Tanigawa, K.; Kodama, H.; Okegawa, Y.; Shikanai, T.; Yamori, W.

Cyclic electron transport (CET) around photosystem I (PSI) is essential for maintaining photosynthetic efficiency by balancing ATP/NADPH production and protecting PSI from photoinhibition. Although the PROTON GRADIENT REGULATION 5 (PGR5)-dependent CET pathway is known to be critical under high or fluctuating light conditions, its role under fluctuating low light remains poorly understood. In natural environments, plants frequently experience prolonged low irradiance interspersed with brief sunflecks, making fluctuating low light a physiologically relevant condition. Here, we investigated Arabidopsis thaliana lines with graded PGR5 expression levels to evaluate the dose-dependent contribution of PGR5 to CET activity, photosynthetic regulation, and growth performance under both low light and fluctuating low light conditions. Moderate increase in the PGR5 protein level enhanced CET activity, accelerated photosynthetic induction, improved PSI protection and increased biomass accumulation under fluctuating low light. In contrast, excessive PGR5 accumulation impaired photosynthetic performance and reduced plant growth, indicating that optimal CET capacity requires precise tuning of PGR5 abundance. These results reveal a non-linear relationship between PGR5 protein levels and photosynthetic performance and demonstrate that moderate enhancement of CET improves plant productivity under fluctuating low light. Our findings highlight the importance of optimizing CET capacity to match dynamic light environments and suggest that fine-tuning PGR5 expression could be a promising strategy for improving crop performance under natural canopy conditions.