Relative contributions of PGR5-and NDH-dependent photosystem I cyclic electron flow in the generation of a proton gradient in Arabidopsis chloroplasts

Respective contributions of PGR5- and NDH-dependent cyclic electron flows around photosystem I for generating the proton gradient across the thylakoid membrane are Delta 30 and Delta 5%. The proton concentration gradient across the thylakoid membrane (Delta pH) produced by photosynthetic electron transport is the driving force of ATP synthesis and non-photochemical quenching. Two types of electron transfer contribute to Delta pH formation: linear electron flow (LEF) and cyclic electron flow (CEF, divided into PGR5- and NDH-dependent pathways). However, the respective contributions of LEF and CEF to Delta pH formation are largely unknown. We employed fluorescence quenching analysis with the pH indicator 9-aminoacridine to directly monitor Delta pH formation in isolated chloroplasts of Arabidopsis mutants lacking PGR5- and/or NDH-dependent CEF. The results indicate that Delta pH formation is mostly due to LEF, with the contributions of PGR5- and NDH-dependent CEF estimated as only Delta 30 and Delta 5%, respectively.