Chloroplasts, ADPGlc PPase

An alternate pathway for starch synthesis has been proposed, which is based on the finding of a putative ADPGlc translocator in the envelope of both amyloplasts and chloroplasts. Akazawa et al. (1991) proposed that ADPglucose is synthesized in the cytosol by the sucrose synthase (rather than in the plastid by the action of the ADPGlc PPase, as is widely accepted), and is then transported into the plastid where it is converted into starch by the starch synthase. A critique of this hypothesis is presented in the chapter, The Site of Starch Synthesis in Nonphotosynthetic Plant Tissues The Amyloplast, where metabolite transport into the plastids is discussed, but it is worth mentioning here that this pathway does not fulfill the criteria mentioned in the preceding—that is, the experimental evidence does not support this alternative pathway. 

Few studies on the localization of the starch biosynthetic enzymes were done before 1978, when it was found that ADPGlc PPase was located exclusively in the chloroplast fraction in both spinach (Mares et al., 1978) and pea (Levi and Preiss, 1978). The first detailed study was done by Okita et al. (1979), in which spinach leaf chloroplasts were isolated either by differential centrifugation (Walker, 1971 see also later) or from protoplasts (Nishimura et al, 1976). These plastid preparations contained essentially all of the activity of the starch biosynthetic enzymes, ADPGlc PPase, starch synthase, and branching enzyme. Subsequently, in guard cells of Commelina communis, Robinson and Preiss (1987) showed that the starch biosynthetic enzymes were present exclusively in the chloroplast fraction. 

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