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., thermo-stable enzymes in organisms that
  tolerate high temperature
• Acclimation inducible responses that enable an
  organism to tolerate an unfavorable or lethal
  change in their 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. nutrient deprivation
• 10. excessive light

• Biotic
• pathogens
• herbivores

4
Plants respond to stresses as individual cells
and as whole organisms stress induced signals
can be transmitted throughout the plant, making
other parts more ready to withstand the stress..

Fig. 22.3, Buchanan et al.
5
Most organisms are adapted to environmental
temperature

• Psychrophiles (lt 20 C)
• Mesophiles ( 20-35 C)
• Thermophiles ( 35-70 )
• 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
6
Heat Stress (or Heat Shock) Response

• Induced by temperatures 10-15oC 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.
7
Thermotolerant growth of soybean seedlings
following a heat shock.
28oC 40oC ? 45oC 45oC
Soybean seedlings.
Fig. 22.42, Buchanan et al.
8
Heat stress effects on protein synthesis in
soybean seedlings (J. Key).
Joe Key
9
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

10
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.
11
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
12
Flooding causes anoxia and an anaerobiotic
response in roots.

• - 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!

Maize (corn) Fig. 22.23
Most of the ANPs are enzymes associated with
glycolysis and fermentation.
13
Aerobic Anoxic
Protein synthesis in aerobic versus anoxic maize
root tips. 5-hour labeling with 3H-leucine and
2-D gel electrophoresis.
Fig. 22.30
14
Enzymes that are up-regulated by anaerobiosis
15
Biotic Stress and Plant Defense Responses

• Pathogen Strategies
• Necrotrophic plant tissue killed and then
  colonized broad host range
• e.g., rotting bacteria (Erwinia)
• Biotrophic plant cells remain alive, narrow
  host range (1 plant species)
• e.g., viruses, nematodes, fungal mildews

16
Major Pathogens

• Viruses - most are RNA viruses w/small genomes,
  which always encode
• Coat protein
• RNA-dependent RNA polymerase
• Movement protein(s)
• Viroids naked, single strands of RNA
  discovered by T.O. Diener
• Bacteria- e.g., Xanthomonas
• Fungi - 4 major groups
• Nematodes - root parasites, also increase
  infection by microorganisms

SS RNA virus Tobacco Mosaic Virus
ds DNA virus Cauliflower Mosaic Virus
Fig. 21.10, Buchanan et al.
17
Plant Defenses

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

18
The Plant Defense Response
Compatible interaction ? disease Incompatible
interaction ? resistance

• 3 aspects of response
• Hypersensitive
• Local
• Systemic

19
Distribution of Oak Wilt in the US
Leaves from Infected tree
Fungus - Ceratocystis fagacearum
Natural root grafts
Sap beetle