Molecular Characterization of Genomic Regions Associated with Resistance to Soybean Rust (Phakopsora pachyrhizi Syd.) in PI 635999

Resistance to soybean rust (SBR) has been identified in many soybean plant introductions (PIs) and is conferred by the single dominant genes Rpp1 – Rpp5.  No US commercial cultivars have been reported to be resistant to SBR, but a soybean accession PI 635999 from Vietnam  has been shown to be resistant to geographically different P. pachyrhizi isolates.  The objectives of this study were to identify and map genomic region(s) associated with the SBR resistance in this PI.  A F_6:7 recombinant inbred line (RIL) population was derived from a Williams 82 x PI 635999 cross.  These RILs were phenotyped in the field in Ha Noi, Vietnam, and Quincy, Florida, in 2008.  Reddish-brown (RB), tan (TAN), Mixed lesion types, and disease severity (%) were recorded to assess responses of the RILs to SBR.  Bulked segregant analysis (BSA) was conducted to detect single gene(s) associated with qualitative resistance.  In addition, the population was also genotyped using a universal soybean single nucleotide polymorphism (SNP) panel (USLP 1.0).  BSA identified four BARC_SNP markers that were positively clustered with the resistant bulk and PI 635999, and which mapped proximate to the known Rpp3 locus on chromosome 6.   Results of the quantitative resistance analysis indicated that besides the Rpp3-related locus, an additional region close to the Rpp4 locus on chromosome 18 was associated with variation in AUDPC values and sporulation.  The data thus indicate that in addition to qualitative inheritance of lesion type, PI 635999 might have quantitative resistance to SBR that impedes development of the disease. To understand the mechanisms underlying the observed resistance, candidate genes will be identified from the associated regions and examined for their potential involvement in resistance to SBR.