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

<jats:p> 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 <jats:italic>in vivo</jats:italic> and <jats:italic>in vitro</jats:italic> by using homozygous loss-of-function mutants in genes coding for chlorophyllide <jats:italic>a</jats:italic> 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 <jats:italic>in vivo</jats:italic> analyses show that <jats:italic>cao</jats:italic> or <jats:italic>oep16</jats:italic> mutants contain a normally structured prolamellar body that contains the protochlorophyllide holochrome. Furthermore, etioplasts from <jats:italic>cao</jats:italic> and <jats:italic>oep16</jats:italic> 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 <jats:italic>in vivo</jats:italic> and <jats:italic>in vitro</jats:italic> as further indicated by protein import studies. </jats:p>