Stress Responses & Gene Expression

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 • Biotic • pathogens • herbivores Abiotic 1. heat 2. cold 3. drought 4. salt 5. wind 6. oxidative 7. anaerobic 8. heavy metals 9. nutrient deprivation 10. excessive light

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 (~ 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. Twowell 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. AerobicAnoxic 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 SS RNA virus: Tobacco Mosaic Virus 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 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