Chlorophyll biosynthesis by mesophyll protoplasts and plastids from etiolated oat (Avena sativa L.) leaves

The uptake of [1-(3)H]geranylgeranyl diphosphate (GGPP) into protoplasts and intact etioplasts and the metabolic interconversion therein was studied after a 2 min pulse of white light. The chlorophyll synthetase reaction, Chlide+GGPP→ChlGG, was taken as a natural probe for the etioplast compartment. This reaction yields labeled ChLGG and, by hydrogenation, labeled ChlP, when [1-(3)H]GGPP receives access to the etioplast stroma. It was found that penetration across the plastid envelope was rapid and that penetration across the plasma membrane of protoplasts, however, was slow. A cellular pool of soluble GGPP was detected. This pool was lost, in part, during preparation of the protoplasts and almost completely during preparation of the etioplasts. The membrane-bound phytol pool of etioplasts could not be replaced by exogenous [(3)H]GG. The endogenous GG and phytol pools of protoplasts, which were larger than those of etioplasts, could be replaced in part by exogenous [(3)H]GGPP. That part of this pool exists as soluble GGPP or as a direct precursor in the cytoplasm is discussed.