NUCLEOTIDES AND NUCLEOTIDE SUGARS IN DEVELOPING MAIZE ENDOSPERMS - SYNTHESIS OF ADP-GLUCOSE IN BRITTLE-1

As part of an in vivo study of carbohydrate metabolism during developm ent of Zea mays L. kernels, quantities of nucleotides and nucleotide s ugars were measured in endosperm extracts from normal, the single-muta nt genotypes shrunken-1 (sh1), shrunken-2 (sh2), and brittle-1 (bf1), and the multiple-mutant genotypes sh1bt1, sh2bt1, and sh1sh2bt1. Resul ts showed that bt1 kernels accumulated more than 13 times as much aden osine 5' diphosphoglucose (ADP-Glc) as normal kernels. Activity of sta rch synthase in bt1 endosperm was equal to that in endosperm extracts from normal kernels. Thus the ADP-Glc accumulation in bt1 endosperm ce lls was not due to a deficiency in starch synthase. ADP-Glc content in extracts of sh1bt1 endosperms was similar to that in bt1, but in extr acts of the sh2bt1 mutant kernels ADP-Glc content was much reduced com pared to bt1 (about 3 times higher than that in normal). Endosperm ext racts from sh1sh2bt1, kernels that are deficient in both ADP-Glc pyrop hosphorylase (AGPase) and sucrose synthase, had quantities of ADP-Glc much lower than in normal kernels. These results clearly indicate that ACPase is the predominant enzyme responsible for the in vivo synthesi s of ADP-Glc in bt1 mutant kernels, but Suc synthase may also contribu te to the synthesis of GDP-Glc in kernels deficient in ACPase.