Regulatory plasticity balances photosynthetic electron flow with enhanced pH-dependent cytochrome b6f control in Arabidopsis.

To avoid photodamage of photosystem I under fluctuating light, plants have evolved multiple photoprotective mechanisms. One key mechanism is photosynthetic control, in which acidification of the thylakoid lumen downregulates electron transport through the cytochrome b6f complex, thereby preventing overreduction of photosystem I. The Arabidopsis proton gradient regulation 5 (pgr5) mutant, which is defective in cyclic electron transport around photosystem I, fails to induce photosynthetic control and consequently suffers severe photosystem I photodamage under fluctuating light. Previously, we showed that introduction of the pgr1 mutation, which enhances the pH sensitivity of the cytochrome b6f complex, partially restored photosystem I oxidation and alleviated photosystem I photodamage in the pgr5 background. However, excessively strong photosynthetic control limits electron transport at relatively low light intensities. To investigate whether a milder enhancement of photosynthetic control can protect photosystem I without compromising photosynthetic performance, we introduced a series of amino acid substitutions into the Rieske subunit of the cytochrome b6f complex using Target-activation-induced cytidine deaminase (AID)-mediated base editing. Among these, the E143K mutation partially oxidized photosystem I and improved photosynthetic induction in the pgr5-2 background more effectively than the pgr1 mutation. Although the E143K mutation had little effect on electron transport parameters in the wild-type background, it significantly reduced the proton motive force. The unexpected reduction in proton motive force suggests that moderately enhanced photosynthetic control can be accommodated without major impairment of photosynthetic electron transport.