Hald Simon, Pribil Mathias, Leister Dario, Gallois Patrick, Johnson Giles N
Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester, M13 9PT, UK.
Biochim Biophys Acta. 2008 Sep;1777(9):1173-83. doi: 10.1016/j.bbabio.2008.04.041. Epub 2008 May 3.
Photosynthetic electron transport can involve either a linear flow from water to NADP, via Photosystems (PS) II and I or a cyclic flow just involving PSI. Little is known about factors regulating the relative flow through each of these pathways. We have examined photosynthetic electron transport through each system in plants of Arabidopsis thaliana in which either the PSI-D1 or PSI-E1 subunits of PSI have been knocked out. In both cases, this results in an imbalance in the turnover of PSI and PSII, such that PSII electron transport is limited by PSI turnover. Phosphorylation of light-harvesting complex II (LHCII) and its migration to PSI is enhanced but only partially reversible and not sufficient to balance photosystem turnover. In spite of this, cyclic electron flow is able to compete efficiently with PSI across a range of conditions. In dark-adapted leaves, the efficiency of cyclic relative to linear flow induced by far-red light is increased, implying that the limiting step of cyclic flow lies in the re-injection of electrons into the electron transport chain. Illumination of leaves with white light resulted in transient induction of a significant non-photochemical quenching in knockout plants which is probably high energy state quenching induced by cyclic electron flow. At high light and at low CO(2), non-photochemical quenching was greater in the knockout plants than in the wildtype. Comparison of PSI and PSII turnover under such conditions suggested that this is generated by cyclic electron flow around PSI. We conclude that, when the concentration of PSI is limiting, cyclic electron flow is still able to compete effectively with linear flow to maintain a high DeltapH to regulate photosynthesis.
光合电子传递既可以是从水到NADP的线性流动，经由光系统（PS）II和I，也可以是仅涉及PS I的循环流动。关于调节通过每条途径的相对流量的因素，人们了解甚少。我们研究了拟南芥植株中通过每个系统的光合电子传递，其中PS I的PS I-D1或PS I-E1亚基已被敲除。在这两种情况下，这都会导致PS I和PS II周转的不平衡，使得PS II电子传递受到PS I周转的限制。光捕获复合体II（LHCII）的磷酸化及其向PS I的迁移增强，但只是部分可逆，不足以平衡光系统的周转。尽管如此，循环电子流在一系列条件下都能够与PS I有效竞争。在暗适应的叶片中，远红光诱导的循环相对于线性流动的效率增加，这意味着循环流动的限制步骤在于将电子重新注入电子传递链。用白光照射叶片会导致敲除植株中短暂诱导出显著的非光化学猝灭，这可能是由循环电子流诱导的高能态猝灭。在高光强和低CO₂条件下，敲除植株中的非光化学猝灭比野生型更大。在这种条件下对PS I和PS II周转的比较表明，这是由围绕PS I的循环电子流产生的。我们得出结论，当PS I的浓度受到限制时，循环电子流仍然能够与线性流动有效竞争，以维持较高的ΔpH来调节光合作用。
