PGR5-dependent cyclic electron transport around PSI contributes to the redox homeostasis in chloroplasts rather than CO₂ fixation and biomass production in rice

The PGR5 (PROTON GRADIENT REGULATION 5) gene that is required for PSI cyclic electron transport in Arabidopsis was knocked down in rice (Oryza sativa). In three PGR5 knockdown (KD) lines, the PGR5 protein level was reduced to 5-8% of that in the wild type, resulting in a 50% reduction in PGRL1 (PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE 1) protein levels. In ruptured chloroplasts, ferredoxin-dependent plastoquinone reduction activity was partially impaired; the phenotype was mimicked by addition of antimycin A to wild-type chloroplasts. As occurred in the Arabidopsis pgr5 mutant, non-photochemical quenching of Chl fluorescence (NPQ) induction was impaired in the leaves, but the electron transport rate (ETR) was only mildly affected at high light intensity. The P700⁺ level was reduced even at low light intensity, suggesting that the PGR5 function was severely disturbed as in the Arabidopsis pgr5 mutant and that the other alternative routes of electrons could not compensate the stromal redox balance. The amplitude of the light-dark electrochromic shift (ECS) signal (ECSt), which reflects the total size of the proton motive force in steady-state photosynthesis, was reduced by 13-25% at approximately the growth light intensity. The CO₂ fixation rate was only slightly reduced in the PGR5 KD lines. Despite the drastic reduction in NPQ and P700⁺ levels, total biomass was only slightly reduced in PGR5 KD lines grown at 370 μmol photons m^-2 s^-1. These results suggest that CO₂ fixation and growth rate are very robust in the face of alterations in the fundamental reactions of photosynthesis under constant light conditions in rice.