Regulation of photosynthesis by cyclic electron transport around photosystem I

Abstract

Cyclic electron transport (CET) around photosystem I (PSI) was discovered more than 60 years ago but it was 2004 when two pathways of CET were proposed. The main pathway depends on the PROTON GRADIENT REGULATION 5 (PGR5) protein, whereas the minor pathway is mediated by the chloroplast NADH dehydrogenase-like (NDH) complex. Although it is still a topic of debate, as to whether PGR5 is really involved in CET, the Arabidopsis pgr5 mutant is severely defective in the ΔpH-dependent regulation of photosynthetic electron transport. PGR5 is necessary to induce a ΔpH-dependent component of nonphotochemical quenching (NPQ) of chlorophyll fluorescence at a high light intensity and this qE mechanism dissipates excessively absorbed light energy safely as heat from photosystem II antennae. The lumenal acidification also downregulates the electron transport at the cytochrome b6f complex. The latter is essential for protecting PSI and the pgr5 mutant cannot survive the fluctuating light intensity. Although the contribution of the NDH complex to the pmf formation is much smaller than that of the PGR5-dependent CET during steady-state photosynthesis, the NDH complex efficiently pumps protons probably in the absence of large pmf. In addition to the size of pmf, it is also necessary to regulate the partitioning pmf components, ΔpH, and membrane potential (ΔΨ) across the thylakoid membrane. Ion channels and transporters localized to the thylakoid membrane involve the regulation and form a regulatory network of photosynthesis with the CET machinery.