The enzyme participates in the flavonoid biosynthesis pathway found in plants. It catalyses the double reduction of anthocyanidins, producing (2R,3R)-flavan-3-ol monomers required for the formation of proanthocyanidins. While the enzyme from the legume Medicago truncatula (MtANR) can use both NADPH and NADH as reductant, that from the crucifer Arabidopsis thaliana (AtANR) uses only NADPH. Also, while the substrate preference of MtANR is cyanidin>pelargonidin>delphinidin, the reverse preference is found with AtANR. cf. EC 1.3.1.112, anthocyanidin reductase [(2S)-flavan-3-ol-forming].