The biosynthesis of chlorophyll is a strictly light-dependent multistep process 
in higher plants. The light-dependent step is catalysed by 
NADPH:protochlorophyllide oxidoreductase (POR, EC.1.6.99.1), which reduces 
protochlorophyllide (Pchlide) to chlorophyllide (Chlide). POR is nucleus-encoded 
and post-translationally imported into plastids. It has been proposed that the 
import of a POR protein isozyme (PORA) is totally dependent on Pchlide and uses 
a novel import pathway. This proposal is based on findings that PORA import only 
occurs in the presence of Pchlide and that the presence of overexpressed 
precursor of Rubisco small subunit (pSS), a protein which is known to use the 
general import pathway, does not outcompete PORA import. Another study 
demonstrated that POR precursor protein (pPOR) can be cross-linked to one of the 
components in the translocation machinery, Toc75, in the absence of Pchlide, and 
that its import can be outcompeted by the addition of the pSS. This indicates 
that pSS and pPOR may use the same translocation mechanism. Thus, POR does not 
necessarily need Pchlide for import--which is in contrast to earlier 
observations--and the exact POR import mechanism remains unresolved. Once in the 
stroma, the POR transit peptide is cleaved off and the mature POR protein is 
associated to the plastid inner membranes. Formation of the correct 
membrane-associated, thermolysin-protected assembly is strictly dependent of 
NADPH. As a final step, the formation of the NADPH-Pchlide-POR complex occurs. 
When POR accumulates in the membranes of proplastids, an attraction of 
monogalactosyl diacylglycerol (MGDG) can occur, leading to the formation of 
prolamellar bodies (PLBs) and the development of etioplasts in darkness.