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.

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 pathogens herbivores Some have common features: e.g., #2-5 can all create water shortages for plant cells.

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: 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 40oC  45oC 45oC Soybean seedlings. 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 up-regulated by anaerobiosis

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

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.

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)

The Plant Defense Response Compatible interaction  disease Incompatible interaction  resistance 3 aspects of response: Hypersensitive Local Systemic

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