[en] The etioplast plastid type of dark-grown angiosperms is defined by the accumulation of the chlorophyll (Chi) precursor protochlorophyllide (Pchlide) and the presence of the paracrystalline prolamellar body (PLB) membrane. Both features correlate with the presence of NADFH:Pchlide oxidoreductase (FOR), a light-dependent enzyme that reduces photoactive Pchlide-F655 to chlorophyllide and plays a key role in chloroplast differentiation during greening. Two differentially expressed and regulated FOR enzymes, FORA and FORB, have recently been discovered in angiosperms. To investigate the hypothesis that etioplast differentiation requires FORA, we have constitutively overexpressed PORA and FORB in the Arabidopsis wild type and in the constitutive photomorphogenic cop1-18 (previously det340) mutant, which is deficient in the PLB and Pchlide-F655. In both genetic backgrounds, FOR overexpression increased PLB size, the ratio of Pchlide-F655 to nonphotoactive Pchl[ide]-F632, and the amount of Pchlide-F655. Dramatically, restoration of either FORA or FORB to the cop1 mutant led to the formation of etioplasts containing an extensive PLB and large amounts of photoactive Pchlide-F655.