Mapping of a soybean rust resistance in PI 594756 at the Rpp1 locus

Abstract

Abstract Asian soybean rust (ASR), caused by the fungus Phakopsora pachyrhizi, is the main disease affecting soybean production in Brazil. The plant introduction PI 594756 is a resistance source that has been employed in breeding for resistance to ASR in this country. This study aimed at investigating the resistance of the PI 594756 to a panel of P. pachyrhizi isolates and mapping its resistance in populations derived from the cross with the susceptible PI 594891. The PI 594756 and resistant varieties were inoculated with seven ASR monosporic isolates. F2 and F2:3 populations were tested against ASR in a greenhouse and used to map a resistance gene to a likely genomic location by means of bulked segregant analysis. Bulks were genotyped with Infinium BeadChips and the genomic region identified was saturated with target GBS (tGBS). PI 594756 presented a unique resistance profile compared to the differential varieties, being resistant to six isolates and immune to one. The resistance was visually monogenic dominant; however, it was classified as incompletely dominant when quantitatively studied. Genetic and QTL mapping placed the PI 594756 gene between chromosome (chr) 18 55,863,741 and 56,123,516. This position is slightly upstream mapping positionsof Rpp1 (PI 200492) and Rpp1-b (PI 594538A). Finally, we performed a haplotype analysis of a panel composed of Brazilian historical germplasm, sources of Rpp genes and resistant varieties and found SNPs that can successfully differentiated the new allele from PI 594756 from Rpp1 and Rpp1-b sources. The haplotype identified can be used as a tool for marker assisted selection.