Redox regulation of PGRL1 at the onset of low light intensity.

PGRL1 regulates photosystem I-cyclic electron flow which transiently activates nonphotochemical quenching at the onset of light. Here, we show that disulfide-based mechanism of PGRL1 regulated this process in vivo at the onset of low light levels. We found that PGRL1 regulation depended on active formation of key regulatory disulfide in the dark, and that PGR5 was required for this activity. The disulfide state of PGRL1 was modulated in plants by counteracting reductive and oxidative components and reached balanced state that depended on light level. We propose that the redox regulation of PGRL1 fine-tunes a timely activation of photosynthesis at the onset of low light.