Changes in chloroplast physiology and whole cell protein profiles of Chlamydomonas mutants under high light and oxidative stress

Britta Förster, Ulrike Mathesius2, Barry Osmond, and Barry Pogson1

1ARC CE in Plant Energy Biology, 2ARC CE of Integrative Legume research, School of Biochemistry & Molecular Biology, The Australian National University, Canberra ACT 0200, Australia

Survival and growth of photosynthetic organisms in strong light and the associated oxidative stress require effective photoprotection and repair mechanisms to ensure maintenance of photosynthetic activity, particularly of the readily photoinactivated photosystem II. We have investigated these mechanisms in very high light resistant (VHLR) mutants that are phenotypically distinguished from wild type only in their ability to sustain fast growth under full sun light levels, normally lethal to C. reinhardtii. Our previous physiological and biochemical investigations of very high light resistant (VHLR) mutants have detected many changes with respect to chloroplast functions under high light stress conditions potentially contributing to enhanced survival, including faster D1 protein synthesis for photosystem II repair, increased photoprotection via the xanthophyll cycle and tolerance to and less accumulation of reactive oxygen species (ROS). At the same time physical and functional downregulation of the photosynthetic apparatus was accompanied by altered electron flow through photosystems. More recently, we explored potential contributions of cyclic electron flow around PSI to enhanced stress tolerance. Preliminary data strongly suggest VHLR related differences. To gain a broader view of cell responses to excess light, we examined constitutive and induced adjustments of the soluble whole cell proteome using comparative proteomics to identify changes potentially necessary for survival under extreme light stress in VHLR mutants and wild type. Protein profiles change markedly in response to light condition or genotype, and identified proteins, such as NAB1 and RB38 (both related to gene expression of photosystem components) as candidates for future investigations. Together, with our map-based cloning and insertional mutants analysis (in progress) that suggests several gene mutation can confer VHLR phenotypes, our findings consolidate the view that VHLR mutants affect regulatory master switches which determine the balance between the induction of photoprotective mechanisms and photosynthetic efficiency.

e-mail address of presenting author: britta.forster@anu.edu.au