PGR5 promotes energy-dependent non-photochemical quenching to enable efficient C ₄ photosynthesis under fluctuating light

PROTON GRADIENT REGULATION 5 (PGR5) is essential for generating proton motive force across thylakoid membranes in C ₃ plants and supporting photoprotection under fluctuating light conditions. It is proposed that this function is achieved by regulating cyclic electron flow around Photosystem I. During the evolutionary transition from C ₃ to C ₄ photosynthesis, the leaf abundance of PGR5 has increased, coinciding with a rise in the cyclic electron flow rate. To investigate the contribution of PGR5 to photoprotection in C ₄ photosynthesis, we generated model C ₄ monocot Setaria viridis with null pgr5 alleles. We show that plants lacking PGR5 struggle to establish proton motive force and energy-dependent non-photochemical quenching (qE) at higher irradiances during instantaneous measurements. This leads to a progressive decline in maximum Photosystem I activity when leaves are exposed to repeated cycles of high irradiance. Additionally, plants without PGR5 exhibit severely reduced growth and photosynthesis compared to wild type plants when grown under fluctuating daylight but not under constant daylight. In the absence of PGR5, a slower-relaxing, zeaxanthin-dependent form of non-photochemical quenching supports growth under fluctuating light, albeit at the cost of reduced photochemical efficiency and assimilation rate. Our findings underscore the role of PGR5 in enabling efficient C ₄ photosynthesis under fluctuating light by establishing proton motive force for the rapid upregulation of qE and preventing photodamage to the electron transport machinery. This study highlights the importance of various non-photochemical quenching mechanisms for C ₄ photosynthesis and emphasises the role of PGR5 in the evolution of C ₄ plants.