Plant stresses and responsesPlant Physiol Biotechnol 3470

March 21, 2006

Chp 21

De Block et al., Plant J. 41:95 (2005)

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

Heat-stressed wheat

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

Stresses are abiotic or biotic

• 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!

ABIOTIC STRESSESEnvironmental, non-

biological• Temperature (high /

low)• Water (high / low)• Salt• Radiation• Chemical

BIOTIC STRESSESCaused by living

organisms• Fungi• Bacteria• Insects• Herbivores• Other

plants/competition

http://www.geo.arizona.edu/gallery/US/tuc_2.html

Fig. 21.1

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 plantAlfalfa plant

Heat stressedrose leaf

– 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 taprootAlfalfa taproot

www.agry.purdue.edu;www.omafra.gov.on.ca;

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 temperature

Gro

wth

rat

e

Topt

Suboptimal growth Ts result in suboptimal plant developmentChilling 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!

• 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

http://cropsoil.psu.edu/Courses/AGRO518/CHILLING.htm

Transition temperature

Biotic stresses are mitigated by plants’ elaborate defense strategies

– 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??

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

Buchanan et al., “Biochemistry & molecular biology of plants,” 2001

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

The hypersensitive reaction initiates a plant immune response

Fig 21.17

• 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

Salicylic acid induces systemic acquired resistance Fig 21.18

• 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

volatilized• Local SA production induces

distal production and SAR via– SA transport in xylem– Methylation into MSA,

volatilization and distal detection