Soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is among the most economically destructive pests of soybean in the United States. The most recent estimate shows that SCN causes a 2 to 6 % annual yield loss worth $1 billion each year. Soybean cyst nematode causes yield reduction by feeding on plant nutrients, retarding root growth, and inhibiting nodulation. It has been proven that rotation with non-host crops and resistant cultivar development are very effective and successful measure allowing soybean production to continue in many growing areas where production could no longer be profitable because of SCN infestations.
It was known that resistance of most commercial soybean cultivars developed in the US may carry two major loci, rhg1 and Rhg4, which have been introduced from a few resistance soybean accessions (Peking and PI88788). However, it is well recognized that growing of the same resistance cultivars for consecutive years may result in genetic vulnerability and SCN population shifts. Thus, broadening the genetic basis of SCN resistance in commercial cultivars is of common and best interests for soybean production in the US.
University of Missouri-Columbia (MU) has a long history in leading soybean research for genetic resistance to the SCN. Screening the USDA soybean germplasm collection, soybean researchers of the National Center for Soybean Biotechnology (NCSB) at MU identified additional plant introductions (PIs) that had resistance to all known races of the SCN. NCSB scientists have developed several genetic populations derived from crosses of resistant PIs and well-adapted cultivars or elite lines in efforts to identify and characterize quantitative trait loci (QTL) or genes responsible for resistance to SCN races. The results of the studies indicated that in addition to two commonly reported LG G- and A2-QTLs associated with rhg1 and Rhg4 loci, many putative QTLs with novel alleles from donor parents have been also identified and mapped in soybean linkage groups (LG). Currently, these mapping populations have been advanced for recombinant inbred lines (RIL), from which near-isogenic lines (NILs) will be developed. Soybean researchers will be conducting fine-mapping using these NILs to narrow down the identified QTL regions to a good resolution, which would be favorable for introgression of resistance genes from donor PIs into elite germplasm and commercial cultivars. Map-based cloning of resistance genes will provide a better understanding of mechanism of host plant resistance to SCN.
Along with the employment of simple sequence repeat (SSR) marker technology for identification and localization of SCN resistance QTL, the availability of soybean bacterial artificial chromosome (BAC) pools and sequence information allow soybean researchers of NCSB to develop a number of single nucleotide polymorphisms (SNP) for genomic regions associated with the SCN resistance, especially for the rhg1 and Rhg4 regions. These SNPs markers are being validated for molecular breeding application. In parallel with the genetic mapping and map-based cloning efforts, molecular basis of pathogenicity and host plant-nematode interactions are being investigated by NCSB scientists. This research is focused on dissecting host plant responses during compatible and incompatible plant-nematode interactions and functional analysis of nematode parasitism genes.