| Wheat endosperm starch represents a major nutrient in the human diet providing an important source of energy. Wheat starch has approximately 75% amylopectin and 25% amylose. Amylose is not readily digested in the small intestine and results in resistant starch, a dietary fiber associated with health benefits. Down regulation of genes involved in amylopectin biosynthesis, such as the Starch Branching Enzyme II (SBEII), can increase amylose and resistant starch content. In each wheat genome, there are two isoforms of this gene, SBEIIa and SBEIIb , which are closely linked. The focus of this dissertation was the characterization of the effects of combined SBEII mutations in common wheat (Triticum aestivum subsp. aestivum ) and durum wheat (T. turgidum subsp. durum ). The first study characterized the effects of combining mutations in five out of the six SBEII genes present in the common wheat variety Lassik. These mutations produced significant increases in amylose (63%) and resistant starch (1057%), reduced total starch (7.8%) and drastically altered starch viscosity parameters. This led to the second study, which quantified the effect of the same five mutations on common wheat grain yield and bread-making quality. The five combined SBEII mutations were associated with significant reductions in grain yield (5.8%), kernel weight (2.8%), flour extraction rate (21.3%) and loaf weight (26.2%) and increases in kernel hardness (25.0%), flour protein (9.5%), flour ash (22.7%), falling number (21.4%), starch damage (31.7%) and crumb 'b' color (36.5%). The third study estimated the effect of four combined SBEII mutations on the utilization and degradability of transitory starch in leaves during the day and in reserve starch in endosperm during germination. The starch synthesized in the leaves of the SBEII mutants during the day showed a 555% higher amylose content than the wild-type. This starch was not efficiently degraded during the night, suggesting that the SBEII mutant plants had reduced access to the synthesized starch.;This limitation was association with significant reductions in aerial biomass and delayed heading time. In summary, this dissertation generated germplasm and information that can be useful to expedite the deployment of high resistant starch wheat. |
