NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) is the key enzyme in 
the light-induced greening of higher plants. A unique light-harvesting 
POR:Pchlide complexes (LHPP) has been found in barley etioplasts, but not in 
other plant species. Why PORs from barley, but not from other plants, can form 
LHPP? And its function is not well understood. We modeled the barley and 
Arabidopsis POR proteins and compared molecular surface. The results confirm the 
idea that barley PORA can form a five-unit oligomer that interacts with a single 
PORB. Chemical treatment experiments indicated that POR complex may be formed by 
dithiol oxidation of cysteines of two adjacent proteins. We further showed that 
LHPP assembly was needed for barley POR functions and seedling greening. On the 
contrary, Arabidopsis POR proteins only formed dimers, which were not related to 
the functions or the greening. Finally, POR complex assembly (including LHPP and 
POR dimers) did not affect the formation of prolamellar bodies (PLBs) that 
function for efficient capture of light energy for photo conversion in 
etioplasts.