes
- secondary metabolytes
2 and 3 for ROS detoxification and osmoprotection
What are heat shock proteins?
'cellular quality control' expressed in response to high temp. maintain/repair proteins damaged by heat targets incorrectly aggregated proteins for degradation. energy costly.
how does thermomorphoenesis occur?
plant senses high temp
generate and transmits a signal to induce morphological adaptations.
how is a small increase in temp sensed?>
HSP70 - heat sensitive promoter. Activity visualised at different temperatures by using HSP70:LUC reporter gene.
also arp6 controls some heat responses. mutant seedlings showed an enhanced temperature response, as if they were under a high temperature when they weren’t. the mutated gene found to be ARP6.
what does ARP6 do?
ARP6 gene is a chromatin remodelling factor. alters histones to change binding of DNA and vulnerability to DNA polymerase.
ARP6 controls histone variant exchange - post translational modifications of histones induce differences in nucleosome structure of histones.
ARP6 can swap variants. eg under high temp, H2AZ is dissociated by SW1/SNF complex, containing ARP6, and replaced by H2A. changes exp of genes associated with that histone.
when is plant growth impaired regarding day and night temps?
when there is a negative difference between day and night temp.
DT
how is PHYB affected by temp?
PHYB activity is dependent on temp.
Pfr ->Pr is accelerated at high temp.
at higher temps, when less Pfr, PIF4 accumulates, and more elongation.
therefore, PHYB acts as a thermosensor.
why might plants grown at high temps show a sh av phenotype?
might be elevating photosynthetic and meristematic tissues away from hot soil, so plant can take advantage of cooling by the moving air.
22 degrees C - observe compact architecture
at 28 degrees - elongated. promotes leaf cooling. thinner leaves, stomata more spaced out, reduce humidity around the pore?
do shade avoidance and heat avoidance share the same molecular mechanisms?
PIF4 regulates high temp signalling
defective petiole elongation and leaf hyponasty response to increased temp.
PIF4 promoter binding is enhanced at high temps.
how can PHYA be manipulated in crops to give desired characteristics?
OE of PHYA in crops - species specific effects.
- tobacco with oat phya under 35S promoter gives improves harvest index.
- OE of arabidopsis PHYA in rice causes dwarfism and increases grain yield by 21%.
- OE of PHYA in creeping bent grass on golf courses is desirable as gives slow growth and high chl content.
why may barley photoperiodic insensitivity be beneficial?
which genes may be responsible?
Winter type barley, requires vernalisation, strong promotion of flowering in response to long days.
Spring type barley, no vernalisation, and weak/strong response to long days.
This allows spring plants to extend their period of vegetative growth and accumulate additional biomass to increase yield, in the long growing season.
ppd-HI locus is determinant of long day sensitivity, mutants delayed flowering in long days.
CONSTANS - flowering time gene. If c expression of this gene is altered, reduces photoperiodic sensitivity.
how can freezing tolerance be enhanced?
example of potato Solanum tubersum
eg potato Solanum tubersum
frost sensitive and cannot cold accumulate.
Add CBF genes from arctic species.
Transformed with arabidopsis CBF genes with 35S promoter. tested by electrolyte leakage assay for freezing tolerance.
However, OE of CBF stunts growth.
instead use different promoter - RD29A, stress induced promoter - enhanced freezing tolerance following cold acclimation and no dwarfism.
how can CBF be manipulated to increase freezing tolerance
OE maize CBF in rice improves stress tolerance.
how can freezing tolerance be measured
3 techniques
whole plant freezing tolerance assays
- grow at ambient temp then acclimate at 4 degrees. Freezing treatment at 6 degrees for 24h. recovery period.
- assess damage, % survival - LT50 - temperature at which 50% of pop dies.
2. Electrolyte leakage assays - leaves cooled with ice and 4 degree water bath and thawed overnight. add water and shaken, measure electrolyte conductivity. then boil to extract all electrolytes and see % leakage at each temp.
3. chl fluorescence. non invasive, measure of PS2 efficiency.
