Krishna K. Niyogi
How Photosynthetic Organisms Prevent Oxidative Damage in High Light
Photosynthetic organisms are extremely responsive to changes in their environment, especially differences in incident light. My laboratory is studying how light and other environmental parameters influence photosynthesis, which comprises some of the most fundamentally important reactions in biology. In nature, photosynthetic organisms experience large variations in incident sunlight on a daily basis. Especially in unfavorable environments, algae and plants often absorb more light energy than they are capable of using for photosynthesis, and this excessive light can cause photo-oxidative damage (bleaching) and loss of photosynthetic efficiency. Photosynthetic organisms have evolved multiple mechanisms to cope with excessive light, and we are trying to identify and dissect these processes by isolating algal and plant mutants. Characterization of the mutants involves a diverse set of techniques, including genetics, physiology, biochemistry, and molecular biology.
The current work on algae is focused on one particular species, Chlamydomonas reinhardtii, a unicellular green alga with many advantages as a model photosynthetic organism. We are studying the cellular processes involved in coping with reactive oxygen species (ROS) produced in excessive light. In all photosynthetic organisms, xanthophyll pigments have an essential role in protection from oxidative damage. The phenotypes of Chlamydomonas xanthophyll mutants are being exploited to analyze photoprotective processes related to singlet oxygen production in the photosystem II light-harvesting antenna.
A major process involved in controlling photosynthesis in excessive light is the downregulation of photosystem II activity by pH- and xanthophyll-dependent dissipation of excess absorbed light energy as heat, measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). We are providing insights into the molecular mechanism of NPQ by isolating npq mutants of the model plant, Arabidopsis thaliana. Characterization of a subset of these mutants has contributed to our knowledge of the role of xanthophyll pigments in NPQ. Other npq mutants identify components besides the xanthophylls that are critical for NPQ, and the resources of the Arabidopsis genome project are being used to facilitate the isolation of the genes affected in these mutants.
The innovative use of genetics to dissect complex problems in the biochemistry and ecophysiology of photosynthesis will enable us to assess the relative importance of different processes for photoprotection. Future studies may allow us to manipulate plant productivity and the ability of plants to grow in different, often adverse, environments.