Estimation of threshold of lumenal pH inducing thermal dissipation of absorbed light energy using an Arabidopsis cytochrome b6f mutant

Abstract

Light-induced lumenal acidification controls the efficiency of light harvesting by inducing thermal dissipation of excess absorbed light energy in photosystem II. Thermal dissipation prevents photo-oxidative damage of the photosynthetic apparatus at high light intensity. To dissect the molecular mechanisms of DpH regulation in vivo, we isolated Arabidopsis mutants with reduced quenching of Chl fluorescence using the imaging system of Chl fluorescence. pgr1 (proton gradient regulation) entirely lacked thermal dissipation at high light intensity. Map-based cloning showed that pgr1 had a point mutation in petC encoding the Rieske subunit of the cytochrome b6f complex. Although the electron transport rate was not affected at low light intensity, it was significantly restricted at high light intensity in pgr1. We estimated maximum lumenal acidification as pH 6.0 in pgr1 from i) the slow kinetics of violaxanthin de-epoxidase, which is closely related to lumenal acidification and ii) the reduced 9-aminoacridine fluorescence quenching. Although the lumenal acidification was insufficient to induce thermal dissipation, photoautotrophic growth was not affected under mild culture conditions in pgr1. These results suggest that thermal dissipation is precisely regulated by lumenal pH to maintain maximum photosynthetic activity. We showed that pgr1 was sensitive to the rapid change in light intensity, demonstrating that maximum activity of the cytochrome b6f complex can induce thermal dissipation which is important for plants to survive in the field.