High temperature inhibition of photosynthesis is recognized as a primary limitation to the growth and productivity of plants in both agricultural and natural environments. With global temperatures predicted to rise, research aimed at understanding and reducing plant susceptibility to heat stress is likely to have a significant impact on future crop yields. Heat stress appears to inhibit photosynthesis primarily by impairing the activity of Rubisco during the rate-limiting step of carbon assimilation in plants. Rubisco relies on an enzyme called activase to maintain its functional potential. During periods of heat stress, activase loses its ability to sustain Rubisco activity, resulting in decreased rates of photosynthesis and plant growth. There is substantial evidence that photosynthesis can acclimate to long term heat stress and that activase may play a central role in the acclimatory response. However, the basis for heat acclimation of photosynthesis is largely undefined. The current project builds on recent work supporting the involvement of activase gene regulation in the acclimation response. The focus of this research is to address specific questions regarding activase gene expression and protein regulation in plants exposed to elevated temperatures, and to begin to define the role of these regulatory processes in acclimation of photosynthesis to heat stress. Specifically, the project addresses the hypothesis that activase regulation during heat stress is influenced by genetic elements in its 3'-untranslated region, and seeks to evaluate activase protein turnover properties in vivo using a unique approach.
Knowledge of the regulatory processes that support activase gene and protein expression during heat stress will greatly facilitate the design of effective strategies to promote heat stress tolerance of photosynthesis in plants, including important crop species. This project will be carried out at Grinnell College in close partnership with a diverse group of undergraduate research students who will participate at all levels in the gathering, analysis and communication of the results of the study. By providing independent research opportunities within a biology curriculum centered on inquiry-based skills, this project will maintain the institution's proven tradition of integrating scientific research into a broad and consequential liberal arts education.
High environmental temperatures inhibit plant growth and reduce crop productivity. With earth’s temperatures predicted to rise, research aimed at understanding and reducing plant susceptibility to heat stress is likely to have a significant impact on future crop yields and global environmental policies. The negative impact of high temperature on plant growth and productivity can largely be attributed to inhibition of photosynthesis. Our work focused on the response of photosynthesis to heat stress at the molecular (DNA) and biochemical (protein) levels. We investigated an important photosynthesis gene called activase, which appears to play a key role in allowing photosynthesis to acclimate to heat stress. The objective of our research was to understand the basic cellular mechanisms that control the expression of activase genes and proteins during heat stress and to what extent those mechanisms contribute to the acclimation of photosynthesis when plants are exposed to elevated temperatures. To do this, we genetically-engineered versions of the activase gene that were different from their natural counterpart, in order to understand the function and importance of specific DNA regions of the gene. These new genes, produced by recombinant DNA technology, were re-introduced into mutant Arabidopsis plants that lacked activase. The resulting transgenic plants comprised a set of tools, which we then used in several key experiments to examine the validity of our hypotheses regarding acclimation of photosynthesis to heat stress. We discovered that heat stress significantly impacted the expression of activase, mainly by stabilizing the levels of both the RNA message and the protein content of this important gene. We discovered that a new isoform of activase is preferentially produced during heat stress and we think this new activase may contribute to the way photosynthesis in plants tolerate heat stress. It appears that the same mechanisms found in our model plant, Arabidopsis, exist in agriculturally-important species, such as cotton. In pursuing this notion, we cloned and sequenced four new activase genes from Pima cotton; these sequences are available for viewing at the National Center for Biotechnology Information. We are currently exploring the practical implications of our work in cultivated varieties of cotton. Broader impacts -- This work was conducted with the full partnership of 12 dedicated undergraduate research students at Grinnell College, and resulted in peer-reviewed publication (Planta 236: 463-476) and public presentation by both the Principal Investigator and students alike. This grant provided foundational research experiences for these students that impacted their educational goals and contributed significantly to their development as scientific scholars. That nearly all of these students are currently enrolled in doctoral programs at various institutions of higher learning demonstrates the value of integrating research into the undergraduate science curriculum. Basic scientific knowledge gained from our work will greatly enhance our ability to design effective strategies to promote heat stress tolerance of photosynthesis in plants, including important crop species. Our work is already being extrapolated to studies in cotton, a widely-produced crop in the United States and abroad, whose productivity is affected by heat stress.
