Regulation of proton-to-electron stoichiometry in photosynthetic electron transport: physiological function in photoprotection

The primary stable products of photosynthetic electron flow are NADPH and ATP. Stoichiometry of their production depends on the ratio of protons pumped across the thylakoid membrane to electrons passed through the electron transport pathway (H+/e− ratio). Flexible requirements of the ATP/NADPH ratio by various assimilatory reactions in chloroplasts must be fulfilled by the H+/e− ratio during the electron flow. In addition to the well-known role of ΔpH during ATP synthesis, ΔpH also functions as a trigger of the down-regulation of photosystem II (PSII) photochemistry. Excessive light energy is safely dissipated as heat by this regulatory process to suppress the generation of toxic reactive oxygen species. Thus, regulation of the H+/e− ratio may function in the photoprotection, as well as in the regulation of the ATP/NADPH production ratio. It has long been the consensus that the H+/e− ratio can be controlled by regulating the proton-transporting Q-cycle in the cytochrome b6f complex and by the cyclic electron flow around photosystem I (PSI). Despite the possible physiological importance and the long history of interest, the molecular identity of Q-cycle regulation and the cyclic electron flow around PSI have been remained unclear. The recent improvements in research tools, including the genetic approach using chlorophyll fluorescence imaging and establishment of the chloroplast transformation technique, are providing new insights into classical topics. In this review, we focus on regulation of the H+/e− ratio especially from the view of photosynthetic regulation.