A key reaction in the biosynthesis of chlorophylls (Chls) a and b from cyanobacteria through higher plants is the strictly light-dependent reduction of protochlorophyllide (Pchlide) a to chlorophyllide (Chlide) a. Angiosperms, unlike other photosynthetic organisms, rely exclusively upon this mechanism to reduce Pchlide and hence require light to green. In Arabidopsis, light-dependent Pchlide reduction is mediated by three structurally related but differentially regulated NADPH:Pchlide oxidoreductases, denoted as PORA, PORB, and PORC. The PORA and PORB genes, but not PORC, are strongly expressed early in seedling development. In contrast, expression of PORB and PORC, but not PORA, is observed in older seedlings and adult plants. We have tested the hypothesis that PORB and PORC govern light-dependent Chl biosynthesis throughout most of the plant development by identifying porB and porC mutants of Arabidopsis, the first higher plant por mutants characterized. The porB-1 and porC-1 mutants lack the respective POR transcripts and specific POR isoforms because of the interruption of the corresponding genes by a derivative of the maize Dissociation (Ds) transposable element. Single por mutants, grown photoperiodically, display no obvious phenotypes at the whole plant or chloroplast ultrastructural levels, although the porB-1 mutant has less extensive etioplast inner membranes. However, a light-grown porB-1 porC-1 double mutant develops a seedling-lethal xantha phenotype at the cotyledon stage, contains only small amounts of Chl a, and possesses chloroplasts with mostly unstacked thylakoid membranes. PORB and PORC thus seem to play redundant roles in maintaining light-dependent Chl biosynthesis in green plants, and are together essential for growth and development.