Drought Stress I Flashcards
Drought
The single most common cause of severe food shortages in developing countries
Dessication
- extreme water loss
- when tissue equilibrates to ψ of air (<=-100MPa)
Avoidance
- maintains constant ψ
- flowering during/after rains in deserts
- succulence
- CAM photosynthesis
- etc.
Tolerance
- equilibrate cellular and ambient ψ: ψ changes
- withstand and recover
Vegetative desiccation tolerance
- ancestral
- most bryophytes are dessication-tolerant
- lost in moss stem lineage
- c0.2% in angiosperms (Craterostigma pumilum)
- most seeds and pollen are dessication tolerant
Principles driving water movement
- high -> low ψ
- root = 0.3
- stem = liquid column, highly cohesive molecules
- air = -100
Short distance water movement
Diffusion, osmosis
Long distance water movement
- tension caused by evaporation
- bulk flow under pressure
Factors affecting water movement @ root
1) soil properties
2) architecture
3) cell wall permeability
4) plasmodesmata
5) aquaporins
Factors affecting water movement @ stem
Xylem anatomy - Tradeoff safety/efficiency
Factors affecting water movement @ leaf
1) leaf vein patterning
2) guard cell density and opening
Xylem structure affects water transport
- larger elements:
- higher conductance + water vol
- embolism
- cavitation
Why is drought a stress?
- Oxidative
- Low energy
- Osmotic
Drought oxidative stress
ROS formation
ROS formation
- membrane damage
- protein aggregation
- photosynthetic impairment
- other cellular functions
Drought as low energy stressor
CO2 limitation
Drought as osmotic stressor
- membranes and cell walls break and adhere to
- protein aggregation/denaturation
Vacuole
- most water stored here
- provides positive turgour pressure that supports tissues
- 0.5-1.5MPa
Water limitation
- < turgour pressure
- tissues wilt
Osmosensing
- 2x component regulatory system
- Arabidopsis histidine kinase I (AHKI)
- ABA
Osmosensing pathway
1) oxygen signal: AHK1 autophopshorulation
2) starts H-D-H-D photo relay activating response regulator(s)
3) drought induced ABA transcription
AHK1
- putative PM osmosensor
- OX = drought tolerance in Arabidopsis
Mechanically activated ion channels
- activated by membrane deformation
- channels turn mechanical force into ion flux
OSCA/TMEM63 family
- hyperosmolarity-gated Ca2+ channels @ PM
- mutant: lacks drought-induced early Ca2+ spikes; more sensitive to osmotic stress
OSCA OX
- increased drought tolerance in different species of Arabidopsis
MCA1/2
Additional PM localised Ca2+ channels linked to drought stress
MSLs, PIEZO, TPK1
- Ca2+ channels localised to organelles membrane
- linked to drought stress
Signal transduction
Can be ABA-(in)dependent
ABA
- basal in all cells
- increases rapidly in leaves upon osmotic stress
- synthesis in plastid + cytoplasm
- from carotenoids
NCED3
Expression induced ABA
Polymerisation-induced BGI
Deconjugates and activates ABA
Plants that cannot close their stomata
- lose turgour and wilt
- e.g. ABA- deficient tomato (notabilis)
Short term responses
- Water balance
- Protection
Water balance
- decrease water loss by stomatal closure
- increase water uptake by osmolyte production (AAs, sugars, proline)
- increased Aquaporin activity
Protection
Against protein misfolding and membrane leakage
- LEA proteins
- chaperones
- antioxidant systems
Direct regulation
Transporters, enzymes
Indirect regulation
Gene expression
No ABA signalling
- ABCG40: not activated
- no PYR/PYL activation
- PP2Cs active
- SnRK2s blocked
- guard cells remain turgid
ABA signalling
- ABCG40 R activated
- PYR/PYL activated
- PP2Cs blocked
- SnRK2s active
- NADPH oxidase activated: ROS
- ROS activate Ca2+
- extracellular ion flux means turgidity lost
- flaccid; pore closes
TF regulation
- 2 pronged approach
- PYR/PYL enters nucleus
- blocks PP2Cs
- SnRK2s phosphorylate TFs
Drought TFs
1) osmolyte production
2) aquaporins
3) antioxidants
4) growth adjustments
Drought-induced transcriptional reprogramming
Thousands of genes
cis-acting promoter elements
- ABRE motif
- DRE motif
ABRE motif
- Present in promoter of 82% dehydration-responsive genes in Arabidopsis
- bound by AREB/ABF TFs
DRE motif
- bound by DREB TFs
- ABA-independent
Osmoticum
- water-soluble, osmotic compound
- does not interfere w metabolism
- contributes to cellular water retention
Proline
- biosynthesis induced by ABA
- synthesised from glutamate in chloroplast
- transported into cytosol
- degraded in mitochondria
Proline functions
1) osmolyte: ^ cell turgour
2) stabiliser: molecular chaperone; protects protein and membrane integrity
3) ROS scavenger
4) redox regulator -> buffers cytosoluc pH to maintain state
Aquaporins
- channel-forming TM proteins
- facilitate water movement across membranes
- regulated (post)-transcriptionally
LEA proteins
- accumulate for protection of cell structures eg membrane
- intrinsically disordered
- water deficit: folded α-helix
LEA functions
1) sequester (H2O, sugar, ions, ROS)
2) molecular chaperone
Antioxidants
- ^ capacity
- scavenge ROS
Enzymatic antioxidants
SOD
APX
GPX
GST
CAT
Non-enzymatic antioxidants
AA, GSH, α-tocopherol, carotenoids, phenolic, flavonoids, proline
