Chloroplast biogenesis: The use of mutants to study the etioplast-chloroplast transition

In angiosperm plants, the etioplast-chloroplast transition is light-dependent. A key factor in this process is the protochlorophyllide oxidoreductase A (PORA), which catalyzes the light-induced reduction of protochlorophyllide to chlorophyllide. The import pathway of the precursor protein prePORA into chloroplasts was analyzed in vivo and in vitro by using homozygous loss-of-function mutants in genes coding for chlorophyllide a oxygenase (CAO) or for members of the outer-envelope solute-channel protein family of 16 kDa (OEP16), both of which have been implied to be key factors for the import of prePORA. Our in vivo analyses show that cao or oep16 mutants contain a normally structured prolamellar body that contains the protochlorophyllide holochrome. Furthermore, etioplasts from cao and oep16 mutants contain PORA protein as found by mass spectrometry. Our data demonstrate that both CAO and OEP16 are dispensable for chloroplast biogenesis and play no central role in the import of prePORA in vivo and in vitro as further indicated by protein import studies.