L15 - Physiology and Molecular Biology of Excess Water Stress Flashcards
Define water-logging and submergence
Water-logging: Height of water covers only the roots
Submergence: Water partially or completely covers the aerial plant tissues
Why is developing flood resistant crops important for food supply?
- 9% of overall hazard impact faced by crop + animal farming = flooding
- Flooding reduces seed stock loss, destroys plants
- IPCC predict frequency and intensity of precipitation to increase
Give the major biological problems of excess water
- Diffusion rates of gases greatly reduced
Reduced CO2 Level:
- Restricts photosynthesis
- Cellular damage due to ROS
Reduced O2 level (Hypoxia)
- ATP not efficiently produced
- Aerobic respiration inhibited
- Low energy for e.g. DNA synthesis, cell division etc…
Give the three categories in which a plant’s flooding adaptive strategies can be placed
1) Flooding avoidance:
- Avoid floods by altered life cycle
2) Flooding Avoidance
- Maintain optimal oxygen level in hypoxia condition
3) Flooding tolerance
- Limit damage despite non-optimal conditions
Describe the flooding avoidance strategy.
Include an example.
- Complete life cycle before flooding
- OR vegetative growth between floods, seeds produced post flooding e.g. Chenopodium rubrum
Describe 3 different examples of flooding avoidance through root development
Root development:
Aerenchyma formation (permanent or induced):
- Intercellular lacunae network helps O2 diffuse from non-submerged section to submerged roots
- Provides air reservoir
Radial O2 Loss (ROL) Barrier:
- Suberin deposited between epidermis and hypodermis prevents O2 loss in root apex
Adventitious Roots:
- Roots developed from hypocotyl or basal stem to gain O2 access
Describe 3 different examples of flooding avoidance through shoot development
Aerenchyma formed in shoots (detailed function in roots section)
Hydrophobic gas films deposited on leaf surface
- Creates air space for gases to diffuse inward
Stem + leaf elongation
- Hyponastic response to reach O2 above rising water
Give 3 mechanisms used in flooding tolerance
1) Glycolysis for ATP
-Glycolysis used when oxidative phosphorylation not possible
- Less efficient = more carbohydrates
2) Fermentation for NAD+ regeneration
- NAD+ e- carrier consumed under glycolysis, not recycled as in O phosphorylation = NAD+ shortage
- Cells regenerate NAD+ from fermentation
- Pyruvate converted into ethanol or lactic acid
- Relevant enzymes ADH and LDH induced during hypoxia
3) Antioxidant mechanism to prevent ROS damage - detailed in L13 and L14
Describe how plants sense excess water
Plants sense O2 and Ethylene levels
O2 perception:
Normoxia:
- Plant Cysteine Oxidases (PCO) take O2 to Ethylene response factor group VII (ERF-VII) causing arginylation for protein degradation via Arg-N degron pathway
Hypoxia:
- PCO cannot modify ERF-VII TFs
- TFs activate gene expression for hypoxia acclimation
Ethylene levels:
- Outwards diffusion of ethylene limited during flooding causing accumulation
- Causes downstream response
- Increases Gibberellin (GA) hormone synthesis, further regulating response
Describe how ERF-VII TFs cause transcriptional activation
- ERF-VII TFs bind to conserved nucleotides, hypoxia responsive promoter element (HRPE), in promoter region
- Downstream genes for hypoxia response activated
Give an example of a plant that uses the “Get Some Air” (Escape) strategy
Outline the mechanisms behind this response
When is this response useful?
South Asian Deep Water Rice
- During seasonal rainfall, deep water rice rapidly elongate stems + leaves to escape submersion
1) Two ERF-VII TFs, SNORKEL1 and SNORKEL2 (SK1/2) cause stem elongation (unknown mechanism)
2) SEMIDWARF1 (SD1) gene for GA biosynthesis triggered by ethylene
- Stronger expression of deep water rice’s SD1 from promoter + intron variations
- Enzyme produces more potent GA version, GA4, which binds a GA receptor more strongly. Exon variation (coding redion) causes this
- Useful when water level expected to increase steadily for extended period
Give an example of a plant that uses the “Hold your breathe” (Quiescence) strategy
Outline the mechanisms behind this response
When is this response useful?
Submergence 1 (SUB1) gene from Indian Dhalputtia
- Poor yield but endures submergence for max. 14 days
- SUB1 introduced to elite Swarna rice in India, Nepal, Bangladesh with no yield penalty + improved flood tolerance
1) SUB1 made from 3 ERF-VIIs.
- SUB1-A strongly induced by ethylene
- SUB1-A increases gene expression + protein stability of GA transcription suppressor, stopping stem growth
- Growth stopped till flood subsides, saving energy
2) Sub1s confer the following:
- Reduced carbohydrate consumption
- Preventing chlorophyll breakdown
- Activating fermentation
- Useful for short-term flash floods
