Arabidopsis mutants lacking DLDG1 and non-photochemical quenching-related proteins reveal the regulatory role of DLDG1 in chloroplast pH homeostasis.

The control of pH in chloroplasts is important to regulate photosynthesis, although details of the precise regulatory mechanisms of H+ homeostasis in chloroplasts are not fully understood. We recently found that the cyanobacterial PxcA homolog DLDG1 is involved in plastidial pH control. PxcA and DLDG1 have been thought to control light-dependent H+ extrusion across the cyanobacterial cytoplasmic and chloroplast envelope membranes, respectively. To investigate DLDG1-dependent pH control in chloroplasts, we crossed the dldg1 mutant with various mutants lacking known non-photochemical quenching (NPQ)-related proteins, such as fluctuating-light acclimation protein 1 (FLAP1), PsbS/NPQ4, and proton gradient regulation 5 (PGR5). Phenotypes of these double mutants revealed that PsbS works upstream of DLDG1, PGR5 affects NPQ independently from DLDG1, and the ΔpH regulation by FLAP1 and DLDG1 are independent of each other.