Does the Arabidopsis Mutant Leak Protons from the Thylakoid Membrane?

Despite generating an obvious mutant phenotype, whether the Arabidopsis () () mutation influences cyclic electron transport (CET) around PSI is a topic of debate. Results of electrochromic shift analysis show that proton conductivity across the thylakoid membrane ( ) in the mutant is enhanced at high light intensity. Given this observation, PGR5 was proposed to regulate ATP synthase activity rather than mediating CET. The originally reported phenotype reflects a smaller proton motive force (pmf) and could be explained by this H leakage model. In this study, we genetically reexamined the high- phenotype of the mutant. Transgenic lines in which flavodiiron protein-dependent pseudo-CET replaced PGR5-dependent CET had wild-type levels of , suggesting that the high- phenotype in plants is caused secondarily by the low pmf. The mutant shows a similar reduction in pmf because of enhanced sensitivity of its cytochrome complex to lumenal acidification. In contrast to the mutant, was lower in the mutant than in the wild type. In the double mutants, was intermediate to values of the respective single mutants. It is unlikely that is upregulated simply in response to a low pmf. We did not observe uncoupling of the thylakoid membrane in the mutant upon monitoring the quenching of 9-aminoacridine fluorescence. We conclude that the parameter may be influenced by other factors not related to the H leakage through ATP synthase. It is unlikely that the mutant leaks protons from the thylakoid membrane.