What enzyme catalyzes the reduction of NADP+ to NADPH in the light reactions?

Ferredoxin-NADP+ reductase catalyzes the reduction of NADP+ to NADPH in the light reactions. It receives electrons from reduced ferredoxin, the carrier that accepts high-energy electrons from photosystem I, and transfers them to NADP+ using a flavin adenine dinucleotide (FAD) cofactor.

FNR is the final major electron-transfer enzyme in linear photosynthetic electron flow. Its NADPH product supplies reducing power for carbon fixation and other biosynthetic reactions in the chloroplast stroma. The enzyme generally transfers two one-electron inputs from ferredoxin to the two-electron carrier NADP+.

A common mix-up is confusing FNR with ferredoxin itself. Ferredoxin carries electrons, while FNR performs the enzymatic transfer that produces NADPH. ATP synthase makes ATP, Rubisco fixes carbon dioxide, and plastocyanin transfers electrons earlier, between cytochrome b6f and photosystem I. FNR can also operate in reverse in some organisms, using NADPH to reduce ferredoxin for other metabolic pathways.