Balancing chloroplast redox status – regulation of FNR binding and release

Working in synchrony, photosynthetic charge separation, electron transfer, and redox reactions generate proton motive force necessary for the synthesis of ATP and funneling of electrons toward stromal reducing equivalent NADPH. The last step of electron transfer from ferredoxin to NADP is catalyzed by ferredoxin-NADP oxidoreductase (FNR). Two proteins, TROL (thylakoid rhodanese-like) and Tic62 (62 kDa component of the translocon at the inner envelope of chloroplasts), have been characterized and shown to form dynamic complexes with FNR. Inactivation of TROL leads to changes in efficiency of electron transfer and induction of non-photochemical quenching. TROL-deficient plants have changed nuclear gene expression with up-regulation of NADPH-dependent malic enzyme, which can form NADPH in an alternative pathway. Thus, NADPH synthesis, mediated by FNR-TROL interaction, may be the source element in metabolic retrograde signal-transduction pathway linking light reactions with nuclear gene expression.