Ferredoxin-NADP+ reductases (FNRs, EC 1.18.1.2) constitute a family of hydrophilic, monomeric enzymes that contain non-covalently bound FAD as prosthetic group. These flavoenzymes deliver NADPH or low potential one-electron donors (ferredoxin, flavodoxin, adrenodoxin) to redox-based metabolisms in plastids, mitochondria and bacteria. The main physiological role of the chloroplast FNR is to catalyze the final step of photosynthetic electron transport, namely, the electron transfer from the ferredoxin (Fd), reduced by photosystem I, to NADP+ (Eqn. 1) (Shin & Arnon, 1965). This reaction provides the NADPH necessary for CO2 assimilation in plants and cyanobacteria. FNRs also participate in others electron transfer metabolic processes as nitrogen fixation, isoprenoid biosynthesis, steroid metabolism, xenobiotic detoxification, oxidative-stress response and iron-sulfur cluster biogenesis (Carrillo & Ceccarelli, 2003, Ceccarelli et al., 2004, Medina & Gomez-Moreno, 2004, Rohrich et al., 2005, Seeber et al., 2005). Eqn. 1 represents the electron flow through FNR as it occurs in the photosynthetic electron chain. However, the physiological direction of the reaction catalyzed by FNRs involved in the other pathways is opposite, i.e. toward the production of reduced Fd. On this basis, FNRs are sometimes classified as autotrophic (photosynthetic FNRs) and heterotrophic (all other FNRs) (Aliverti et al., 2008, Arakaki et al., 1997).