Does the Arabidopsis proton gradient regulation5 Mutant Leak Protons from the Thylakoid Membrane?

Despite generating an obvious mutant phenotype, whether the Arabidopsis (Arabidopsis thaliana) proton gradient regulation5 (pgr5) 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 (g _H ⁺) in the pgr5 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 pgr5 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-g _H ⁺ phenotype of the pgr5 mutant. Transgenic lines in which flavodiiron protein-dependent pseudo-CET replaced PGR5-dependent CET had wild-type levels of g _H ⁺, suggesting that the high-g _H ⁺ phenotype in pgr5 plants is caused secondarily by the low pmf. The pgr1 mutant shows a similar reduction in pmf because of enhanced sensitivity of its cytochrome b ₆ f complex to lumenal acidification. In contrast to the pgr5 mutant, g _H ⁺ was lower in the pgr1 mutant than in the wild type. In the pgr1 pgr5 double mutants, g _H ⁺ was intermediate to g _H ⁺ values of the respective single mutants. It is unlikely that g _H ⁺ is upregulated simply in response to a low pmf. We did not observe uncoupling of the thylakoid membrane in the pgr5 mutant upon monitoring the quenching of 9-aminoacridine fluorescence. We conclude that the g _H ⁺ parameter may be influenced by other factors not related to the H⁺ leakage through ATP synthase. It is unlikely that the pgr5 mutant leaks protons from the thylakoid membrane.