Excess Light-Induced Molecular Responses of Chloroplast Rieske FeS Protein

Under drought stress in the presence of high light, limited entry of CO2 into the leaf causes the suppression of photosynthetic carbon fixation, and the electron transport chain is liable to be over-reduced, leading to the increased risks of oxidative damages in photosynthetic apparatus. To gain insights into the regulation of photochemistry under such conditions, we analyzed changes in the proteome of the membrane fraction in the leaves of wild watermelon under drought stress. From twodimensional gel electrophoresis (2DE) analysis and mass spectrometry, we found the number of spots for the chloroplast Rieske FeS protein, a subunit of cytochrome b 6 f complex (Cyt b 6 f), increased under the stress. We showed that the Rieske protein is encoded by single gene in wild watermelon, and the total amount of the Rieske protein was not changed under drought stress. These results suggest that the Rieske FeS protein would undergo posttranslational modification under stress. Western blot analysis using 2DE revealed that there were multiple spots for the Rieske protein with very similar molecular weights but with different isoelectric points under unstressed conditions. The new spots of the Rieske protein with more acidic pI values emerged under drought stress, and these spots disappeared upon resuming irrigation. These new spots were also observed in the leaf which was subjected to high light stress. It is proposed that the post-translational modification of the Rieske FeS protein may regulate electron transport to avoid over-reduction under stress conditions.