Drought Stress I Flashcards
1
Q
Drought
A
The single most common cause of severe food shortages in developing countries
2
Q
Dessication
A
- extreme water loss
- when tissue equilibrates to ψ of air (<=-100MPa)
3
Q
Avoidance
A
- maintains constant ψ
- flowering during/after rains in deserts
- succulence
- CAM photosynthesis
- etc.
4
Q
Tolerance
A
- equilibrate cellular and ambient ψ: ψ changes
- withstand and recover
5
Q
Vegetative desiccation tolerance
A
- ancestral
- most bryophytes are dessication-tolerant
- lost in moss stem lineage
- c0.2% in angiosperms (Craterostigma pumilum)
- most seeds and pollen are dessication tolerant
6
Q
Principles driving water movement
A
- high -> low ψ
- root = 0.3
- stem = liquid column, highly cohesive molecules
- air = -100
7
Q
Short distance water movement
A
Diffusion, osmosis
8
Q
Long distance water movement
A
- tension caused by evaporation
- bulk flow under pressure
9
Q
Factors affecting water movement @ root
A
1) soil properties
2) architecture
3) cell wall permeability
4) plasmodesmata
5) aquaporins
10
Q
Factors affecting water movement @ stem
A
Xylem anatomy - Tradeoff safety/efficiency
11
Q
Factors affecting water movement @ leaf
A
1) leaf vein patterning
2) guard cell density and opening
12
Q
Xylem structure affects water transport
A
- larger elements:
- higher conductance + water vol
- embolism
- cavitation
13
Q
Why is drought a stress?
A
- Oxidative
- Low energy
- Osmotic
14
Q
Drought oxidative stress
A
ROS formation
15
Q
ROS formation
A
- membrane damage
- protein aggregation
- photosynthetic impairment
- other cellular functions
16
Q
Drought as low energy stressor
A
CO2 limitation
17
Q
Drought as osmotic stressor
A
- membranes and cell walls break and adhere to
- protein aggregation/denaturation
18
Q
Vacuole
A
- most water stored here
- provides positive turgour pressure that supports tissues
- 0.5-1.5MPa
19
Q
Water limitation
A
- < turgour pressure
- tissues wilt
20
Q
Osmosensing
A
- 2x component regulatory system
- Arabidopsis histidine kinase I (AHKI)
- ABA
21
Q
Osmosensing pathway
A
1) oxygen signal: AHK1 autophopshorulation
2) starts H-D-H-D photo relay activating response regulator(s)
3) drought induced ABA transcription
22
Q
AHK1
A
- putative PM osmosensor
- OX = drought tolerance in Arabidopsis
23
Q
Mechanically activated ion channels
A
- activated by membrane deformation
- channels turn mechanical force into ion flux
24
Q
OSCA/TMEM63 family
A
- hyperosmolarity-gated Ca2+ channels @ PM
- mutant: lacks drought-induced early Ca2+ spikes; more sensitive to osmotic stress
25
OSCA OX
- increased drought tolerance in different species of Arabidopsis
26
MCA1/2
Additional PM localised Ca2+ channels linked to drought stress
27
MSLs, PIEZO, TPK1
- Ca2+ channels localised to organelles membrane
- linked to drought stress
28
Signal transduction
Can be ABA-(in)dependent
29
ABA
- basal in all cells
- increases rapidly in leaves upon osmotic stress
- synthesis in plastid + cytoplasm
- from carotenoids
30
NCED3
Expression induced ABA
31
Polymerisation-induced BGI
Deconjugates and activates ABA
32
Plants that cannot close their stomata
- lose turgour and wilt
- e.g. ABA- deficient tomato (notabilis)
33
Short term responses
1. Water balance
2. Protection
34
Water balance
- decrease water loss by stomatal closure
- increase water uptake by osmolyte production (AAs, sugars, proline)
- increased Aquaporin activity
35
Protection
Against protein misfolding and membrane leakage
- LEA proteins
- chaperones
- antioxidant systems
36
Direct regulation
Transporters, enzymes
37
Indirect regulation
Gene expression
38
No ABA signalling
- ABCG40: not activated
- no PYR/PYL activation
- PP2Cs active
- SnRK2s blocked
- guard cells remain turgid
39
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
40
TF regulation
- 2 pronged approach
- PYR/PYL enters nucleus
- blocks PP2Cs
- SnRK2s phosphorylate TFs
41
Drought TFs
1) osmolyte production
2) aquaporins
3) antioxidants
4) growth adjustments
42
Drought-induced transcriptional reprogramming
Thousands of genes
43
cis-acting promoter elements
- ABRE motif
- DRE motif
44
ABRE motif
- Present in promoter of 82% dehydration-responsive genes in Arabidopsis
- bound by AREB/ABF TFs
45
DRE motif
- bound by DREB TFs
- ABA-independent
46
Osmoticum
- water-soluble, osmotic compound
- does not interfere w metabolism
- contributes to cellular water retention
47
Proline
- biosynthesis induced by ABA
- synthesised from glutamate in chloroplast
- transported into cytosol
- degraded in mitochondria
48
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
49
Aquaporins
- channel-forming TM proteins
- facilitate water movement across membranes
- regulated (post)-transcriptionally
50
LEA proteins
- accumulate for protection of cell structures eg membrane
- intrinsically disordered
- water deficit: folded α-helix
51
LEA functions
1) sequester (H2O, sugar, ions, ROS)
2) molecular chaperone
52
Antioxidants
- ^ capacity
- scavenge ROS
53
Enzymatic antioxidants
SOD
APX
GPX
GST
CAT
54
Non-enzymatic antioxidants
AA, GSH, α-tocopherol, carotenoids, phenolic, flavonoids, proline
