NADPH:protochlorophyllide oxidoreductase (POR) catalyzes the light-dependent 
reduction of protochlorophyllide. To elucidate the physiological function of 
three differentially regulated POR isoforms (PORA, PORB and PORC) in Arabidopsis 
thaliana, we isolated T-DNA tagged null mutants of porB and porC. The mature 
seedlings of the mutants had normal photosynthetic competencies, showing that 
PORB and PORC are interchangeable and functionally redundant in developed 
plants. In etiolated seedlings, only porB showed a reduction in the photoactive 
protochlorophyllide and the size of prolamellar bodies (PLBs), indicating that 
PORB, as well as PORA, functioned in PLB assembly and photoactive 
protochlorophyllide formation in etiolated seedlings. When illuminated, the 
etiolated porB seedling was able to green to a similar extent as the wild type, 
whereas the greening was significantly reduced under low light conditions. 
During greening, high light irradiation increased the level of PORC protein, and 
the greening of porC was repressed under high light conditions. The porB, but 
not porC, etiolated seedling was more sensitive to the far-red block of greening 
than the wild type, which is caused by depletion of endogenous POR proteins 
resulting in photo-oxidative damage. These results suggest that, at the onset of 
greening, PLBs are important for efficient capture of light energy for 
photoconversion under various light conditions, and PORC, which is induced by 
high light irradiation, contributes to photoprotection during greening of the 
etiolated seedlings.