L13 - Physiology and Molecular Biology of Temperature Responses

Question 1

What’s the definition of plant stress?

How much ability do plants have to regulate their temperatrures?

Answer 1

• Any environmental condition that prevents the plant from achieving its full genetic potential under ideal growth conditions
• Mostly only moderate regulation through transpiration

Question 2

Give the 3 key effects of temperature stress

Answer 2

1) Disruption of protein stability and enzymatic reactions.

2) Alterations in membrane fluidity - can uncouple multi-protein complexes, disrupt e- flow etc…

3) Changes nucleic acid structures: high temp. can destabilise, low temp can over-stabilise. Both interfere w/ transcription, translation etc…

Question 3

Give the two types of low temperature stress

Answer 3

Chilling stress - exposure to low temp. above freezing point

Freezing stress - exposure to low temp. below freezing point

Question 4

Give the potential effect and the plant response to chilling stress

Answer 4

• Fatty acid tails of phospholipids in the cell membranes become rigid = gel like state with limited permeability.

Response:
- Chilling-tolerant plants increase unsaturated FAs
- Double bonds create kinks in membrane, reducing packing density of acyl chains, increasing membrane fluidity

Question 5

Give the potential effects and the plant responses to freezing stress

Answer 5

Potential effect - ice formation in cells

Response 1:
- Freeze-acclimated plants secrete antifreeze proteins (AFPs) to prevent ice growth by inhibiting crystallisation process around ice nucleus.
- Freezing point of water lowered.

Response 2:
- Increased concentrations of osmotically active substances in cell sap lower freezing temp

Question 6

Outline the three strategies within heat stress resistance

Answer 6

Escape: Prevent encountering stress conditions. Often through phenology: dormancy, early flowering etc…

Avoidance: Lowering intracellular temperatures through developmental changes called photomorphogenesis

Tolerance: Acclimating to elevated intracellular temperatures + protecting cells

Question 7

Describe the impacts of warm temperatures on the early stages of plant growth and vegetative development

Answer 7

Seed germination:
- High temp. can delay germination
- E.g. Arabidopsis requires period of cold to break dormancy
- Used so seeds germinate in spring, not hot summer

Vegetative development traits:
- Thermomorphogenesis adaptations (similar to photmorphogenesis) (Arabidopsis example) :
1) Elongation of hypotocyls and petioles to increase leaf elevation from soil
2) Leaf hyponasty, upward curvature of leaves
3) Open rosette leaves
4) Deep roots

Overall, cooling capacity enhanced through transpiration - can elevate water shortage

Duration of vegetative development:
- Vegetative development phase altered (longer or shorter, depends on plants)
- Longer can = more C invested in growth rather than seeds
- Shorter can = less time for C fixation needed for leafs

Question 8

Outline the signalling pathway for thermomorphogenesis in 4 steps

Answer 8

1) Temp. change detected by 3 receptors: phyB, ELF3, PIF7
2) Transcription factors activated: PIF4, PIF7
3) Hormone biosynthesis and signal transduction involving auxins + brassinosteroids (BR) promoted
4) Asymmetric cell elongation = thermomorphogenesis traits

Question 9

Describe the thermosensor phy B:

Answer 9

Phytochrome B (phyB):
- exists in two reversible forms, inactive form Pr and active form Pfr.
- Pr converted to Pfr by red light. Pfr returned to Pf by far-red light (not photosynthesised)
- Pfr inhibits PIF4 TF, stopping hypocotyl elongation that searches for sun
- Warm temp. promotes dark reversion (return to Pr), phyB = negative regulator of thermomorphogenesis

Question 10

Describe the thermosensor PIF7:

Answer 10

PIF7 transcription factor:
- At lower temp. PIF7 TF mRNA forms hairpin structure in 5’ untranslated region (UTR) before first start codon.
- Hairpin inhibits translation of PIF7 TF
- Higher temp. causes conformational change, opens hairpin, allowing translation and expression of thermomorphogenesis genes

Question 11

Describe the thermosensor ELF3:

Answer 11

ELF3:
- ELF3 = Transcriptional repressor, must be deactivated to increase PIF4 expression
- Undergoes phase separation to form inactive condensate at higher temps.

Question 12

Describe the impacts of warm temperatures on flowering and reproductive development

Answer 12

• Speeds up flowering timing
• Plants require thermal sun to trigger flowering
• Global warming = earlier flowering
• Heat waves disrupt reproductive development.
• Pollen number, germination and fertilisation rates decreased.
• Overall fewer seeds

Question 13

List all the problems associated with heat stress

Answer 13

• Membrane fluidity
• Accumulation of ROS
• Reduced turgor pressure = reduced structural integrity
• Misfolded proteins

Question 14

Sketch a diagram summarising the mechanisms for heat stress resistance for hot and scorching conditions.

Answer 14

See diagram on pg 11

Question 15

Describe the two secondary messengers that start the heat acclimation process at moderate hot temperatures

Answer 15

Calcium:
- Heat-induced Ca signals cause changes in gene expression
- Mediated by calmodulin and calcium dependent protein kinases (Ca binding proteins)

ROS:
- Ca signalling activates NADPH oxidase to produce ROS
- ROS activated calcium channels for calcium influx stimulated
- Positive feedback look amplifies signal

Question 16

What do the secondary messengers activate? Describe.

Answer 16

Heat Shock Factors (HSF):
- Transcription factors that bind to heat shock elements in promoter of Heat Shock Proteins (HSP)
- Genes induced: HSPs, ROS scavenging enzymes, membrane lipid composition adjustment

Question 17

What response is triggered by very high temperatures? Describe.

Answer 17

Unfolded protein response (UPR):
- Elevated levels of unfolded or misfolded proteins detected in Endoplasmic Reticulum (ER)
- bZIP TFs transported from ER to nucleus to activate gene expression
- Genes to degrade misfolded proteins activated: ER-associated degradation (ERAD)
- Genes for heat acclimation activated: HSPs and ROS scavenging enzymes, membrane lipid composition adjustment

Question 18

Describe the genes induced by both heat acclimation and the UPR pathway

Answer 18

Heat Shock Proteins (HSPs):
- Molecular chaperones, assist w/ refolding of proteins

ROS scavenging enzymes:
- Scavenge ROS to minimise damage
- E.g. ascorbate peroxidase, peroxidase

Membrane lipid composition adjustment:
- Reduce amount of unsaturated fatty acids in membrane.
- Lowers membrane fluidity.
- E.g. Fatty acid desaturase

Question 19

Draw a diagram summarising all the high temperature resistance mechanisms, classifying each as avoidance, acclimation or tolerance.

Answer 19

See diagram on pg 15

Question 20

Give an example of a heat resilient crop

Answer 20

Introgressed Asian Rice:
- African rice (Oryza glaberrima) has lower yield but beter heat resistance than Asian Rice (Oryza sativa).
- ThermoTolerance 1 (TT1) gene introduced to Asian from African via introgression.
- Compared to TT1 (Asian), TT1 (African) more strongly activated by heat + enhanced ability to degrade heat denatured proteins.

• Decreased abundance of proteins in introgressed Asian compared to normal Asia.
• Introgressed Asian showed no decrease in yield at optimum temp. but higher yield than non-introgressed Asian during heat stress.