Stress Responses & Gene Expression
• plants must
adapt to
stresses
because of
their
sedentary
lifestyle
Fig. 22.2, Buchanan et al.
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
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
1. pathogens
2. herbivores
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.
Most organisms are adapted to environmental
temperature:
1.
2.
3.
4.
Psychrophiles (< 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
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.
Thermotolerant growth of soybean seedlings following a
heat shock.
28oC
Soybean seedlings.
40oC  45oC
45oC
Fig. 22.42, Buchanan et al.
Heat stress effects on protein synthesis in soybean
seedlings (J. Key).
Joe Key
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
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.
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
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.
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
Enzymes
that are upregulated by
anaerobiosis
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
Major Pathogens
Viruses - most are RNA viruses w/small
genomes, which always encode:
1. Coat protein
2. RNA-dependent RNA polymerase
3. Movement protein(s)
SS RNA virus: Tobacco
Mosaic Virus
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
ds DNA virus:
Cauliflower Mosaic Virus
Fig. 21.10, Buchanan et al.
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)
The Plant Defense Response
Compatible interaction  disease
Incompatible interaction 
resistance
3 aspects of response:
1. Hypersensitive
2.
Local
3.
Systemic
Distribution of Oak Wilt in the US
Leaves from Infected tree
Fungus - Ceratocystis fagacearum
Natural root grafts
Sap beetle