A rice Wx gene encoding a granule-bound starch synthase I (GBSSI) was introduced into the null-mutant waxy (wx) rice, and its effect on endosperm starches was examined. The apparent amylose content was increased from undetectable amounts for the non-transgenic wx cultivars to 21.6-22.2% of starch weight for the transgenic lines. The increase was in part due to a significant amount of extra-long unit chains (ELCs) of amylopectin (7.5-8.4% of amylopectin weight), that were absent in the non-transgenic wx cultivars. Thus, actual amylose content was calculated to be 14.9-16.0% for the transgenic lines. Only slight differences were found in chain-length distribution for the chains other than ELCs, indicating that the major effect of the Wx transgene on amylopectin structure was ELC formation. ELCs isolated from debranched amylopectin exhibited structures distinct from amylose. Structures of amylose from the transgenic lines were slightly different from those of cv. Labelle (Wx(a)) in terms of a higher degree of branching and size distribution. The amylose and ELC content of starches of the transgenic lines resulted in the elevation of pasting temperature, a 50% decrease in peak viscosity, a large decrease in breakdown and an increase in setback. As yet undetermined factors other than the GBSSI activity are thought to be involved in the control of formation and/or the amount of ELCs. Structural analysis of the Wx gene suggested that the presence of a tyrosine residue at position 224 of GBSSI correlates with the formation of large amounts of ELCs in cultivars carrying Wx(a).

Long chains (LC) were fractionated by 1-butanol precipitation from isoamylase-debranched amylopectins of rice, maize, wheat, buckwheat, and sweet potato. The structure of the precipitate (LCppt) was characterized by high-performance size-exclusion chromatography (HPSEC) with pre-column labeling with 2 aminopyridine. HPSEC showed that LCppt corresponded to the LC fraction of unit-chain distribution of amylopectin. LCppt had a number-average degree of polymerization of 330 490 and an average-number of chains of 1.2 1.4, indicating that the structure of LCppt is very similar irrespective of its botanical sources and is distinct from amylose. HPSEC revealed that size distribution of LCppt was also similar among the specimens examined. The distribution was distinct from and much narrower than that of amylose. Still, LCppt consisted of a significant amount (by weight) of chains as long as amylose. LCppt contained 5.1 9.4 mol% of branched molecules, which had 5 chains per molecule (except for 10 of indica rice IR36), indicating incomplete debranching of the side chains on LC by isoamylase. The structure of the isoamylase-resistant branches is unknown. In this respect, LCppt and amylose appear to have a common branched structure, which is unusual in starch α-glucans.