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 deprivation10. excessive light

Biotic 1.pathogens2.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. Psychrophiles (< 20 °C)2. Mesophiles (~ 20-35 °C)3. Thermophiles ( ~35-70 °)4. 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 response2. 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.

28oC 40oC 45oC 45oC

Fig. 22.42, Buchanan et al.Soybean seedlings.

Thermotolerant growth of soybean seedlings following a heat shock.

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.

Maize (corn)

Fig. 22.23

- 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!

Most of the ANPs are enzymes associated with glycolysis and fermentation.

Fig. 22.30

Protein synthesis in aerobic versus anoxic maize root tips.5-hour labeling with 3H-leucine and 2-D gel electrophoresis.

Aerobic Anoxic

Enzymes that are up-regulated by anaerobiosis

Biotic Stress and Plant Defense Responses

Pathogen Strategies1. Necrotrophic – plant tissue killed and then colonized;

broad host rangee.g., rotting bacteria (Erwinia)

2. Biotrophic – plant cells remain alive, narrow host range (1 plant species)

e.g., viruses, nematodes, fungal mildews

Major PathogensViruses - most are RNA viruses w/small

genomes, which always encode: 1. Coat protein2. RNA-dependent RNA polymerase 3. 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

SS RNA virus: Tobacco 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

3 aspects of response:1. Hypersensitive

2. Local

3. Systemic

Compatible interaction diseaseIncompatible interaction resistance

Distribution of Oak Wilt in the US

Fungus - Ceratocystis fagacearum

Leaves from Infected tree

Natural root grafts Sap beetle