L14 - Physiology and Molecular Biology of Water Deficit Flashcards
Outline how drought is a serious threat to food security.
- Drought accounts for nearly half of the overall natural hazards affecting crop and animal production.
- On-third of the world’s food produced on irrigated land indicates this problem
- Climate change will increase unpredictability, frequency and intensity of drought
What are the main functions of water in plants?
How much of the water taken up by plants is lost through transpiration?
- Largest portion of cell volume in plants
- Regulated turgor pressure
- Needed for photosynthesis
- 97% lost through transpiration
- But transpiration drives mineral and water movement through plant
- Cools plant during excess light
Give the 4 main problems plants experience with water deficit
- Stomatal closure to reduce water loss inhibits photosynthesis
- Reduced photosynthesis increases ROS
- Water potential more negative outside the cell, reduction in turgor = reduced cell growth
- Reduced transpiration = nutrient uptake and movement inhibited
Give the three plant drought resistance strategies
- Drought escape
- Drought avoidance
- Drought tolerance
Give three ways plants escape drought
- Early maturity
- Rapid plant development
- Seasonal growth before drought
What is the objective of drought avoidance?
Give the two main drought avoidance strategies
- Maintain optimal relative water content in dry conditions
- Minimise water loss: stomatal closure, leaf rolling, leaf epidermal structure (e.g. trichomes, thicker cuticle)
- Improved water access through root growth e.g. longer main root, reduced lateral roots
What is the objective of drought tolerance
Outline the three main mechanisms of drought tolerance
- Minimising damage given below optimum water levels in plant
Osmotic adjustment:
- Compatible solutes (neutral) accumulated in cytoplasm to reduce water potential in cell
- Water enters from soil, maintaining turgor pressure
Antioxidant defense mechanisms against ROS:
- Carotenoids scavenge ROS
- ROS detoxifying enzymes e.g. catalases
- Late Embryo Abundant (LEA) proteins accumulate, sequestering ROS
Beneficial microbes:
- E.g. arbuscular mycorrhizal fungi helps nutrient uptake + drought resistance
Give the three steps in the sensing and signalling of water defecit
1) Sensing e.g. via osmotic sensing
2) Hormone synthesis for signalling e.g. phytohormone abscisic acids (ABA)
3) Causes TFs to launch global gene expressions for drought resistance response
Outline the likely environmental factor used by plants to sense water deficit.
Give three potential sensing mechanisms
Osmotic sensing - Negative water potential outside cell causes water efflux = cell shrinkage
1) Osmotic imbalance: hypertonic environment outside cell causes conformational changes of receptor
2) Mechanosensitive channel: reduced turgor pressure = altered physical tension of plasma membrane, sensed
3) Cell wall integrity: Changes in turgor pressure detected at cell wall by plasma membrane-localised receptor kinases
Describe the plant response after the initial sensing of a water deficit
- Roots sense deficit then Abscisic acid (ABA) released from plastids into transpiration stream.
- ABA targeted to guard cells in leaves, promoting stomata closure.
Describe what happens after hormone synthesis in the drought response of plants
Provide an example
- TFs triggered to bind to particular conserved DNA sequences upstream of transcription start site
- Expression of drought resistant genes increased
- E.g TFs DREBs (Drought Responsive Element-Binding) binds to conserved core sequence DRE (Dehydration responsive element)
Give two examples of improved crop resilience against drought
- Rice variety Sahbhagi dhan (SD) flowers early and is drought tolerant (conventionally bred)
- Grown by more than 300,000 farmers in E. India
- Elite rice variety IR64 introgressed w/ deep rooted rice variety KP.
- Deep Root1 (DRO1) gene transferred to IR64.
- DRO1-NIL showed larger root angle than IR64 due to more asymmetric root cell elongation in gravitropism
- Better under drought, equal under no drought
- DRO1 lost during modern breeding due to irrigation practices
