2 ISOFORMS OF THE GBSSI CLASS OF GRANULE-BOUND STARCH SYNTHASE ARE DIFFERENTIALLY EXPRESSED IN THE PEA PLANT (PISUM-SATIVUM L.)

In addition to the GBSSI isoform of starch synthase described previous ly, the pea plane contains a second, granule-bound isoform, GBSSIb, GB SSI is abundant in pea embryos and Rhizobium root nodules, is present at low levels in pods and is absent from leaves. Mutations at the lam locus eliminate GBSSI from all of these organs. GBSSIb is present in p ods, leaves and nodules and is unaffected by mutations at the lam locu s. GBSSI and GBSSIb are very similar ire molecular mass, primary seque nce, activity and antigenic properties. GBSSIb, like GBSSI, can synthe size amylose in the presence of malto-oligosaccharides in isolated sta rch granules. However, its role in vivo is unclear. The lam mutation e liminates amylose from the starch of embryos but does not effect the r elatively small amounts of amylose-like material in the starch of pods , leaves and nodules, The significance of these results for understand ing of the regulation of amylose synthesis is discussed.