9506254 Grossman Photon flux densities that are in excess of what a plant can use for photochemistry result in the activation of processes that help protect the photosynthetic apparatus against photodamage; specifically, plants can dissipate much of the excess excitation energy as heat. This nonradiative energy dissipation, which is measured as nonphotochemical quenching (NPQ) of chlorophyll fluorescence, is probably the result of several processes. One of these processes, the generation of zeaxanthin (which may directly quench excited chlorophyll molecules) via the xanthophyll cycle, occurs in the light- harvesting antenna complexes. Other energy dissipating processes, such as charge recombination between P680+ and QA- and a cyclic electron flow around photosystem II (PSII), are associated with the reaction centers. The objective of this grant is to elucidate the mechanisms underlying these processes and their relative importance for photoprotection, using Chlamydomonas as a model organism. Mutants have been generated by insertional mutagensis, which will allow for the relatively facile isolation of the genes of interest. Physiological characterizations of the mutant strains grown in high or low light have revealed several novel classes of mutants, including those that appear to be defective in 1) the xanthophyll cycle, 2) other mechanisms of energy dissipation, 3) the regulation of NPQ, and 4) scavenging of oxygen radicals. It is proposed to continue physiological, biochemical and genetic analyses of the mutant strains that have already been isolated. These studies will help us evaluate the importance of known photoprotective processes, discover specific mechanisms that govern these processes and perhaps even unveil novel mechanisms that contribute to photoprotection.
