Characterization of starch branching enzyme II mutants in durum wheat

Wheat (Triticum ssp.) starch consists of approximately 70 to 80% amylopectin, a highly branched D-glucose polysaccharide, and 20 to 30% amylose, a linear helical chain of D-glucose with few branching points. High-amylose starch is associated with increased levels of resistant starch, an important component of dietary fiber with numerous health benefits. Amylose content in the wheat grain can be increased by down regulating starch branching enzyme II (SBEII) genes involved in amylopectin biosynthesis. In cereals there are two SBEII paralogs ( SBEIIa and SBEIIb) and the natural variation in these genes is limited in wheat. The primary focus of this dissertation was quantifying the effects of mutant SBEII alleles in durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] on amylose and resistant starch content, pasta quality and agronomic performance. The first study showed that mutant SBEIIa alleles increased amylose content by 22% and resistant starch by 115%. It also showed that the SBEIIb paralog was less than 2 cM from SBEIIa. The combination of SBEIIa and SBEIIb mutant alleles by recombination resulted in substantially greater effects than the single-homoeolog mutants in amylose content (66%) and resistant starch (~750%). The combined SBEII mutations had pleiotropic effects on grain weight (5% reduction) and pasta quality (increased pasta firmness, and increased ash content which reduced color scores). Finally, a rat feeding study demonstrated beneficial gastrointestinal fermentation responses associated with the inclusion of grain from the SBEII mutants in the diet. In summary, the work presented in this dissertation provides valuable information regarding the benefits and limitations associated with SBEII mutations that can be used to accelerate the deployment of commercial wheat varieties carrying high-amylose and resistant starch traits.