Stress Responses

1
Stress Responses Gene Expression

• plants must adapt to stresses because of their
  sedentary lifestyle

Fig. 22.2, Buchanan et al.
2
Adaptation and Acclimation

• Adaptation - evolutionary changes that enable an
  organism to exploit a certain niche. These
  include modification of existing genes, as well
  as gain/loss of genes.
• e.g., thermophilic enzymes in organisms that
  tolerate high temperature
• Acclimation inducible response(s) that allows
  an organism to tolerate an unfavorable or lethal
  (to some plants) change in the environment.
• e.g., heat shock response

3
Types of Stress

• Abiotic
• 1. heat
• 2. cold
• 3. drought
• 4. salt
• 5. wind
• 6. oxidative
• 7. anaerobic
• 8. heavy metals
• 9. wounding
• 10. nutrient deprivation
• 11. excessive light
• Biotic
• 12. pathogens
• 13. herbivores
• Focus on heat, cold, anaerobiosis, oxidative,
  and biotic stress

4
Plants respond to stresses as individual cells
and as whole organisms. Stress-induced signals
can be transmitted throughout plant, making other
parts more ready to withstand stress.
Fig. 22.3, Buchanan et al.
5
Heat Stress (or Heat Shock) Response

• Discovered in Drosophila, polytene chromosome
  puffing
• Specific response to temperatures 10-15oC above
  normal growth temp.
• Ubiquitous
• Conserved
• Rapid
• Transient
• Dramatic change in pattern of protein synthesis

6
Heat stress effects on protein synthesis in
soybean seedlings (J. Key).
7
Heat stress/shock protein synthesis in the
cyanobacterium Synechococcus.
8
Generalized order of events in heat shock response

• Initial events
• Inhibition of protein synthesis
• Inhibition of transcription RNA processing
• Induction of new hs (hsp) mRNAs
• Pre-existing cellular mRNAs still present but
  not translated (initiation factor eIF4A B
  deactivated)

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Phase II events

• Partial restoration of protein synthesis, mainly
  translation of hsp mRNAs
• Accumulation of hsp (heat shock proteins)
• Gradual resumption of normal cellular protein
  synthesis
• Decline in hsp synthesis

10
Function Thermotolerance

• Enables organisms to survive high temp.
• A sub-lethal heat shock allows organisms to
  survive a lethal treatment.
• Production of hsps correlate with acquired
  thermotolerance.
• Some mutants (yeast) and transgenic plants with
  altered expression of certain hsps dont show
  thermotolerance.

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28oC 40oC ? 45oC 45oC
Soybean seedlings.
Fig. 22.42, Buchanan et al.
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Heat Shock Proteins (hsp)

• 100, 90, 70, and 60 kDa
• Low molecular weight (LMW) hsp 27, 20-22,
  15-18 kDa
• all induced within 30 min.
• more LMW hsp in plants
• 2-Dimensional gel electrophoresis and molecular
  cloning indicates most hsps are families of
  related proteins, particularly hsp70 and the LMW
  hsps

13
HSP functions

• 1 LMW hsp is ubiquitin, which tags proteins to
  be degraded by the proteasome
• hsp90, hsp70, and hsp60 involved in protein
  folding "molecular chaperones"
• hsp100 may promote translation of hsp mRNAs, via
  CAP-independent mechanism

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HSP70, a chaperonin

• Essential gene
• Homologues found in cytoplasm, ER lumen,
  mitochondria, and chloroplasts
• function in protein targeting and assembly in
  normal (non-stressed) cells, hydrolyze ATP
• Constitutive heat-induced (cytoplasmic) forms
• the heat-induced form first appears in the
  nucleolus, then goes to cytoplasm (may protect
  pre-ribosomes from heat stress?)
• Also, some hsp70s are light-induced chloroplast
  hsp70 helps protect PSII from light/heat damage
  in Chlamy

15
(No Transcript)
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HSP60 (cpn60)

• first one termed "molecular chaperone
• Discovered as the RuBPCase LS binding protein
  that participates in RuBPCase assembly
• in eucaryotes, only in mitochondria and plastids
  
• similar to E. coli GroEL gene
• exists as abundant 720 kD complex with two
  subunits of 61 and 60 kDa (ATPase)
• associates with cpn10 (GroES homologue)
• facilitates folding/ assembly of other proteins

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LMW HSPs

• highly heat-induced
• 4 nuclear gene families
• Class I cytoplasmic
• Class II cytoplasmic
• Chloroplast localized
• Endomembrane localized (ER)
• found in organelles only in plants
• function mostly unknown
• aggregate in vivo into "heat shock granules"

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HSP regulation

• most work on LMW hsp in plants
• induction is mainly transcriptional but also
  translational control (hsp mRNAs preferentially
  translated)
• genes induced coordinately, but not equally in
  all tissues
• light can also induce some LMW hsps

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Soybean Hsp mRNAs that are strongly induced.
Temp. oC
20
Cis-acting transcriptional regulatory elements

• HSE (heat shock elements) in the 5' regions
• 10-15 bp partial palindromes
• multiple copies required
• also found in other HS genes (e.g., hsp 70)
• similar to HSEs in animals

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HSEs in plants and animals.
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Heat-shock transcription factor (HSF)

• studied mostly in animals and yeast
• Binds to HSEs
• Contains leucine zipper motifs
• Binds DNA as a trimer
• Activity is induced by heat, and phosphorylation
• Activity Repressed by HSP70

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Fig. 22.43, Buchanan et al.
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Fig. 22.44, Buchanan et al.