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 cyanobacteriaGloeobacter violaceusPCC7421 andSynechocystissp. PCC6803 function in plastids. First, we found that theG. violaceusPOR shows a higher affinity to its substrate protochlorophyllide than theSynechocystisPOR but a similar affinity to plant PORs. Secondly, the reduced size of prolamellar bodies caused by a knockdown mutation of one of thePORgenes,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 parentPORAknockdown 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.