Flavodiiron Protein Substitutes for Cyclic Electron Flow without Competing CO₂ Assimilation in Rice

Abstract

Flavodiiron protein (FLV) mediates photoreduction of O₂ to H₂O. It is conserved from cyanobacteria to gymnosperms but not in angiosperms. The introduction of a moss (<i>Physcomitrella patens</i>) <i>FLV</i> (<i>PpFLV</i>) gene into Arabidopsis (<i>Arabidopsis thaliana</i>) made photosystem I (PSI) resistant to fluctuating light. Here, we used the same strategy with three rice (<i>Oryza sativa</i>) genotypes. PpFLV in the wild-type rice background functioned as an efficient PSI electron sink and increased resistance to PSI photodamage under fluctuating light. The introduction of PpFLV into the <i>PGR5</i>-RNAi mutant [defective in PROTON GRADIENT REGULATION5 (PGR5)-dependent cyclic electron transport around PSI, CET-PSI], the <i>crr6</i> mutant [defective in chloroplast NAD(P)H-dehydrogenase-like complex (NDH)-dependent CET-PSI], and the <i>PGR5</i>-RNAi <i>crr6</i> double mutant (double defective in CET-PSI activity) alleviated PSI photodamage under fluctuating light. Furthermore, PpFLV substituted for the function of PGR5- and NDH-dependent CET-PSI without competing for CO₂ assimilation under constant light, as there was no difference in CO₂ assimilation per Rubisco content and biomass production was recovered to the wild-type level. Thus, the exogenous FLV system could act not only as a safety valve under fluctuating light, but also generate a proton motive force for balancing the ATP/NADPH production ratio during steady-state photosynthesis.