Higuchi Mieko, Noguchi Takumi, Sonoike Kintake
Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8562, Japan.
Biochim Biophys Acta. 2003 Jul 10;1604(3):151-8. doi: 10.1016/s0005-2728(03)00044-6.
Oxygen evolution is inhibited when leaves of chilling-sensitive plants like cucumber are treated at 0 degrees C in the dark. The activity is restored by moderate illumination at room temperature. We examined the changes in the redox state of the Mn-cluster in cucumber leaves in the processes of dark-chilling inhibition and subsequent light-induced reactivation by means of thermoluminescence (TL). A TL B-band arising from S(2)Q(B)(-) charge recombination in PSII was observed upon single-flash illumination of untreated leaves, whereas four flashes were required to yield the B-band after dark-chilling treatment for 24 h. This three-step delay indicates that over-reduced states of the Mn-cluster such as the S(-2) state were formed during the treatment. Fitting analysis of the flash-number dependence of the TL intensities showed that the Mn-cluster was more reduced with a longer period of the treatment and that S(-3) was the lowest S-state detectable in the dark-chilled leaves. Measurements of the Mn content by atomic absorption spectroscopy showed that Mn atoms were gradually released from PSII during the dark-chilling treatment but re-bound to PSII by illumination at 30 degrees C. Thus, dark-chilling inhibition of oxygen evolution can be ascribed to the disintegration of the Mn-cluster due to its over-reduction. The observation of the S(-3) state in the present in vivo system strongly suggests that S(-3), which has been observed only by addition of exogenous reductants into in vitro preparations, is indeed a redox intermediate of the Mn-cluster in the processes of its disintegration and photoactivation.
当像黄瓜这样的冷敏感植物的叶片在黑暗中于0摄氏度处理时，氧气释放受到抑制。在室温下适度光照可恢复其活性。我们通过热发光（TL）研究了黄瓜叶片在黑暗低温抑制及随后光诱导再激活过程中锰簇氧化还原状态的变化。未处理叶片单闪光照射时观察到源于PSII中S(2)Q(B)(-)电荷复合的TL B带，而黑暗低温处理24小时后则需要四次闪光才能产生B带。这种三步延迟表明在处理过程中形成了诸如S(-2)状态的锰簇过度还原状态。对TL强度的闪光次数依赖性进行拟合分析表明，处理时间越长，锰簇还原程度越高，且S(-3)是黑暗低温处理叶片中可检测到的最低S状态。通过原子吸收光谱法测量锰含量表明，在黑暗低温处理过程中锰原子逐渐从PSII释放，但在30摄氏度光照下又重新结合到PSII上。因此，黑暗低温对氧气释放的抑制可归因于锰簇因过度还原而解体。在当前体内系统中观察到S(-3)状态强烈表明，仅在体外制剂中添加外源还原剂时才观察到的S(-3)确实是锰簇在其解体和光激活过程中的氧化还原中间体。
