The regulation of the chloroplast proton motive force plays a key role for photosynthesis in fluctuating light

•During photosynthesis, a proton motive force (pmf) forms across thylakoid membranes.•The lumenal pH component of the pmf has important regulatory functions.•Specific pmf alterations rapidly adjust photosynthesis to changes in light intensity.•Defects in pmf regulation impair photosynthesis in dynamic light environments.

Plants use sunlight as their primary energy source. During photosynthesis, absorbed light energy generates reducing power by driving electron transfer reactions. These are coupled to the transfer of protons into the thylakoid lumen, generating a proton motive force (pmf) required for ATP synthesis. Sudden alterations in light availability have to be met by regulatory mechanisms to avoid the over-accumulation of reactive intermediates and maximize energy efficiency. Here, the acidification of the lumen, as an intermediate product of photosynthesis, plays an important role by regulating photosynthesis in response to excitation energy levels. Recent findings reveal pmf regulation and the modulation of its composition as key determinants for efficient photosynthesis, plant growth, and survival in fluctuating light environments.