Phototransformation of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). chlorophyllide a + NADP(+) = H(+) + NADPH + protochlorophyllide a Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Part of the FLU-containing chloroplast membrane complex composed of FLU, CRD1, PORB, PORC, CHLP and HEMA1 (PubMed:22212719). Interacts with CDF1 in chloroplast (PubMed:24151298). Expressed in flowers, upper leaves, rosette and cauline leaves, stem. Not detectable in non-photosynthetic tissues such as roots and seeds. Expressed in seedlings and adult plant. Expressed in etiolated seedlings. Circadian-regulation. Not regulated by light. No visible phenotype at the levels of the whole plant or chloroplast ultrastructure; due to the redundancy with PORC. Porb and porc double mutants have a seedling-lethal pale-yellow xantha phenotype at the cotyledon stage, contain only small amounts of Chla, and possess chloroplasts with mostly unstacked thylakoid membranes. The presence of TOC33 is required for the import of PORA into plastids in cotyledons and true leaves. Belongs to the short-chain dehydrogenases/reductases (SDR) family. POR subfamily.