Chloroplast Lipid Composition: Connecting Biophysics to Photosynthetic Function

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Life on Earth depends on plant photosynthesis, so it is vital that we fully understand all the component systems that contribute to photosynthetic function. Despite considerable research effort over the last twenty years, we still do not know how the lipid composition of the photosynthetic (thylakoid) membrane affects photosynthesis. This project aims to discover the particular lipid requirements for chloroplast biogenesis and function. The project will use the full range of biochemical, molecular and genetic techniques to investigate mutants of the model plant, Arabidopsis, in which different changes in membrane lipid composition are associated with dramatic phenotypes. The knowledge gained will help scientists predict the likely consequences of environmental changes on plant growth and crop productivity. Eventually it will be possible to manipulate membrane lipid composition of plants to better suit particular environmental conditions. Because photosynthesis is central to plant productivity, the extensive temperature-dependent effects observed in the mutants have wide implications for understanding the temperature responses of plants, and agricultural production systems. The project will provide opportunities for graduate and undergraduate students to receive training and experience in plant biology research relevant to agriculture, forestry and other fields of plant production. In particular, the project participants will contribute to an outreach program that uses experiential learning and peer mentoring to encourage Native American high school students to enroll in STEM disciplines in college.This proposal focuses on two new classes of mutants with distinct phenotypes. In the fab1 mutant, an increase in 16:0 results in the collapse of photosynthesis after 2-3 weeks at 2°C, and to destruction of chloroplasts. A collection of suppressor mutations that allow survival of fab1 at 2°C have been isolated. Cloning and characterization of these will provide information about the photosynthetic processes that are most susceptible to the membrane alterations in fab1 plants. The project has generated an allelic series of fad2 fad6 mutants containing 5% to 46% polyunsaturated fatty acids (vs. 77% in wild type) that provide the first opportunity to explore, in detail, the relationship between thylakoid unsaturation and chloroplast function. Analyses indicate that an increase in leakage of protons through the thylakoid membrane is one possible defect in the mutants. The project has also discovered alterations in the capacities of different pathways of thylakoid protein transport in other mutants and these investigations will be extended to include the fad2 fad6 mutants. The results obtained will permit the development and testing of mechanistic models that relate the physiological phenotypes of the mutants to the biophysics of membrane lipid structure. The complementary paths of investigation described in this proposal promise important advances in the application of biophysical principles to understanding how lipid molecules contribute to the functioning of chloroplasts and the responses of plants to environmental change.