A cytosolic ADP-glucose pyrophosphorylase is a feature of graminaceous endosperms, but not of other starch-storing organs

The occurrence of an extra-plastidial isoform of ADP-glucose (Glc) pyrophos phorylase (AGPase) among starch-storing organs was investigated in two ways . First, the possibility that an extra-plastidial isoform arose during the domestication of cereals was studied by comparing the intracellular distrib ution of enzyme activity and protein in developing endosperm of noncultivat ed Hordeum species with that previously reported for cultivated barley (Hor deum vulgare). As in cultivated barley, the AGPase of H. vulgare subsp. spo ntaneum and Hordeum murinum endosperm is accounted for by a major extra-pla stidial and a minor plastidial isoform. Second, the ratio of ADP-Glc to UDP -Glc was used as an indication of the intracellular location of the AGPase activity in a wide range of starch-synthesizing organs. The ratio is expect ed to be high in organs in which UDP-Glc and ADP-Glc are synthesized primar ily in the cytosol, because the reactions catalyzed by AGPase and UDP-Glc p yrophosphorylase will be coupled and close to equilibrium. This study revea led that ADP-Glc contents and the ratio of ADP-Glc to UDP-Glc were higher i n developing graminaceous endosperms than in any other starch-storing organ s. Taken as a whole the results indicate that an extra-plastidial AGPase is important in ADP-Glc synthesis in graminaceous endosperms, but not in othe r starch-storing organs.