Identifying Gene(s) Conferring Quantitative Disease Resistance Against the Soybean Pathogen Phytophthora Sojae Using RNAi Gene Knockdown of Candidate Genes within a Quantitative Trait Locus.

Phytophthora sojae is the second most damaging soybean [Glycine max (L.) Merr.,]  pathogen, annually causing an excess of $200 million in losses in the US. Genetic resistance is the preferred management method to limit losses due to P. sojae. Two types of resistance are available to soybean growers including single, qualitative resistance genes (Rps-mediated) or quantitative disease resistance (QDR). Rps-mediated resistance has been more commonly used in breeding programs because it is conferred by single, dominantly inherited genes, and results in complete resistance. Unfortunately, the effectiveness of Rps-mediated resistance can be short-lived as Rps genes only confer resistance to specific races of P. sojae, resulting in selection and adaption of virulent populations. QDR does not have the same limitations as Rps-mediated resistance. Though QDR acts to reduce rather than eliminate disease, QDR is broad-spectrum and expected to be more durable. Historically, less emphasis has been placed on introducing QDR traits. To correct this, resistance screenings and mappings studies have been performed to identify and characterize sources of QDR. These efforts have resulted in the identification of 33 regions possessing alleles which confer QDR against P. sojae. However, no genes controlling QDR have been isolated from within these regions. The objective of this research is to identify the gene(s) putatively controlling resistance within a resistance quantitative trait loci (QTL) on chromosome 18, which spans 166 kb and contains 19 positional candidates. These genes will be functionally analyzed using RNAi gene knockdown followed by P. sojae infection and resistance ratings. Successful identification of the gene(s) controlling resistance will provide molecular markers for breeding efforts, a greater understanding of how QDR is being conferred against P. sojae, and provide precedence for candidate gene selection in future functional analysis studies.