9.2 Temperature stress: heat and cold

Plants face temperature extremes that can disrupt their normal functions. Heat stress triggers protective mechanisms like heat shock proteins and membrane adaptations. Cold stress prompts acclimation responses, including antifreeze proteins and dehydrins.

Understanding these responses is crucial for plant survival in changing climates. From cellular protection to whole-plant adaptations, temperature stress responses showcase plants' remarkable ability to cope with environmental challenges.

Heat Stress Response

Cellular Protection Mechanisms

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• Heat shock proteins (HSPs) are rapidly synthesized in response to heat stress
  • Act as molecular chaperones to prevent protein denaturation and aggregation
  • Assist in refolding of denatured proteins to maintain cellular function
  • Major classes include HSP60, HSP70, HSP90, and small HSPs
• Membrane fluidity increases at high temperatures due to changes in lipid composition
  • Leads to increased permeability and potential loss of cellular contents
  • Plants adapt by altering fatty acid saturation levels to maintain optimal fluidity
  • Involves enzymes such as desaturases and elongases to modify membrane lipids
• Reactive oxygen species (ROS) production is enhanced under heat stress conditions
  • Includes superoxide radicals, hydrogen peroxide, and hydroxyl radicals
  • Can cause oxidative damage to proteins, lipids, and DNA
  • Plants employ antioxidant defense systems to scavenge ROS (superoxide dismutase, catalase, ascorbate peroxidase)
  • Compatible solutes (proline, glycine betaine) also contribute to ROS detoxification

Cold Acclimation and Vernalization

Adaptive Responses to Low Temperatures

• Cold acclimation is the gradual acquisition of freezing tolerance upon exposure to low non-freezing temperatures
  • Involves changes in gene expression, metabolite accumulation, and membrane composition
  • Enhances survival during subsequent freezing events by minimizing cellular damage
  • Examples include increased synthesis of cryoprotectants (sugars, proline) and antifreeze proteins
• Vernalization is the promotion of flowering by prolonged exposure to cold temperatures
  • Required by many winter annual and biennial plants to transition from vegetative to reproductive growth
  • Involves epigenetic silencing of floral repressors (FLC in Arabidopsis) by chromatin modifications
  • Ensures flowering occurs under favorable conditions in spring after winter cold exposure
• CBF/DREB transcription factors play a central role in cold acclimation and freezing tolerance
  • Bind to C-repeat/Dehydration-Responsive Elements (CRT/DRE) in promoters of cold-responsive genes
  • Rapidly induced by low temperatures to activate downstream genes involved in cold protection
  • Overexpression of CBF/DREB factors enhances freezing tolerance in various plant species (Arabidopsis, tomato, barley)

Cold Stress Protection

Mechanisms to Mitigate Freezing Damage

• Antifreeze proteins (AFPs) accumulate in cold-acclimated plants to prevent ice crystal growth
  • Bind to ice crystal surfaces and inhibit their expansion, reducing cellular damage
  • Present in various overwintering plants (winter rye, carrot, bittersweet nightshade)
  • Can be induced by both cold acclimation and vernalization treatments
• Dehydrins are a class of late embryogenesis abundant (LEA) proteins that protect cells from dehydration stress
  • Accumulate in response to cold, drought, and salinity stress to stabilize membranes and proteins
  • Act as molecular chaperones to prevent protein aggregation under dehydration conditions
  • Examples include COR15a in Arabidopsis and WCOR410 in wheat, induced by low temperatures
• Chilling injury occurs in sensitive plants exposed to low non-freezing temperatures (0-15°C)
  • Leads to membrane dysfunction, electrolyte leakage, and reduced photosynthetic capacity
  • Symptoms include wilting, chlorosis, necrosis, and pitting of fruits and vegetables (tomato, cucumber, banana)
  • Can be mitigated by pre-conditioning treatments, controlled atmosphere storage, and genetic improvement of chilling tolerance