Functional Analysis of Two Isoforms of Leaf-Type Ferredoxin-NADP + -Oxidoreductase in Rice Using the Heterologous Expression System of Arabidopsis

Ferredoxin-NADP+-oxidoreductase (FNR) mediates electron transfer between ferredoxin (Fd) and NADP+ and therefore it is a key enzyme that provides the reducing power used in the Calvin cycle. Other than FNR, nitrite reductase, sulfite reductase, glutamate synthase and Fd-thioredoxin reductase also accept electrons from Fd, an electron carrier protein in the stroma. Therefore, the regulation of electron partitioning in the chloroplast is important for photosynthesis and other metabolic pathways. The regulatory mechanism of electron partitioning, however, remains to be elucidated. We found, by taking advantage of a gain-of-function approach, that expression of two rice full-length cDNAs (fl-cDNAs) of leaf-type FNR (OsLFNR1, OsLFNR2) led to altered chlorophyll fluorescence and growth in Arabidopsis and rice. We revealed that overexpression of the OsLFNR1 and OsLFNR2 fl-cDNAs resulted in distinct phenotypes despite the high sequence similarity between them. Expression of OsLFNR1 affected the nitrogen assimilation pathway without inhibition of photosynthesis under normal conditions. On the other hand, OsLFNR2 expression led to impairment of photosynthetic linear electron transport as well as Fd-dependent cyclic electron flow around PSI. The endogenous protein level of OsLFNR was found to be suppressed in both OsLFNR1 and OsLFNR2 overexpressing rice plants, leading to changes in the stoichiometry of the two LFNR isoforms within the thylakoid and soluble fractions. Thus, we propose that the stoichiometry of two LFNR isoforms plays an important role in electron partitioning between carbon fixation and nitrogen assimilation.