PGRL1A redox states alleviate photoinhibition in Arabidopsis during step changes in light intensity

Non-motile plants have evolved regulatory mechanisms to maintain homeostasis for optimal growth. Responses to environmental changes in light are particularly important not only during the diurnal transition from night to day but also to react to light changes caused by passing clouds or by wind. Thioredoxins rapidly orchestrate redox control during environmental change by modifying cysteine residues. Here, we assign a function to regulatory cysteines of PGRL1A, a constituent of the ferredoxin-dependent cyclic electron flow (Fd-CEF) pathway and show their role in the regulation of proton motive force (PMF) and nonphotochemical quenching (NPQ). During step increase of low light intensity (10-60 μE*m -2 *s -1 ), the intermolecular disulfide of the PGRL1A 59-kDa complex is reduced transiently within seconds to the 28 kDa form. In contrast, step increases to higher light intensity (60-600 μE*m -2 *s -1 ) stimulated a stable partially reduced redox state in PGRL1A. Measurements of NPQ, PMF and resultant photosynthetic controls Y(ND) and Y(NA) were found to correlate with the redox state of PGRL1A during step increases in light intensity but not in PGRL1mutant plants pgrl1ab or PGRL1A cysteine mutant ( PGRL1A C1,2A ). Continuous light regimes did not affect mutant growth; however, fluctuating regimes of light intensity showed significant growth reduction in the mutants. Inhibitors of photosynthesis placed control of the PGRL1A redox state as dependent on the penultimate ferredoxin redox state that fuels reducing equivalents to the large set of chloroplasts thioredoxins. Our results showed that redox state changes in PGRL1A are crucial to the optimization of photosynthesis and are regulated by the photosynthetic electron flux.