Takeuchi Ko, Harimoto Shintaro, Maekawa Shu, Miyake Chikahiro, Ifuku Kentaro
Graduate School of Agriculture, Kyoto University, Kyoto, Japan.
Graduate School for Agricultural Science, Kobe University, Kobe, Japan.
Physiol Plant. 2025 Jul-Aug;177(4):e70392. doi: 10.1111/ppl.70392.
Photosystem I (PSI) can be photoinhibited by excessive electron flow from Photosystem II (PSII), causing serious growth inhibition due to PSI's limited repair capacity. In contrast, PSII is more prone to photoinhibition under environmental stress, but it can recover efficiently. Consequently, PSII photoinhibition is considered a protective mechanism that mitigates PSI over-reduction. However, this photoprotective role under environmental stress remains unexplored in intact plants without using mutants or chemical treatments. To address this, we examined the relationship between PSII photoinhibition and PSI protection under two representative stresses that selectively induce PSI photoinhibition: chilling stress and fluctuating light, using A. thaliana and cucumber plants. Under chilling stress, A. thaliana exhibited marked PSII photoinhibition and maintained active PSI, whereas cucumber showed insufficient PSII downregulation and suffered from PSI photoinhibition. In addition, when fluctuating light treatment was applied to plants with various Fv/Fm (the maximum quantum yield of PSII), plants with reduced Fv/Fm maintained an oxidized PSI, and PSI photoinhibition progressed slowly. The susceptibility to PSI photoinhibition under fluctuating light strongly correlated with Fv/Fm, providing clear evidence that PSII photoinhibition protects PSI. Interestingly, even in plants where P700 remained oxidized due to PSII photoinhibition, the Fe-S clusters remained reduced during saturation pulses. However, the re-oxidation of reduced Fe-S clusters was enhanced in PSII-photoinhibited plants, suggesting that charge recombination to P700 with reduced components on the PSI acceptor side or accelerated processes downstream of ferredoxin would suppress ROS generation downstream of PSI. This study clarifies how PSII photoinhibition suppresses PSI photoinhibition and prevents ROS-induced damage under environmental stresses.
光系统I（PSI）会因来自光系统II（PSII）的过量电子流而受到光抑制，由于PSI的修复能力有限，这会导致严重的生长抑制。相比之下，PSII在环境胁迫下更容易受到光抑制，但它能够高效恢复。因此，PSII光抑制被认为是一种减轻PSI过度还原的保护机制。然而，在不使用突变体或化学处理的完整植物中，这种环境胁迫下的光保护作用仍未得到探索。为了解决这个问题，我们使用拟南芥和黄瓜植株，研究了在两种选择性诱导PSI光抑制的代表性胁迫（冷胁迫和波动光）下，PSII光抑制与PSI保护之间的关系。在冷胁迫下，拟南芥表现出明显的PSII光抑制并维持活跃的PSI，而黄瓜则表现出PSII下调不足并遭受PSI光抑制。此外，当对具有不同Fv/Fm（PSII的最大量子产率）的植物进行波动光处理时，Fv/Fm降低的植物维持PSI氧化状态，PSI光抑制进展缓慢。波动光下对PSI光抑制的敏感性与Fv/Fm密切相关，这清楚地证明了PSII光抑制保护PSI。有趣的是，即使在由于PSII光抑制而P700保持氧化状态的植物中，在饱和脉冲期间Fe-S簇仍保持还原状态。然而，在PSII光抑制的植物中，还原的Fe-S簇的再氧化增强，这表明与PSI受体侧的还原成分发生电荷复合或铁氧还蛋白下游的加速过程会抑制PSI下游的ROS生成。这项研究阐明了PSII光抑制如何在环境胁迫下抑制PSI光抑制并防止ROS诱导的损伤。
