In angiosperms the strictly light-dependent reduction of protochlorophyllide to 
chlorophyllide is catalyzed by NADPH:protochlorophyllide oxidoreductase (POR). 
The Arabidopsis thaliana genome encodes three structurally related but 
differentially regulated POR genes, PORA, PORB and PORC. PORA is expressed 
primarily early in development-during etiolation, germination and greening. In 
contrast, PORB and PORC are not only expressed during seedling development but 
also throughout the later life of the plant, during which they are responsible 
for bulk chlorophyll synthesis. The Arabidopsis porB-1 porC-1 mutant displays a 
severe xantha (highly chlorophyll-deficient) phenotype characterized by smaller 
prolamellar bodies in etioplasts and decreased thylakoid stacking in 
chloroplasts. Here we have demonstrated the ability of an ectopic PORA 
overexpression construct to restore prolamellar body formation in the porB-1 
porC-1 double mutant background. In response to illumination, light-dependent 
chlorophyll production, thylakoid stacking and photomorphogenesis are also 
restored in PORA-overexpressing porB-1 porC-1 seedlings and adult plants. An 
Arabidopsis porB-1 porC-1 double mutant can therefore be functionally rescued by 
the addition of ectopically expressed PORA, which suffices in the absence of 
either PORB or PORC to direct bulk chlorophyll synthesis and normal plant 
development.