Induced Mutations in the Starch Branching Enzyme II (SBEII) Genes Increase Amylose and Resistant Starch Content in Durum Wheat

Wheat (Triticum aestivum L.) provides approximately one fifth of the calories consumed by the human population.  Since starch is the major component of the wheat kernel (~50-70% of its dry weight), any improvements in its nutritional composition have the potential to deliver benefits to a large number of people. Starch in the wheat grain consists of approximately 70-80% amylopectin and 20-30% amylose. Amylopectin is a highly-branched, readily digested polysaccharide, whereas amylose has few branches and forms complexes that resist digestion and mimic dietary fiber (resistant starch). As with fiber, increased consumption of resistant starch has been associated with reduced risk of diseases such as diabetes, obesity, heart disease, and cancers of the colon and rectum. Down-regulation of the starch branching enzyme II (SBEII) gene by RNA interference (RNAi) was previously shown to increase amylose content in both hexaploid and tetraploid wheat. We generated ethyl methane sulphonate (EMS) mutants for the SBEIIa-A and SBEIIa-B homoeologs in the tetraploid durum wheat variety Kronos (T. turgidum ssp. durum L.). Single-gene mutants showed non-significant increases in amylose and resistant starch content, but a double mutant combining a SBEIIa-A knock-out mutation with a SBEIIa-B splice-site mutation showed a 22% increase in amylose content (P<0.0001) and a 115% increase in resistant starch content (P<0.0001). In addition, we obtained mutants for the A and B genome copies of the paralogous SBEIIb gene, mapped them 1-2 cM from SBEIIa, and generated double SBEIIa-SBEIIb mutants to study the effect of the SBEIIb gene in the absence of SBEIIa.