NYB (Nanchong Yellow Barley) is a Chl-less barley mutant, which is controlled by a recessive nuclear gene. It is the only protochlorophyllide oxidoreductases (POR)-less barley mutant known in the world. The putative mechanism of the mutation and its Chl synthesis and plastid development are studied here. Neither PORC nor an additional copy of porB could be detected in barley. porB mRNAs are normally expressed and correctly spliced in the mutant. However, the import of PORA, PORB, LHCIIb1 (light harvesting complex II b1) and SSU (small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase) proteins to the plastid was greatly hampered in the mutant. We presume that a common translocon is mutated in NYB. The content of the supramolecular light-harvesting POR complex LHPP (light-harvesting NADPH:protochlorophyllide oxidoreductase:protochlorophyllide) and the density of prolamellar bodies in etioplasts are decreased in the mutant. However, no further oxidative damage could be observed for the de-etiolated mutant seedlings after a dark to light shift. Development of the plastid is arrested (less stacking) in NYB, and the mutant becomes more yellowish in high-light conditions, with dwarfing of seedlings and decreased yield. The physiological significance and developmental roles of POR proteins and LHPP in barley cells are discussed.