Ion antiport accelerates photosynthetic acclimation in fluctuating light environments

Many photosynthetic organisms globally, including crops, forests and algae, must grow in environments where the availability of light energy fluctuates dramatically. How photosynthesis maintains high efficiency despite such fluctuations in its energy source remains poorly understood. Here we show that Arabidopsis thaliana K + efflux antiporter (KEA3) is critical for high photosynthetic efficiency under fluctuating light. On a shift from dark to low light, or high to low light, kea3 mutants show prolonged dissipation of absorbed light energy as heat. KEA3 localizes to the thylakoid membrane, and allows proton efflux from the thylakoid lumen by proton/potassium antiport. KEA3’s activity accelerates the downregulation of pH-dependent energy dissipation after transitions to low light, leading to faster recovery of high photosystem II quantum efficiency and increased CO 2 assimilation. Our results reveal a mechanism that increases the efficiency of photosynthesis under fluctuating light. Plants must respond rapidly to unpredictable variations in light intensity to maximize photosynthetic efficiency. Here Armbruster et al. identify a potassium antiporter that is critical for accelerating proton fluxes across thylakoid membranes and minimizing energy loss in fluctuating light conditions.