1. Plant stresses and responses
De Block et al., Plant J. 41:95 (2005)
Plant stresses and responses
Plant Physiol Biotechnol 3470
March 21, 2006
Chp 21

2. Plants are sessile and must deal with stresses in place
Plants cannot avoid stress after germination
How plants deal with stress has implications in
Ecology: Stress responses help explain geographic distribution of species
Crop science: Stress affects productivity
Physiology and biochemistry: Stress affects the metabolism of plants and results in changes in gene expression
From engineering, stresses cause strains (responses of stressed objects) = changes in gene expression and metabolism in plants
Biological stress difficult to define/quantify:
What is “normal” metabolism?
Limitations to yield?
Where on gradient of availability of limiting resource does stress begin?
Varies by species, ecotype
Heat-stressed wheat

3. Stresses are abiotic or biotic
Fig. 21.1
ABIOTIC STRESSES
Environmental, non-biological
Temperature (high / low)
Water (high / low)
Salt
Radiation
Chemical
BIOTIC STRESSES
Caused by living organisms
Fungi
Bacteria
Insects
Herbivores
Other plants/competition
Stresses cause responses in metabolism and development
Injuries occur in susceptible plants, can lead to impeding flowering, death
Ephemeral plants avoid stress
Mexican poppies in US desert SW
Only bloom after wet winter
Die before summer returns
Preferable!

4. Plants must be stress resistant to survive
Avoidance also possible by morphological adaptations
Deep tap roots in alfalfa allow growth in arid conditions
Desert CAM plants store H2O in fleshy photosynthetic stems
Stress resistant plants can tolerate a particular stress
Resurrection plants (ferns) can tolerate dessication of protoplasm to <7% H2O  can rehydrate dried leaves
Plants may become stress tolerant through
Alfalfa plant
Adaptation: heritable modifications to increase fitness
CAM plants’ morphological and physiological adaptations to low H2O environment
Acclimation: nonheritable physiological and biochemical gene expression
Cold hardening induced by gradual exposure to chilling temps, a/k/a cold-hardy plants
Alfalfa taproot
Heat stressed
rose leaf

5. Let’s walk through one each of important abiotic and biotic stresses
View how they affect metabolism
Determine how the plant responds to counter the stress
ABIOTIC STRESS: Temperature
Plants exhibit a wide range of Topt (optimum temperature) for growth
We know this is because their enzymes have evolved for optimum activity at a particular T
Properly acclimated plants can survive overwintering at extremely low Ts
Environmental conditions frequently oscillate outside ideal T range
Deserts and high altitudes: hot days, cold nights
Three types of temperature stress affect plant growth
Chilling, freezing, heat
Growth rate
Topt
Growth temperature

6. Suboptimal growth Ts result in suboptimal plant development
Chilling injury
Common in plants native to warm habitats
Peas, beans, maize, Solanaceae
Affects
seedling growth and reproduction
multiple metabolic pathways and physiological processes
Cytoplasmic streaming
Reduced respiration, photosynthesis, protein synthesis
Patterns of protein expression
Transition
temperature
Membrane fluidity affects permeability!
Initial metabolic change precipitating metabolic shifts thought to be alteration of physical state of cellular membranes
Temperature changes lipid and thus membrane properties
Chilling sensitive plants have more saturated FAs in membranes: these congeal at low temperature (like butter!)
Liquid crystalline  gel transition occurs abruptly at transition temperature

7. Biotic stresses are mitigated by plants’ elaborate defense strategies
Buchanan et al., “Biochemistry & molecular biology of plants,” 2001
BIOTIC STRESS: Pathogen (e.g., fungus) invasion
Plant reaction to invading pathogens center around the hypersensitive reaction
The hypersensitive reaction initiates many changes in plant physiology and biochemistry
Defenseless
Wild type
Early activation of defense related genes to synthesize pathogenesis related (PR) proteins
Protease inhibitors to stop cell wall lysis by specific enzymes expressed by pathogen
Bacterial cell wall lytic enzymes (chitinase, glucanase)
Change cell wall composition
Express enzymes providing structual support to cell walls via synthesis of lignin, suberin, callose, glycoproteins
Synthesize secondary metabolites to isolate and limit the pathogen spread
These include isoflavonoids, phytoalexins
Apoptosis at invasion site to physically cut off rest of plant
Sequential or parallel events??

8. How does the plant recognize and defend itself against pathogens?
Plant disease has an underlying genetic basis
Pathogens may be more or less potentially infectious to a host
virulent on susceptible hosts
avirulent on non-susceptible hosts
Pathogens carry avirulence (avr) genes and hosts carry resistance (R) genes
The normal presence of both prevents pathogens from attacking the plant
Infection occurs when pathogen lacks avr genes or plant is homozygous recessive for resistance genes (rr)
In these cases, the plant cannot initiate the hypersensitive reaction
This is bad news!
The plant requires this response to survive!
Note the communication between pathogen and plant
Pathogen: avr genes may code for proteins that produce elicitors
bits of pathogen: polysaccharides, chitin, or bits of damaged plant: cell wall polysaccharides
Plant: R genes may be elicitor receptors

9. The hypersensitive reaction initiates a plant immune response
The long term plant resistance to a pathogen is similar to a mammalian immune response
This is known as systemic acquired resistance (SAR)
Secondary metabolites induced by the hypersensitive reaction initiate changes in metabolism in other plant organs through control of signal transduction chains
Hours to days: capacity to resist pathogens spreads throughout plant
Immune capacity = SAR
SAR signaling involves salicylic acid (SA), a natural secondary metabolite
SA both induces pathogenesis related gene expression and enhances resistance to infection by plant viruses
Fig 21.17

10. Salicylic acid induces systemic acquired resistance
Fig 21.18
volatilized
Local SA production induces distal production and SAR via
SA transport in xylem
Methylation into MSA, volatilization and distal detection
High constitutive SA levels result in plants with high ability to withstand pathogens
Mechanism by which SA induces SAR unknown
Jasmonic acid also mediates disease and insect resistance
JA also mediates other developmental responses: PGR?
All stress affects photosynthesis: productivity and survival
Knowledge of how stress is perceived and transduced central to understanding plant metabolism