Institute of Plant Biology, Biological Research Centre, Szeged, Hungary.
Universit� Paris-Saclay, CEA, CNRS, Institute for Integrative Biology of the Cell (I2BC) 91191 Gif-sur-Yvette, France.
Plant Cell. 2021 May 31;33(4):1286-1302. doi: 10.1093/plcell/koab008.
Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons for life on Earth. The photochemical reaction center of PSII is known to possess two stationary states. In the open state (PSIIO), the absorption of a single photon triggers electron-transfer steps, which convert PSII into the charge-separated closed state (PSIIC). Here, by using steady-state and time-resolved spectroscopic techniques on Spinacia oleracea and Thermosynechococcus vulcanus preparations, we show that additional illumination gradually transforms PSIIC into a light-adapted charge-separated state (PSIIL). The PSIIC-to-PSIIL transition, observed at all temperatures between 80 and 308 K, is responsible for a large part of the variable chlorophyll-a fluorescence (Fv) and is associated with subtle, dark-reversible reorganizations in the core complexes, protein conformational changes at noncryogenic temperatures, and marked variations in the rates of photochemical and photophysical reactions. The build-up of PSIIL requires a series of light-induced events generating rapidly recombining primary radical pairs, spaced by sufficient waiting times between these events-pointing to the roles of local electric-field transients and dielectric relaxation processes. We show that the maximum fluorescence level, Fm, is associated with PSIIL rather than with PSIIC, and thus the Fv/Fm parameter cannot be equated with the quantum efficiency of PSII photochemistry. Our findings resolve the controversies and explain the peculiar features of chlorophyll-a fluorescence kinetics, a tool to monitor the functional activity and the structural-functional plasticity of PSII in different wild-types and mutant organisms and under stress conditions.
光系统 II（PSII）利用太阳能氧化水，并为地球上的生命提供电子。已知 PSII 的光化学反应中心具有两种稳定状态。在开放状态（PSIIO）下，吸收单个光子会引发电子转移步骤，将 PSII 转化为电荷分离的闭合状态（PSIIC）。在这里，通过在 Spinacia oleracea 和 Thermosynechococcus vulcanus 制剂上使用稳态和时间分辨光谱技术，我们表明，额外的光照会逐渐将 PSIIC 转化为适应光的电荷分离状态（PSIIL）。PSIIC 到 PSIIL 的转变在 80 到 308 K 之间的所有温度下都可以观察到，它是可变叶绿素-a 荧光（Fv）的很大一部分原因，并且与核心复合物中的微妙、暗可逆重组、非冷冻温度下的蛋白质构象变化以及光化学和光物理反应速率的显著变化有关。PSIIL 的建立需要一系列光诱导事件，这些事件产生快速重组的初级自由基对，这些事件之间有足够的等待时间-这指向了局部电场瞬变和介电弛豫过程的作用。我们表明，最大荧光水平 Fm 与 PSIIL 相关，而不是与 PSIIC 相关，因此 Fv/Fm 参数不能等同于 PSII 光化学的量子效率。我们的发现解决了争议，并解释了叶绿素-a 荧光动力学的特殊特征，这是一种监测不同野生型和突变体生物以及在胁迫条件下 PSII 功能活性和结构功能可塑性的工具。
