Molecular Genetic Dissection of the Regulatory Network of Proton Motive Force in Chloroplasts.

This article focuses on the regulation of the proton motive force (pmf) across the thylakoid membrane in chloroplasts, which drives ATP synthesis and modulates photosynthetic electron transport to balance efficient photosynthesis with photoprotection. The pmf consists of membrane potential (∆Ψ) and proton concentration gradient (∆pH), with ∆pH-dependent acidification of the thylakoid lumen triggering photosynthetic control by downregulating electron transport through the cytochrome b6f complex to protect photosystem I (PSI) from photodamage. Two main cyclic electron transport (CET) pathways around PSI—one dependent on PROTON GRADIENT REGULATION 5 (PGR5) and another mediated by the NADH dehydrogenase–like (NDH) complex—contribute to pmf modulation, with PGR5-dependent CET playing a key role under high light and fluctuating conditions, and NDH-dependent CET functioning primarily under low light. Regulation of ion transporters such as the H+/K+ antiporter KEA3 adjusts the balance between ∆Ψ and ∆pH components of pmf, optimizing photosynthetic control and non-photochemical quenching (NPQ). Despite advances from genetic and biochemical studies, the precise molecular mechanisms underlying PGR5 function, ion movement regulation, Cyt b6f complex pH sensing, and ATP synthase activity remain incompletely understood, highlighting the complexity and adaptability of photosynthetic regulation.