Twenty One Years of Molecular Marker-Assisted Breeding for Resistance to Soybean Sudden Death Syndrome

Novel tools to improve resistance to sudden death syndrome (SDS) and the underlying Fusarium root rot (FRR) caused by Fusarium virguliforme (Aoki) have been developed for soybean [Glycine max (L.) Merr.]. Between eighteen and thirty resistance loci have been identified due to clustering the real number is unclear. Many were confirmed over the past twenty one years (named Rfs1 to Rfs18). To select the beneficial alleles of 8 to 10 loci per cross needed for optimal resistance is a difficult task for plant breeders. Resistance mechanisms to FRR and SDS provide only partial protection. Crops contend with many Fusaria, a group with a wide host range and flexible hemibiotrophic lifestyle. Full resistance is absent among the leguminacea, brassicacea, cucurbitacea, and solanacea. This poster focuses on the use of plant genomics resources to aid breeding selection for resistance to FRR and SDS. The SDS is a combination of two diseases. The first, FRR,  includes rotted roots and toxin-restricted root development. Resistances include variations in infection severity, infection frequency, and rot severity. The second is caused by toxins translocated from infected roots to the shoots. Leaf scorch, supra-petiolar abscission, pod abortion and early plant maturity are consequences of many toxin to target interactions. Breeding for combined FRR and SDS resistance has begun using a set of exciting new tools for pathogen quantification in roots. Resistance genes were proven, including GmRLK18-1 (Glyma_18_02680) Rfs2 through trangenics, and also MIPs1a (EC 5.5.1.4; Rfs3) by mutation. NILS and SNPs were used to confirm two more genes on the same chromosome 18 (Rfs and Rfs1). Some candidate genes were identified. The new tools provide an opportunity for new breeding initiatives. This review aims to inform these new programs of the core discoveries from the past 21 yrs, to incorporate best practices from old and new initiatives.