Molecular mechanism for the dissipation of excess photon energy in chloroplasts

点击翻译按钮获取中文摘要

When plant leaves are exposed to sunlight in excess of its utilizing capacity for coィイD22ィエD2-fixation, the excess photons produce active oxygens and cause the photoinhibition of photosynthesis. The superoxide and hydrogen peroxide photogenerated in photosystem I (PSI) are effectively scavenged, and their interactions with the stromal enzymes, PSI complex and the scavenging enzyme are suppressed. The term of water-water cycle is proposed for this scavenging system, and its molecular mechanism and physiological functions are overviewed. The water-water cycle can dissipate the excess photon energy in addition to the scavenging of active oxygens, and its high flux rate is accounted for by the FAD enzyme monodehydroascorbate reductase-catalyzed photoreduction of OィイD22ィエD2 in chloroplasts.Stress by the excess photon in chloroplasts is relaxed also by the cyclic electron flow around PSI. Participation of NAD(P)H dehydrogenase (NDH) complex as an electron-mediator between PSI and the intersystem plastoquinone is shown, and the molecular properties of NDH from cyanobacteria and chloroplasts are revealed. NDH-less transformant of tobacco shows a low cycle electron flow and is damaged the stress of excess photons.