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 versus 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)

9
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.
• Germinating seeds have naturally high
  thermotolerance and have a lot of hsps
• A sub-lethal heat shock allows organisms to
  survive a lethal treatment.
• Production of hsps correlates with acquired
  thermotolerance.
• Some mutants (yeast) and transgenic plants with
  altered expression of certain hsps dont show
  thermotolerance.

11
Thermotolerant growth of soybean seedlings
following HS.
28oC 40oC ? 45oC 45oC
Soybean seedlings.
Fig. 22.42, Buchanan et al.
12
(major) 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
• Chloroplast translation elongation factor eF-Tu
  (nuclear-encoded) (also has chaperone act.)
• hsp90, hsp70, and hsp60 involved in protein
  folding "molecular chaperones"
• hsp100 promotes translation of hsp and other
  mRNAs (Ferredoxin), via a CAP- independent
  mechanism (can be knocked out, plants grow but
  have reduced thermotolerance)

14
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
Chaperones bind short hydrophobic and/or
positively charged stretches of amino acids.
16
HSP60 (cpn60)

• First one termed "molecular chaperone.
• Discovered as the RuBPCase LS binding protein
  that participates in 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.

17
Movies! http//people.cryst.bbk.ac.uk/ubcg16z/c
haperone.html
18
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 (some are also appear to
  be chaperones)
• aggregate in vivo into "heat shock granules
• Maybe processing denatured proteins

19
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 and hsp70

20
Soybean Hsp mRNAs that are strongly induced.
Temp. oC
21
Cis-acting transcriptional regulatory elements

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

22
HSEs in plants and animals.
23
Heat-shock transcription factor (HSF)

• Studied mostly in animals and yeast, but also
  plants.
• Binds to HSEs.
• Contains leucine zipper motifs.
• Binds DNA as a trimer
• Activity is induced by heat, and
  phosphorylation.
• Activity repressed by HSP70.

24
Tomato HSFA2 has NLS and NES - Cytoplasmicnucleus
distribution part of control
Fig. 22.43, Buchanan et al.
25
Fig. 22.44, Buchanan et al.