Drought Stress II Flashcards
Long term responses
Water balance (source-sink relations)
Water balance
- inducing water uptake
- hydraulic accumulation
- reducing water loss
- drought escape
Induction of water uptake
1) increased primary root growth
2) root tropisms and patterning towards water
Hydraulic acclimation
- decreased xylem diameter
Decreased water loss
- increased epidermal wax production
- leaf abscission
- decreased stomatal density
- decreased leaf surface
- decreased shoot growth
Drought escape
- induction of reproductive phase transition
ABA alters xylem development
- endodermal ABA signalling inhibits metaxyken formation
- protoxylem
- miRNA165/166
Protoxylem
- smaller
- carries < water
- less embolism
- thinner cell walls
- ^ water entry - embolised vessels when drought subsides
Suberisation
Protected inner cell layers (vasculature)
Suberin
- Hydrophobic biopolymer
- insulates
- minimised water loss
ABA induces endodermal suberisation
- GPAT6 -> C16 Suberin biosynthesis
- stronger in Arabidopsis accession from drier environments
Water flow is affected by root architecture
And root architecture affects water flow
Optimal root architecture depends on
1) soil type
2) water depth
3) nutrient distribution
4) micro-organisms
Important root traits
1) primary root length(s)
2) lateral root density, elongation and angle
3) crown/adventitious roots
4) osmotropism
Drought induced root growth strategies
1) xerotropism
2) hydrotropism
3) xerobranching
4) hydropatterning
Xerotropism
- primary root growth
- deeper and moister soil
- enhanced gravitropism
- increased auxin sensitivity
Hydrotropism
- growth reorientation towards water
- ABA-mediated
Hydrotropism expts
1) ABA signalling in cortex
2) gravitropism feedback inhibition
3) ABA-mediated differential elongation in cortex ; drives “bending”
- split agar hydrotropism assay
Xerobranching
- repression of lateral root formation in air-filled soil
- ABA
Hydropatterning
- formation of lateral roots on the side w moister soil
- auxin
Reducing water loss
- decrease shoot growth, leaf surface and stomatal density
- increase epidermal wax and senescence
Enhanced water absorption
- increased root growth
- requires resource allocation
If drought stimulus is severe enough,
All resources are allocated to meristems for preservation (growth arrest)
Drought escape
- accelerate reproductive phase
- early flowering
- ABA-mediated photoperiodic floral induction
No stress flowering
1) inductive photoperiod
2) circadian clock + light
3) kinase + TF regulation
4) Florigen accumulation
5) apex = flowering
Drought escape mechanism
Non-inductive photoperiod hijacked by ABA
How to help agriculture combat drought?
- technology
- biology
Monitoring methods
- inform management practices
- efficient water utilisation
Soil monitoring + improved irrigation practices
- Soil humidity sensors
- Drip irrigation
- Partial root-zone drying
Partial root-zone drying
- plants are watered on alternating sides; the each half of the root system alternates: wet/dry
- often results in equivalent harvests
- smaller leaf sizes
Partial root-zone drying hypothesis
- wet roots: hydrate shoot
- dry roots: partial stomatal closure; water conservation
Plant monitoring
1) thermal imaging: drones/satellites (aerial)
2) generate drought maps
3) precision farming
How does plant monitoring work?
1) drought
2) stomatal closure
3) decreased transpiration
4) increased leaf surface temperature detected in IR
5) irrigate that patch!
Photosynthesis-related measurements
- GE
- chlorophyll fluorescence
- inform plant water status; stress responses
Using biology to combat drought
1) osmotic adjustment
2) stomata conductance regulation
3) water use efficiency ^
4) photosynthetic rate ^
5) delayed senescence
6) root architecture
7) reproductive phase regulation
Strategies for using biology to combat drought:
1) chemical application: osmoprotectants, growth effectors
2) induce drought memory: seed priming
3) generate drought-tolerant varieties: molecular breeding and genetic engineering
Stress priming
In seeds, seedlings, plants
Osmopriming
- osmolytes + antioxidant accumulation
- plant-water relations
- root architecture depends
Osmopriming had improved drought tolerance in
1) wheat
2) barley
3) sorghum
4) caraway
Genetic strategies
- identify relevant genes, then manipulate them
Osmotic adjustments
- mannitol
- proline
- glycine betaine
Protective proteins
- chaperones
- dehydrins
Transport proteins
- vascular Na+/K+ antiporter
- aquaporins
Forward genetics
- phenotype -> gene
- mutant/natural variant carries particular phenotype
- identify gene: mapping, sequencing, validation
- (elucidate corresponding protein function)
Reverse genetics
- gene -> phenotype
- specific gene of unknown function
- phenotype analysis on corresponding mutations shy
- elucidation of corresponding protein function
Process of identifying relevant gene by reverse genetics
- Gene X = drought-induced, highly conserved across species
- Hypothesis: Gene X contributes to drought tolerance
- Inactivate + OX
- Compare performance to confirm hypotheses
Considerations when identifying relevant genes
- does it work similarly in crop of interest?
- field conditions?
- GMO?
- drawbacks?
CspA/B maize
- bacterial RNA chaperone
- increased yields in maize under late-season drought
- no growth penalty under normal conditions
Maize GE
- bacterial cold shock proteins
- decreased senescence, increased photosynthesis and chlorophyll
Soybean GE
- Hahb4
- decreased senescence
- increased photosynthesis
Hahb4
stress-inducible (via ethylene sensitivity) HD-ZIP TF
T6P
- impacts sink strength
- ability to import, assimilate and use
- insulin/like
How does T6P work: reaction sequence
hexose-P -TPS-> T6P -TPP-> trehalose
How does T6P work: consequences
- activated by sucrose
- activated biosynthesis, respiration, growth
- blocks photosynthesis, starch and other C source anabolism
OX TPP in developing ears
- low T6P: “starvation”
- sucrose import
- increase yield
- in control and drought conditions
Breeding approaches
- wild populations have greater genetic diversity
- many ancestral traits are not represented in modern varieties
- can still be found in germplasm collections
Conventional breeding
- v. long, tedious
- phenotype-based
- genotype changes unknown
- requires germplasm diversity
How does conventional breeding work ?
- breed high yielders + drought-resistant plants
- search for progeny combining both parental traits
Molecular breeding
- genotype based, via molecular markers
- QTL linked to trait(s)
- mapping and GWAS
- no GMO; minimal genome change
- no need to phenotype; locus identifies trait
- proceed only w relevant plants: highest % of relevant loci
Gene editing
- if you know causal gene/mutation
- highly precise, flexible, fast
Examples of gene editing for drought
- maize ARG058
- tomato SINPRI
- Arabidopsis AREB1
- rice OSSAPK2
How does gene editing work
1) DNA modification within plant cells
2) minimal genome changes and selection time
3) inducible, cell-type specific modifications minimise trade offs
