Regulation and levels of the thylakoid K⁺/H⁺ antiporter KEA3 shape the dynamic response of photosynthesis in fluctuating light

Crop canopies create environments of highly fluctuating light intensities. In such environments, photoprotective mechanisms and their relaxation kinetics have been hypothesized to limit photosynthetic efficiency and therefore crop yield potential. Here, we show that overexpression of the Arabidopsis thylakoid K⁺/H⁺ antiporter KEA3 accelerates the relaxation of photoprotective energydependent quenching after transitions from high to low light in Arabidopsis and tobacco. This, in turn, enhances PSII quantum efficiency in both organisms, supporting that in wild-type plants, residual light energy quenching following a high to low light transition represents a limitation to photosynthetic efficiency in fluctuating light. This finding underscores the potential of accelerating quenching relaxation as a building block for improving photosynthetic efficiency in the field. Additionally, by overexpressing natural KEA3 variants with modification to the C-terminus, we show that KEA3 activity is regulated by a mechanism involving its lumen-localized C-terminus, which lowers KEA3 activity in high light. This regulatory mechanism finetunes the balance between photoprotective energy dissipation in high light and maximum quantum yield in low light, likely to be critical for efficient photosynthesis in fluctuating light conditions.