1. Stress Responses & Gene Expression
plants must adapt to stresses because of their sedentary lifestyle
Plants have adapted through evolutionary modifications to resist certain stressors (abiotic and biotic). Most can also acclimate to certain stressors to promote their survival in the short term; some species (strains) are better than others at acclimating.
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
Some have common features: e.g., #2-5 can all create water shortages for plant cells.

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 (< 20 °C)
Mesophiles (~ °C)
Thermophiles ( ~35-70 °)
Hyperthermophiles ( °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 , Buchanan et al.

7. Thermotolerant growth of soybean seedlings following a heat shock.
28oC oC  45oC oC
Soybean seedlings.
Fig , 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 ~ proteins
mRNAs for other proteins there but not translated well!
Maize (corn)
Fig
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

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 , 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
Strong association with urban areas in these regions; maybe easier to spread from tree to tree?
Natural root grafts
Sap beetle