KEA3 is a thylakoid membrane localized K+/H+ antiporter that regulates photosynthesis by modulating two components of proton motive force (pmf), the proton gradient (δpH) and the electric potential (δψ). We identified a mutant allele ofKEA3, disturbed proton gradient regulation (dpgr) based on its reduced non-photochemical quenching (NPQ) in artificial (CO2-free with low O2) air. This phenotype was enhanced in the mutant backgrounds of PSI cyclic electron transport (pgr5andcrr2-1). In ambient air, reduced NPQ was observed during induction of photosynthesis indpgr, the phenotype that was enhanced after overnight dark adaptation. In contrast, the knockout allele ofkea3-1exhibited a high-NPQ phenotype during steady state in ambient air. Consistent with thiskea3-1phenotype in ambient air, the membrane topology of KEA3 indicated a proton efflux from the thylakoid lumen to the stroma. Thedpgrheterozygotes showed a semidominant and dominant phenotype in artificial and ambient air, respectively. Indpgr, the protein level of KEA3 was unaffected but the downregulation of its activity was probably disturbed. Our findings suggest that fine regulation of KEA3 activity is necessary for optimizing photosynthesis.Significance Statement
A thylakoid membrane-localized K+/H+ antiporter (KEA3) regulates photosynthesis by modulating the proton gradient (δpH) and the electric potential (δψ). Here, by characterizing a point mutant allele of KEA3 and a knockout allele, we suggest that fine regulation of KEA3 activity is necessary for optimizing photosynthesis in fluctuating light conditions.