The evolution of abiotic stress tolerance: A physiological and gene expression approach to the relationship between high light and heat stress responses in Boechera

As sessile organisms, plants must be able to complete their life cycle wherever a seed germinates. It is well established that not all plants can tolerate all climates and conditions. Much of what is known about plant tolerance to abiotic stress factors, such as heat and light, is known from studies of model species. However, differential tolerance to stress, that is variation in stress responses, clearly has a major role in shaping biogeographical patterns. The goals of this study are to examine the patterns of stress responses in five Boechera species and to compare these patterns to that of Arabidopsis. Further, the patterns of biochemical and physiological variation found in this study will be evaluated in light of phylogenetic relationships and geographical distribution.;Plants in the genus Boechera are found in a variety of habitats ranging from hot deserts to alpine mountains. Previous work has shown that Boechera species have different tolerances to heat stress (HS). This project tested the impacts of high light (HL) and combined stress of HS and HL. Stress treatments include basal (direct impact) and acquired (plants pretreated with less intense stress before higher intensity stress).;Plants were grown at 22°C and 150muE for 7-10 days. For HS, plants were treated for 3 hours at 38-45°C. For HL treatment plants were exposed to 1500muE for 3 hours. The combined treatments were both HS at 38-45°C and HL at 1500muE. Acquired pretreatments included pretreatments of 38°C and 38°C/1500muE, followed by 3 hour stress treatments from 41°C-45°C, at 150muE and 1500muE.;Plant responses to the stress treatments were quantified via: (1) whole-organism response, (2) photosynthetic, and (3) gene expression. The whole organismal response indicates the amount of cell death that occurred. The photosynthetic response was measured by Pulse-Amplitude-Modulation as chlorophyll fluorescence, a measure of plant photosystem II activity. Gene expression response was quantified by measuring the changes in expression of stress-induced genes using quantitative real-time PCR. The genes of interest include: reactive oxygen detoxifiers (APX2), light-induced proteins (ELIP2), heat shock transcription factors (HsfA2), and genes important for energy dissipation during photosynthesis (AVDE1).;High light alone was found to have had a small but significant impact, and there is a strong effect on both photosynthesis and plant growth when HL and HS are combined. Importantly, it was found that Boechera species do not have identical tolerance to combined stresses, both in basal and acquired stress treatments. Interestingly, a combined pretreatment gives protection against single and combined stresses in some Boechera species but not to others. B. depauperata and B. californica were found to be the most tolerant of stress, while A. thaliana and B. perennans were the least tolerant. B. arcuata and the endangered B. johnstonii had intermediate tolerance to stresses. There was no evidence to suggest that these differences were driven by the phylogenetic relationships of these species. Gene expression also shows that the species do not have identical responses to stresses on a molecular level. Particularly, gene expression of APX2 and HsfA2 varies between A. thaliana and the Boechera species. There is differential gene expression between the species and between stress treatments as well. The tolerances and gene expression patterns are indicative of adaptation of the species to their specific environments.