Plant stresses and responses

1. Plant stresses and responses De Block et al., Plant J. 41:95 (2005) 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 www.grainscanada.gc.ca • 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! http://www.geo.arizona.edu/gallery/US/tuc_2.html

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 www.agry.purdue.edu; www.omafra.gov.on.ca; 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 Topt Growth rate Growth temperature

6. Suboptimal growth Ts result in suboptimal plant development Transition temperature 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 Membrane fluidity affects permeability! http://cropsoil.psu.edu/Courses/AGRO518/CHILLING.htm • 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 Defenseless Wild type 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 • 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 inducespathogenesis 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