heat/cold tolerance

Question 1

when does chilling stress and injury occur?

what happens

Answer 1

>0 degrees.
reduced growth rate and ph,
increased respiration
membrane injury and loss of permeability, electrolyte leakage, less turgor and surface lesions
meristem damage.

Question 2

when does freezing stress and injury occur?

what happens

Answer 2

<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.

Question 3

7 adaptations to cold

Answer 3

1. short stature - protective in strong winds
2. shallow roots
3. small waxy leaves retain moisture
4. hairs and buds trap warm air
5. hibernating buds
6. photoperiodism - produce flowers quickly in short growing seasons
7. heliotropism - position flowers and leaves towards sun

Question 4

how do plants sense the cold

Answer 4

maybe through membrane fluidity increases with temp. although not much info known.
there is likely to be multiple mechanisms

Question 5

what are 3 strategies to survive the cold

Answer 5

freezing tolerance
freezing avoidance
cold acclimation

Question 6

2 mechanisms in freezing tolerance

Answer 6

1. 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.
2. promotion of extracellular freezing. dehydrin proteins involved in moving water to extracellular spaces to prevent cell freezing

Question 7

2 mechanisms in freezing avoidance

Answer 7

1. accumulate compatible solutes so cells dont freeze. lowers freezing point of cell contents
2. 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.

Question 8

what is cold acclimation

Answer 8

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.

Question 9

how is cold acclimation regulated?

why is this beneficial

Answer 9

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.

Question 10

what increase is there upon cold exposure?

Answer 10

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

Question 11

what is CBF

how do they help protect against cold?

Answer 11

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.

Question 12

what is ICE1

Answer 12

gene which is activated by sumoylation of SIZI, to upregulate CBF TFs.
ICE1 is degraded by HOS1, but HOS1 is inhibited by cold.

Question 13

what does OE of CBF do?

Answer 13

constitutive freezing tolerance, short stature.

Question 14

what does CBF1 do?

Answer 14

upregulates CA2ox1

increases CA catabolism, reducing plant growth.

Question 15

what is a phenotype of CAMTA mutants?

Answer 15

CAMTA 1/3 double mutants have impaired cold acclimation and freezing tolerance

Question 16

why is high temp damaging?

Answer 16

reduced: growth, ph, crop quality, yield
increased ROS, water loss,
improper development, altered phenology, inhibited seed production.

Question 17

desert adaptations

Answer 17

1. To conserve water: spikes, waxy cuticle
2. Stem modifications
3. root adaptations - short, branched, water doesnt penetrate deep into soil. alt, may have deep tap root to reach ground water.
4. physiology - quick dev and seed set.
5. CAM photosynthesis

Question 18

avoidance and tolerance of heat stress

Answer 18

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.

Question 19

3 heat stress proteins required for thermal tolerance

Answer 19

1. chaperones/heat shock proteins
2. osmolytes
3. secondary metabolytes

2 and 3 for ROS detoxification and osmoprotection

Question 20

What are heat shock proteins?

Answer 20

'cellular quality control'
expressed in response to high temp. 
maintain/repair proteins damaged by heat
targets incorrectly aggregated proteins for degradation. 
energy costly.

Question 21

how does thermomorphoenesis occur?

Answer 21

plant senses high temp

generate and transmits a signal to induce morphological adaptations.

Question 22

how is a small increase in temp sensed?>

Answer 22

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.

Question 23

what does ARP6 do?

Answer 23

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.

Question 24

when is plant growth impaired regarding day and night temps?

Answer 24

when there is a negative difference between day and night temp.
DT

Question 25

how is PHYB affected by temp?

Answer 25

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.

Question 26

why might plants grown at high temps show a sh av phenotype?

Answer 26

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?

Question 27

do shade avoidance and heat avoidance share the same molecular mechanisms?

Answer 27

PIF4 regulates high temp signalling
defective petiole elongation and leaf hyponasty response to increased temp.
PIF4 promoter binding is enhanced at high temps.

Question 28

how can PHYA be manipulated in crops to give desired characteristics?

Answer 28

OE of PHYA in crops - species specific effects.

1. tobacco with oat phya under 35S promoter gives improves harvest index.
2. OE of arabidopsis PHYA in rice causes dwarfism and increases grain yield by 21%.
3. OE of PHYA in creeping bent grass on golf courses is desirable as gives slow growth and high chl content.

Question 29

why may barley photoperiodic insensitivity be beneficial?

which genes may be responsible?

Answer 29

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.

Question 30

how can freezing tolerance be enhanced?

example of potato Solanum tubersum

Answer 30

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.

Question 31

how can CBF be manipulated to increase freezing tolerance

Answer 31

OE maize CBF in rice improves stress tolerance.

Question 32

how can freezing tolerance be measured

3 techniques

Answer 32

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.