Graduate Thesis Or Dissertation
 

A historical evaluation of high molecular weight glutenins and the dwarfing (Rht) gene in Triticum aestivum in selected genotypes grown in the United States

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  • Wheat (Triticum aestivum) is an important crop worldwide, however, in recent years concerns have been raised regarding two health issues, celiac disease and gluten sensitivity, and their relationship to wheat consumption. This increase has led to questions regarding potential changes over time in the composition of gluten (glutenin and gliadin) in wheat, especially after incorporation of the reduced height alleles (Rht-D1b and Rht-B1b) to increase wheat productivity. Questions raised include: 1) Have there been changes in the frequencies of glutenin alleles in hard or soft-grained wheat since the incorporation of the Rht alleles and 2) Are potential changes in wheat gluten a result of breeding for increased dough strength? To answer these questions, 124 top production acreage cultivars from the Pacific Northwest (PNW) and elsewhere across the United States were collected. All but one cultivar (19th century) were developed and grown in the 20th and 21st centuries. PNW soft-winter wheat cultivars were included as there has been systematic selection pressure applied to decrease dough strength in this wheat class whereas other regions have selected for increased dough strength. Plants of each cultivar were grown under greenhouse conditions in a completely randomized design with four replications. For Rht testing, DNA was collected from plants at the two leaf stage. Two Kompetitive Allele Specific PCR (KASP) single nucleotide polymorphism (SNP) assays were performed to detect the dwarfing mutant alleles (Rht-D1b or Rht-B1b) or wild-type non-dwarfing alleles (Rht-D1a or Rht-B1a). The first assay was for Rht-B1 located on chromosome 4B. The second assay was for Rht-D1, which is located on chromosome 4D. To determine the composition of high molecular weight glutenins, grain was harvested at maturity and micro fluidic capillary electrophoresis was used. Analysis showed that there had been a shift in glutenin subunits after 1999 in both hard and soft wheat cultivars. No significant change was found at the Glu-A1 and Glu-B1 loci in hard wheat. However, at the Glu-A1 locus in soft wheat, subunit Ax1 decreased in frequency over time while the null allele increased over time. At the Glu-B1 locus, subunit Bx7 showed an increase over time. After 1999, the Glu-D1 locus in hard wheat contained the Dx5+Dy10 combination at a higher frequency than previously observed. For soft wheat the Dx2+Dy12 subunit combination was the only combination present. KASP assays showed that the dwarfing alleles of the Rht gene were in only one cultivar in the 1950's, however, by the 1960's half of the PNW cultivars contained a dwarfing Rht gene. By the year 2000 all PNW cultivars tested contained a dwarfing gene. Cultivars from other states had a slower incorporation of this gene; as of the year 2000 dwarfing alleles were found at 73% frequency. With the introduction of the gene, glutenin subunit patterns did not shift from prior years.
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