Title

Ecological and evolutionary variation in heat-shock proteins

Date of Award

2003

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Advisor(s)

Scott A. Heckathorn

Keywords

Stress response, Photosynthesis, Thermotolerance, Chenopodium album, Solidago altissima, HSP

Subject Categories

Botany | Ecology and Evolutionary Biology | Life Sciences | Plant Sciences

Abstract

In spite of the ubiquitous nature of heat-shock proteins (Hsps), and their central role in thermotolerance, there is variation in the patterns of expression of Hsps. While much is now known about Hsp function, it remains unclear why we see such variation. A primary objective of my work was to understand variation in the heat-shock response (HSR), and examine the relationship of the HSR to the thermal environment. In particular, I focussed on the temperature set-points of the HSR and magnitude of Hsp expression. In a comprehensive analysis of the HSR, I demonstrate a strong relationship between the set-points of the HSR and growth temperature in diverse species, across a broad range of temperatures. Deviation from this relationship was related to the ecological and evolutionary history of organisms. The HSR also exhibits plasticity in response to changing growth temperatures, and I describe general patterns of plasticity in the HSR. In examining variation in Hsp content in Chenopodium album , I show that contrary to expectations, populations from more stressful habitats had lower Hsp content and induced thermotolerance. Among populations, acclimation to higher growth temperatures decreased Hsp content. Organisms from stressful habitats that require frequent induction of the HSR may down-regulate the HSR, and instead rely on basal mechanisms of thermotolerance. An in vitro assay was used to explicitly determine the adaptive significance of natural variation in Hsps. I demonstrate natural variation in protection of photosynthetic electron transport by chloroplast small Hsps (csHsp), and show that csHsps can account for most of the induced thermotolerance observed. In the final study, I examine the influence of light on Hsp expression in the field. I show that Hsp content is greater in Solidago altissima plants in the sun, than in the shade. In accordance with these results, I demonstrate a significant effect of light and temperature on Hsp accumulation in the laboratory. These results add to the surprisingly small number of studies that have examined Hsp accumulation in field plants, confirm previous results on the effect of light, and importantly, show that light influences Hsp accumulation in plants in their natural habitat.

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