Regulatory plasticity balances photosynthetic electron flow with enhanced pH-dependent cytochrome b 6 f control

To avoid photodamage of photosystem I (PSI) 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 (Cyt b6f) complex, thereby preventing over-reduction of PSI. The Arabidopsis proton gradient regulation 5 (pgr5) mutant, which is defective in cyclic electron transport around PSI, fails to induce photosynthetic control and consequently suffers severe PSI photodamage under fluctuating light. Previously, we showed that introduction of the pgr1 mutation, which enhances the pH sensitivity of the Cyt b6f complex, partially restored PSI oxidation and alleviated PSI 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 PSI without compromising photosynthetic performance, we introduced a series of amino acid substitutions into the Rieske subunit of the cytochrome b6f complex using Target-AID-mediated base editing. Among these, the E143K mutation partially oxidized PSI 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.