Collaboration between NDH and KEA3 Allows Maximally Efficient Photosynthesis after a Long Dark Adaptation

In angiosperms, the NADH dehydrogenase-like (NDH) complex mediates cyclic electron transport around PSI (CET). K⁺ Efflux Antiporter3 (KEA3) is a putative thylakoid H⁺/K⁺ antiporter and allows an increase in membrane potential at the expense of the ∆pH component of the proton motive force. In this study, we discovered that the chlororespiratory reduction2-1 (crr2-1) mutation, which abolished NDH-dependent CET, enhanced the kea3-1 mutant phenotypes in Arabidopsis (Arabidopsis thaliana). The NDH complex pumps protons during CET, further enhancing ∆pH, but its physiological function has not been fully clarified. The observed effect only took place upon exposure to light of 110 µmol photons m^-2 s^-1 after overnight dark adaptation. We propose two distinct modes of NDH action. In the initial phase, within 1 min after the onset of actinic light, the NDH-dependent CET engages with KEA3 to enhance electron transport efficiency. In the subsequent phase, in which the ∆pH-dependent down-regulation of the electron transport is relaxed, the NDH complex engages with KEA3 to relax the large ∆pH formed during the initial phase. We observed a similar impact of the crr2-1 mutation in the genetic background of the PROTON GRADIENT REGULATION5 overexpression line, in which the size of ∆pH was enhanced. When photosynthesis was induced at 300 µmol photons m^-2 s^-1, the contribution of KEA3 was negligible in the initial phase and the ∆pH-dependent down-regulation was not relaxed in the second phase. In the crr2-1 kea3-1 double mutant, the induction of CO₂ fixation was delayed after overnight dark adaptation.