Plants must cope with stresses in place as they are sessile organisms. The document discusses various biotic and abiotic stresses plants encounter and their responses. It explains that plants have developed mechanisms of avoidance, tolerance and resistance to deal with stresses. The stress responses are important for ecology, agriculture and understanding plant physiology. The hypersensitive response is a key defense strategy against pathogen invasion and results in systemic acquired resistance throughout the plant.

1. Plant stresses and responses
De Block et al.,
Plant J. 41:95
(2005)

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
Heat-stressed wheat
• 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

3. 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 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
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
Heat stressed
rose 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
taproot
www.agry.purdue.edu;
www.omafra.gov.on.ca;

11. Most organisms are adapted to environmental
temperature:
1. Psychrophiles (< 20 °C)
2. Mesophiles (~ 20-35 °C)
3. Thermophiles ( ~35-70 °)
4. Hyperthermophiles (70-110 °C)
Groups 1,3 & 4 are a.k.a. “Extremophiles”
But can also acclimate to “extreme” shifts, if they
are not permanent, and not too extreme.
Two well studied acclimation responses are:
1. the Heat Shock response
2. Cold acclimation

18. Heat Stress (or Heat Shock) Response
• Induced by temperatures ~10-15o
C above normal
• Ubiquitous (conserved), rapid & transient
• Dramatic change in pattern of protein synthesis
– induction (increase) of HSPs
– most HSPs are chaperones (chaperonins) that
promote protein re-folding & stability
• HSP induction mediated by a bZIP factor, HSF
Fig. 22.43, Buchanan et al.

19. 28o
C 40o
C  45o
C 45o
C
Fig. 22.42, Buchanan et al.
Soybean seedlings.
Thermotolerant growth of soybean seedlings following a
heat shock.

20. Heat stress effects on protein synthesis in soybean
seedlings (J. Key).
Joe Key

29. Cold Acclimation (CA) involves:
• Increased accumulation of small solutes
– retain water & stabilize proteins
– e.g., proline, glycine betaine, trehalose
• Altered membrane lipids, to lower gelling temp.
• Changes in gene expression [e.g., antifreeze proteins,
proteases, RNA-binding proteins (?)]
• Many cold-regulated promoters have DRE/C-elements
• Activated by CBF1
transcription factor

30. Role of ABA (stress hormone)
• ABA – Abscisic acid, phytohormone
induced by wilting, closes stomata
by acting on guard cells
• Positive correlation between CA and
[ABA]
• Treat plants with ABA, and they will
be somewhat cold hardened
However, ABA does not induce all genes that cold will.
Conclusion: there are ABA-regulated and non-ABA regulated
changes that are induced by cold.

31. Plants vary in ability to tolerate flooding
Plants can be classified as:
• Wetland plants (e.g., rice, mangroves)
• Flood-tolerant (e.g., Arabidopsis,
maize)
• Flood-sensitive (e.g., soybeans,
tomato)
Involves developmental/structural, cellular
and molecular adaptations.
Pneumatophores in mangrove

32. Flooding causes anoxia and an
anaerobiotic response in roots.
Maize (corn)
- Shift carbohydrate metabolism
from respiration to anaerobic
glycolysis
- Protein synthesis affected:
results in selective synthesis of
~10-20 proteins
-mRNAs for other proteins there
but not translated well!

35. Enzymes that are
up-regulated by
anaerobiosis

36. • 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

37. 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

38. Biotic Stress and Plant Defense
Responses
Pathogen Strategies
1. Necrotrophic – plant tissue killed and then colonized;
broad host range
e.g., rotting bacteria (Erwinia)
2. Biotrophic – plant cells remain alive, narrow host range
(1 plant species)
e.g., viruses, nematodes, fungal mildews

39. Plant Defenses
1) Physical barriers: cuticle, thorns, cell
walls
2) Constitutively produced chemicals (e.g.,
phytoalexins) and proteins (e.g., Ricin)
3) Induced responses (a.k.a., the Plant
Defense Response)

40. The Plant Defense Response
3 aspects of response:
1. Hypersensitive
2. Local
3. Systemic
Compatible interaction  disease
Incompatible interaction 
resistance

41. 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

42. 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

43. 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

44. 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

45. Name - Mohd Tahir Awan
M.Sc - Botany
Jaipur National University , Jaipur
Email : meettahirmtajs@gmail.com
Phone : 09982899978 , 09596951795
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