Regulatory network of proton motive force: contribution of cyclic electron transport around photosystem I.

Cyclic electron transport around photosystem I (PSI) generates ∆pH across the thylakoid membrane without net production of NADPH. In angiosperms, two pathways of PSI cyclic electron transport operate. The main pathway depends on PGR5/PGRL1 proteins and is likely identical to the historical Arnon's pathway. The minor pathway depends on chloroplast NADH dehydrogenase-like (NDH) complex. In assays of their rates in vivo, the two independent pathways are often mixed together. Theoretically, linear electron transport from water to NADP(+) cannot satisfy the ATP/NADPH production ratio required by the Calvin-Benson cycle and photorespiration. PGR5/PGRL1-dependent PSI cyclic electron transport contributes substantially to the supply of ATP for CO2 fixation, as does linear electron transport. Also, the contribution of chloroplast NDH cannot be ignored, especially at low light intensity, although the extent of the contribution depends on the plant species. An increase in proton conductivity of ATP synthase may compensate ATP synthesis to some extent in the pgr5 mutant. Combined with the decreased rate of ∆pH generation, however, this mechanism sacrifices homeostasis of the thylakoid lumen pH, seriously disturbing the pH-dependent regulation of photosynthetic electron transport, induction of qE, and downregulation of the cytochrome b 6 f complex. PGR5/PGRL1-dependent PSI cyclic electron transport produces sufficient proton motive force for ATP synthesis and the regulation of photosynthetic electron transport.