Improving Durum Wheat Quality Via Mutagenesis of Starch Synthase IIa.

In cereal seeds, starch biosynthesis involves a variety of enzymes.  Amylopectin is produced from the cooperation of soluble starch synthases, starch branching and de-branching enzymes whereas amylose is produced exclusively by granule bound starch synthase I.  Mutations in one or more durum wheat (Triticum turgidum spp. durum) non-granule bound starch synthases led to decreased amylopectin and increased amylose content.  Increased amylose is an important characteristic as it can increase pasta quality by increasing firmness as well as lowering glycemic index.  The impact of mutations in starch synthase IIa-A (ssIIa-A) and ssIIa-B on durum starch was investigated.  A screen of durum accessions starch granule proteins (SGPs) identified two lines missing SSIIa-A1 (or SGP-A1) which were determined to carry the same 29 bp deletion in the first exon of ssIIa-A.  The SGP-A1 nulls were each crossed with the durum cultivar ‘Mountrail’ and F₅ derived SGP-A1 null progeny lines were treated with EMS.  In each EMS population, one line was discovered that had a missense mutation in ssIIa-B and was missing SGP-B1.  Each of the SGP-1 double nulls was found to have significant increases in amylose content and reduced binding of SGP-2 and SGP-3 to the interior of starch granules.  The impact of the loss of SGP-1 on other starch biosynthetic genes expression was investigated using RNA-Seq.  Transcript levels of several starch biosynthetic genes were significantly higher in SGP-1 double nulls compared to Mountrail.  The SGP-1 double nulls described here produced firmer pasta with an altered glycemic response.