Plant Stress Responses Flashcards
Stress
- suboptimal environmental condition that adversely affects plant growth & development
- normal part of life; varies in diversity and severity
- reduces yields >50% (potential maximum vs yield losses)
Abiotic:
- water: drought, flooding
- temperature: heat, chilling, freezing
- light: high, shading, UV
- nutrient: deficiency, excess (micro and macro)
- salinity
- heavy metal toxicity + xenobiotics: pesticides, herbicides, air pollutants
- mechanical: touch, gravity, pressure, wounding
Biotic:
- pathogens: viruses, viroids, bacteria, fungi, oomycetes, nematodes, phytopalms
- pests: herbivores, insects, mites etc.
- plants: parasites, interspecific (weeds), intraspecific
Acclimation
- individual level
- caused by local stress acting on genetically determined physiological responses
- non-heritable (except epigenetics)
- elastic (reversible)
- timescale: short term (mins-> hours), long term (days-> months)
Adaptation
- population level
- stress-driven natural selection acting on allelic variation
- heritable
- plastic (irreversible)
- multiple generations (evolutionary)
Stress response:
1) time of exposure
2) intensity
3) plant genotype
4) physiological condition
5) developmental stage/organ
Types of stress response curves
- essential + non-essential environmental factors
Essential environmental factors
- optimal range
- sub- (deficiency), supra- (excess)
Non-essential environmental factors
- immediate adverse effect (e.g. some Biotic stresses, wounding)
- tolerance (low intensity; e.g. some biotic stress, salinity, xenobiotics)
Strategies of stress responses
Susceptibility -> resistance;
- tolerance: endure
- avoidance; reduce exposure
- both lead to survival
- if unable to recognise/respond: death
Drought tolerance
Selaginella lepidophylla “resurrection plants” lose 90% cellular water
Freezing tolerance
Antifreeze protein accumulation to control ice crystal growth
Drought avoidance
- Ursinia in Namaqualand, S. Africa
- germination in wet season/after rainfall
Freezing avoidance
Senecio keniodendron (giant groundsel) folds leaves to insulate meristem
Phases of stress acclimation
- Homeostasis = no stress
- Alarm
- Acclimation/ acute damage
- Maintenance / exhaustion
- Recovery / system failure
Stress priming
- short and long term memory
- trans-generational under debate
Stress priming depends on
1) physical marks on chromatin (DNA methylation, histone modifications)
2) diffusible signal conc (hormone, TFs)
- avoids costs associated w constitutive gene expression
Memory genes
Higher expression during stress
Stress responses can
- involve extra/intracellular single d
- be specific/general
Process of stress response
1) immediate (e.g. stomata closure)
2) gene expression, metabolism
3) physiology
4) growth, development
5) acclimation
General responses
- common signals to different stresses
- ROS, low energy, Ca2+
- MAPKs, energy-sensing SnRK1-TOR, Ca2+-dependent kinases
General protection
- tissue repair + removal of damaged components
- removal/compartmentalisation of unnecessary factors
- ROS detoxification
- resource redistribution
Specific responses
- to particular stress types
- signals + components
Specific protection
E.g.
- antifreeze accumulation
- heavy metal compartmentalisation
- defence compounds
ROS
- damage proteins, lipids, DNA and cellular dysfunction
- e.g. prevent stomata closure during drought
- produced in several cellular compartments
- dedicated enzyme systems to neutralise
- “intentionally” produced in the apoplast
ROS production
- < CO2
- < C assimilation rates
- < NADPH recycling
- No NADP for e’s
- E’s + excited chlorophyll react w O2
Enzymatic antioxidants
- SOD
- APX (ascorbate peroxidase)
- GPX (guaiacol peroxidase)
- GST (glutathione-S-transferase)
- CAT
Apoplast
- stress triggers ^ in chronic Ca2+
- different stresses activate different channels: Calcium signatures; amplitude, oscillations, duration
- Ca2+ in external envrt
Ca2+ signals
- calmodulin (CaM)
- CaM-like proteins (CMLs)
- Ca2+-dependent protein kinases (CDPKs)
- calcineurin B-like proteins (CBLs) + associated CIPK kinases
Ca2+ signals affect
- metabolism
- transcription
- transport
- et al
Long-distance signalling via ROS/Ca2+ waves:
- signals propagate from stressed cells -> entire plant within ss, mins
- via: plasmodesmata, apoplast, vasculature
Stress signals
- ROS/Ca2+ waves
- electric currents
- hydraulic signals
- pH
- eATP
- phytohormones
- hormone-like peptides
- miRNAs
Systemic ROS/Ca2+ wave propagation and
1) ROS-induced calcium release (RICR)
2) Calcium-induced calcium release (CICR)
RICR
- apoplastic ROS activate PM calcium channels
- ^ cytosolic Ca2+ conc
- Ca2+ -> plasmodesmata
CICR
- membrane component activation (extracellular ROS production)
- initiates same cascades
- neighbouring cell communication; self-propagating
ROS/Ca2+ waves
- alert cells to impending stress
- triggered by different abiotic stresses/ pathogen infection / wounding
- local signal -> whole plant
- co-signals convey specificity
ROS/Ca2+ Co-signals
- pH
- membrane potential
- metabolites
- hormones
- eATP
Systemic ROS
2’-7’-dichlorodihydrofluorescin diacetate
Ca2+
Flourescence
Different stresses lower energy production
- shading, pollution, drought, salinity, heat, cold, flooding
- SnRK1: crucial for stress tolerance
SnRK1, SNF1, AMPK
- ^ energy production (catabolism, Autophagy)
- < energy consumption (anabolism, translation/growth)
Low sucrose sugar status (stress!)
- SnRK1 inhibits TOR kinase (growth agonist)
High sucrose sugar status (optimal)
- sucrose inhibits SnRK1
Plants have to balance
- growth (anabolism, developmental progression)
- survival (catabolism, nutrient mobilisation, stress responses)
Tradeoff:
^SR = <G
Energy sensing pathways
1) direct target phosphorylation (e.g. enzymes)
2) transcriptional regulation
3) translational regulation
4) autophagy
TOR
- induced translation initiation S6 kinase
- induced rRNA transcription (ribosome biogenesis)
- suppresses autophagy (ATG13 inactivation)
SnRK1
- transcriptional reprogramming
- bZIP, MYC2 TF phosphorylation
- represses ribosomal proteins
- aa degradation, gluconeogensis
- induces autophagy (ATG1, ATG6)
- represses enzymes
- isoprenoid biosynthesis, nitrate assimilation, starch synthesis
Hormonal component of stress:
Abiotic: ABA
Biotic: SA, JA, ET
Stress combination
- synergistic
- neutral
- antagonistic effects
- abiotic stresses activate different can influence biotic outcome
Drought in Solanum dulcamara
- Increased R to insect herbivore
- flooding had no effect
Drought in A. Thaliana
- decreased resistance to Pseudomonas syringae PV tomato DC3000
