In angiosperms, chlorophyll biosynthesis is light dependent. A key factor in 
this process is protochlorophyllide oxidoreductase (POR), which requires light 
to catalyze the reduction of protochlorophyllide to chlorophyllide. It is 
believed that this protein originated from an ancient cyanobacterial enzyme that 
was introduced into proto-plant cells during the primary symbiosis. Here we 
report that PORs from the cyanobacteria Gloeobacter violaceus PCC7421 and 
Synechocystis sp. PCC6803 function in plastids. First, we found that the G. 
violaceus POR shows a higher affinity to its substrate protochlorophyllide than 
the Synechocystis POR but a similar affinity to plant PORs. Secondly, the 
reduced size of prolamellar bodies caused by a knockdown mutation of one of the 
POR genes, PORA, in Arabidopsis could be complemented by heterologous expression 
of the cyanobacterial PORs. Photoactive protochlorophyllide in the etioplasts of 
the complementing lines, however, was retained at a low level as in the parent 
PORA knockdown mutant, indicating that the observed formation of prolamellar 
bodies was irrelevant to the assembly of photoactive protochlorophyllide. This 
work reveals a new view on the formation of prolamellar bodies and provides new 
clues about the function of POR in the etioplast-chloroplast transition.