Disturbed H+/e- ratio in Arabidopsis mutant pgr6 defective in cytochrome bf complex activity

Abstract

Photosynthetic light harvesting is regulated by thylakoidal pH that is generated by electron transport in chloroplasts. Concomitant with the electron transport, protons are uptaked to the lumen side of thylakoid membranes by cytochrome bf complex. To dissect the regulatory mechanism of the H+/e- ratio, we isolated Arabidopsis mutants by screening of reduced non-photochemical quenching of chlorophyll fluorescence. The mutant, pgr6 (proton gradient regulation), cannot induce NPQ sufficiently, although electron transport is unaffected. To specify the pgr6 mutation, the double mutant was created with pgr1 that cannot induce NPQ at high light intensity due to the limited electron transport at cytochrome bf complex. The electron transport of the pgr6 pgr1 double mutant was affected even at low light intensity. This synergistic phenotype indicates that the pgr6 mutation affects the cytochrome bf complex activity. We speculate that PGR6 activates cytochrome bf complex to uptake H+ into the lumen efficiently, implicating that H+/e- ratio is regulated in cytochrome bf complex.