Molecular-assisted breeding for improved carbohydrate profiles in soybean seed.

KEY MESSAGE: Two independent variant raffinose synthase 3 (RS3) alleles produced an equivalent phenotype and implicated the gene as a key contributor to soybean seed carbohydrate phenotype. Soybean is an important crop because the processed seed is utilized as a vegetable oil and a high protein meal typically used in livestock feeds. Raffinose and stachyose, the raffinose family of oligosaccharides (RFO) carbohydrate components of the seed, are synthesized in developing soybean seeds from sucrose and galactinol. Sucrose is considered positive for metabolizable energy, while RFO are anti-nutritional factors in diets of monogastric animals such as humans, poultry, and swine. To increase metabolizable energy available in soybean seed meal, prior research has been successful in deploying variant alleles of key soybean raffinose synthase (RS) genes leading to reductions or near elimination of seed RFO, with significant increases in seed sucrose. The objective of this research was to investigate the specific role of variants of the RS3 gene in a genomic context and improve molecular marker-assisted selection for the ultra-low (UL) RFO phenotype in soybean seeds. The results revealed a new variant of the RS3 allele (rs3 snp5, rs3 snp6) contributed to the UL RFO phenotype when mutant alleles of RS2 were present. The variant RS3 allele identified was present in about 15% of a small set of soybean cultivars released in North America. A missense allele of the RS3 gene (rs3 G75E) also produced the UL RFO phenotype when combined with mutant alleles of RS2. The discoveries reported here enable direct marker-assisted selection for an improved soybean meal trait that has the potential to add value to soybean by improving the metabolizable energy of the meal.