Moderate overexpression of PROTON GRADIENT REGULATION 5 improves photosynthetic performance and plant growth under fluctuating low light in Arabidopsis thaliana.

Cyclic electron transport around photosystem I (PSI) plays an important role in regulating photosynthesis by balancing ATP/NADPH production and maintaining PSI function under fluctuating light. Although PGR5-dependent photosynthetic regulation has been extensively studied under high or fluctuating light, its physiological significance under prolonged low irradiance interrupted by transient increases in light remains less well understood. Here, we analyzed Arabidopsis thaliana lines with different levels of PGR5 accumulation to investigate how PGR5 abundance influences photosynthetic regulation and plant performance under controlled fluctuating light conditions. Moderate PGR5 accumulation was associated with faster photosynthetic induction, improved PSI redox regulation, and greater biomass accumulation under the 100 μmol photons m-2 s-1 fluctuating light regime. In contrast, the absence of PGR5 severely impaired photosynthetic performance, particularly under the 600 μmol photons m-2 s-1 fluctuating light regime, whereas excessively high PGR5 accumulation did not provide additional physiological benefits. These results indicate that photosynthetic responses varied according to PGR5 abundance, but the relationship was not simply proportional across all genotypes. Our findings demonstrate that maintaining an appropriate level of PGR5 is important for efficient photosynthetic regulation under fluctuating light conditions. This study provides new physiological insights into the role of PGR5 under dynamic light environments and establishes a framework for future studies aimed at optimizing photosynthesis under naturally fluctuating irradiance.