Govindachary Sridharan, Bigras Caroline, Harnois Johanne, Joly David, Carpentier Robert
Groupe de Recherche en Biologie Végétale, Université du Québec à Trois-Rivières, C.P. 500, Trois-Rivières, QC, Canada, G9A 5H7.
Photosynth Res. 2007 Nov-Dec;94(2-3):333-45. doi: 10.1007/s11120-007-9199-4. Epub 2007 Jul 17.
This study provides evidence for enhanced electron flow from the stromal compartment of the photosynthetic membranes to P700+ via the cytochrome b6/f complex (Cyt b6/f) in leaves of Cucumis sativus L. submitted to chilling-induced photoinhibition. The above is deduced from the P700 oxidation-reduction kinetics studied in the absence of linear electron transport from water to NADP+, cyclic electron transfer mediated through the Q-cycle of Cyt b6/f and charge recombination in photosystem I (PSI). The segregation of these pathways for P700+ rereduction were achieved by the use of a 50-ms multiple turnover white flash or a strong pulse of white or far-red illumination together with inhibitors. In cucumber leaves, chilling-induced photoinhibition resulted in approximately 20% loss of photo-oxidizible P700. The measurement of P700+ was greatly limited by the turnover of cyclic processes in the absence of the linear mode of electron transport as electrons were rapidly transferred to the smaller pool of P700+. The above is explained by integrating the recent model of the cyclic electron flow in C3 plants based on the Cyt b6/f structural data [Joliot and Joliot (2006) Biochim Biophys Acta 1757:362-368] and a photoprotective function elicited by a low NADP+/NAD(P)H ratio [Rajagopal et al. (2003) Biochemistry 42:11839-11845]. Over-reduction of the photosynthetic apparatus results in the accumulation of NAD(P)H in vivo to prevent NADP+-induced reversible conformational changes in PSI and its extensive damage. As the ferredoxin:NADP reductase is fully reduced under these conditions, even in the absence of PSII electron transport, the reduced ferredoxin generated during illumination binds at the stromal openings in the Cyt b6/f complex and activates cyclic electron flow. On the other hand, the excess electrons from the NAD(P)H pool are routed via the Ndh complex in a slow process to maintain moderate reduction of the plastoquinone pool and redox poise required for the operation of ferredoxin:plastoquinone reductase mediated cyclic flow.
本研究为黄瓜叶片在冷害诱导的光抑制条件下，光合膜基质腔中的电子通过细胞色素b6/f复合体（Cyt b6/f）增强向P700+流动提供了证据。上述结论是根据在不存在从水到NADP+的线性电子传递、通过Cyt b6/f的Q循环介导的循环电子传递以及光系统I（PSI）中的电荷复合情况下所研究的P700氧化还原动力学推导得出的。通过使用50毫秒的多次周转白色闪光或强白色或远红光脉冲以及抑制剂，实现了P700+再还原的这些途径的分离。在黄瓜叶片中，冷害诱导的光抑制导致约20%的可光氧化P700损失。在不存在线性电子传递模式的情况下，由于电子迅速转移到较小的P700+库中，循环过程的周转极大地限制了P700+的测量。通过整合基于Cyt b6/f结构数据的C3植物循环电子流的最新模型[若利奥和若利奥（2006年）《生物化学与生物物理学报》1757:362 - 368]以及低NADP+/NAD(P)H比值引发的光保护功能[拉贾戈帕尔等人（2003年）《生物化学》42:11839 - 11845]来解释上述情况。光合装置的过度还原导致体内NAD(P)H积累，以防止NADP+诱导的PSI中可逆的构象变化及其广泛损伤。由于在这些条件下铁氧还蛋白:NADP还原酶完全被还原，即使在不存在PSII电子传递的情况下，光照期间产生的还原型铁氧还蛋白也会结合在Cyt b6/f复合体的基质开口处并激活循环电子流。另一方面，来自NAD(P)H库的过量电子通过Ndh复合体以缓慢过程进行传递，以维持质体醌库的适度还原以及铁氧还蛋白:质体醌还原酶介导的循环流动运行所需的氧化还原平衡。
