Angiosperms require light for chlorophyll biosynthesis because one reaction in 
the pathway, the reduction of protochlorophyllide (Pchlide) to chlorophyllide, 
is catalyzed by the light-dependent protochlorophyllide oxidoreductase (POR). 
Here, we report that Cell growth defect factor1 (Cdf1), renamed here as 
chaperone-like protein of POR1 (CPP1), an essential protein for chloroplast 
development, plays a role in the regulation of POR stability and function. 
Cdf1/CPP1 contains a J-like domain and three transmembrane domains, is localized 
in the thylakoid and envelope membranes, and interacts with POR isoforms in 
chloroplasts. CPP1 can stabilize POR proteins with its holdase chaperone 
activity. CPP1 deficiency results in diminished POR protein accumulation and 
defective chlorophyll synthesis, leading to photobleaching and growth inhibition 
of plants under light conditions. CPP1 depletion also causes reduced POR 
accumulation in etioplasts of dark-grown plants and as a result impairs the 
formation of prolamellar bodies, which subsequently affects chloroplast 
biogenesis upon illumination. Furthermore, in cyanobacteria, the CPP1 homolog 
critically regulates POR accumulation and chlorophyll synthesis under high-light 
conditions, in which the dark-operative Pchlide oxidoreductase is repressed by 
its oxygen sensitivity. These findings and the ubiquitous presence of CPP1 in 
oxygenic photosynthetic organisms suggest the conserved nature of CPP1 function 
in the regulation of POR.