Cloning and using a QTL for insect resistance in soybean
Ortega, Maria Andrea
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Soybean is one of the world’s most important feed crops. Leaf damage caused by insects reduces yield and seed quality. Development of plants that resist insect damage is the most economical and environmentally sustainable way to control insects. Resistance to leaf-chewing insects is available in the soybean accessions PI 229358 and PI 227687; it is conferred by the quantitative trait loci QTL-M, QTL-H, and QTL-G in PI 229358, and QTL-E in PI 227687. Chapter two details the characterization of novel combinations of insect resistance QTLs, ME and MGHE, that provide high levels of resistance against leaf-chewing insects, and the combination of ME with Bt (cry1Ac) that enhances the effect of Bt. Chapter three describes the development of the elite germplasm lines Benning-ME and Benning-MGHE. This chapter provides the graphical genotypes used to determine the QTL introgressions in each NIL, and KASP markers for marker-assisted selection of QTLs. Chapter four covers the evaluation of insect-resistant soybean meal for broiler chickens. In a 21-day feeding trial, the performance broiler was equivalent for Benning, BenningM, and BenningMGH diets. There is no indication that meal produced from soybean seed carrying QTL-M, QTL-G, and QTL-H would not be as safe as the insect-susceptible Benning soybean cultivar when used for animal feed. Chapter five details the cloning and validation of GmORUGA, the gene for QTL-M, which is by far the major determinant of leaf-chewing insect resistance in soybean. Resistance corresponds to a mutation of TGG (275W) to TGA (stop), which leads to a truncated protein. Resistance is achieved by the loss-of-function of GmORUGA. Complementing a QTL-M resistant line with the susceptible GmORUGA allele restores susceptibility, and silencing GmORUGA in susceptible lines results in resistance. GmORUGA is a genistein 7-O-glucosyltransferase induced after caterpillar damage. The functional enzyme in susceptible soybean contributes to maintaining a constant concentration of genistin after insect attack. This enzyme is not active in QTL-M plants, and genistin is reduced and condensed tannins (CT) increases, presumably through redirection of metabolic flux to produce CTs in lieu of genistein. This is the first report of an insect resistance gene that operates via the isoflavone pathway.