Restriction of Alternative Pathway or Cytochrome Pathway of Mitochondrial Electron Transport Makes Mesophyll Protoplasts of Pea Susceptible to Photoinhibition at Even Moderate Light

摘要

Mitochondrial oxidative metabolism optimizes and protects photosynthesis against abiotic stress. The present study was made to assess the importance of alternative or cytochrome pathways of mitochondrial electron transport in such protection. Photosynthesis in mesophyll protoplasts and leaf discs of pea was monitored under moderate or high light, with mitochondrial metabolism modulated by suitable inhibitors. Mesophyll protoplasts pre-illuminated at a moderate light intensity of up to 1000 mu mol m~(-2) s~(-1), showed no significant photoinhibition. However, when the alternative pathway was inhibited with SHAM, photoinhibition was induced at moderate light intensities. Antimycin A (inhibitor of cytochrome pathway) had similar effect but to a less extent than that of SHAM. At high light (>2000 mu mol m~(-2) s~(-1)), the control itself showed photoinhibition, which was further aggravated by SHAM and antimycin A. Similar pattern was observed in the system of leaf discs. Western blot analysis revealedan increase in the levels of the active (~30 kDa) form of alternative oxidase (AOX) in the light-treated leaf discs, compared to the dark exposed ones. Further, an additional (possibly light stress responsive) isoform of AOX was detected in extracts of leaf discs exposed to high light. We propose that the alternative pathway of mitochondrial electron transport along with cytochrome path, helps to dissipate photochemically generated excess redox, and thus helps in photoprotection at both moderate and high light intensities.