Fine‐tuned regulation of the K⁺/H⁺ antiporter KEA3 is required to optimize photosynthesis during induction

Summary KEA 3 is a thylakoid membrane localized K + /H + antiporter that regulates photosynthesis by modulating two components of proton motive force (pmf), the proton gradient (∆ pH ) and the electric potential (∆ψ). We identified a mutant allele of KEA 3 , disturbed proton gradient regulation ( dpgr ) based on its reduced non‐photochemical quenching ( NPQ ) in artificial ( CO 2 ‐free with low O 2 ) air. This phenotype was enhanced in the mutant backgrounds of PSI cyclic electron transport ( pgr5 and crr2‐1 ). In ambient air, reduced NPQ was observed during induction of photosynthesis in dpgr , the phenotype that was enhanced after overnight dark adaptation. In contrast, the knockout allele of kea3‐1 exhibited a high‐ NPQ phenotype during steady state in ambient air. Consistent with this kea3‐1 phenotype in ambient air, the membrane topology of KEA 3 indicated a proton efflux from the thylakoid lumen to the stroma. The dpgr heterozygotes showed a semidominant and dominant phenotype in artificial and ambient air, respectively. In dpgr , the protein level of KEA 3 was unaffected but the downregulation of its activity was probably disturbed. Our findings suggest that fine regulation of KEA 3 activity is necessary for optimizing photosynthesis.