Reactions of 52 soybean germplasm accessions with Rpp3 alleles to a panel of 13 Phakopsora pachyrhizi (soybean rust) isolates from the southern United States

Authors: PAUL, CHANDRA - University Of Illinois; HARRIS, DONNA - University Of Georgia; LI, ZENGLU - University Of Georgia; BOLLICH, PATRICIA - Louisiana State University; Walker, David

Submitted to: Journal of General Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2020
Publication Date: 1/2/2021
Citation: Paul, C., Harris, D.K., Li, Z., Bollich, P.A., Walker, D.R. 2021. Reactions of 52 soybean germplasm accessions with Rpp3 alleles to a panel of 13 Phakopsora pachyrhizi (soybean rust) isolates from the southern United States. Journal of General Plant Pathology. 87:55-70. https://doi.org/10.1007/s10327-020-00972-w.
DOI: https://doi.org/10.1007/s10327-020-00972-w

Interpretive Summary: Soybean rust, caused by Phakopsora pachyhizi, is a persistent threat to soybean grown in the southern United States. More than 120 soybean plant introductions (PIs) with resistance to the fungus have been identified. Genes for resisance to soybean rust (Rpp genes) have been mapped at seven different genetic loci, but 52 out of PIs investigated have a resistance gene at the Rpp3 locus. The reactions of these 52 PIs to 13 strains of rust from the southern United States were compared to the reactions of PI 462312 (original source of the Rpp3 gene) and 'Hyuuga' (a variety with an Rpp5 gene and an Rpp3 gene) to determine which PIs might carry novel gene alleles or combinations of genes for rust resistance. Thirty-two PIs had reaction patterns similar to PI 462312, eight had patterns more similar to Hyuuga, and 12 had reaction patterns unlike either. Those 12 PIs are the most likely of the 52 to carry unique forms of the Rpp3 gene. The results of this study will be useful to breeders seeking sources of resistance genes that will provide new soybean varieties with broad resistance to P. pachyrhizi in soybean-producing regions of the southern United States.

Technical Abstract: A high percentage of the soybean plant introductions (PIs) resistant to soybean rust (SBR; Phakopsora pachyrhizi) populations and isolates from the United States carry a resistance allele at the Rpp3 locus on soybean chromosome (Chr) 6 that was originally identified in PI 462312. The degree of genotypic similarity among the alleles in these PIs, and between the alleles in the PIs and the Rpp3 gene in PI 462312 and the SBR-resistant cultivar ‘Hyuuga’, is not known, but 37 of 52 resistant PIs were previously reported to have a marker haplotype identical to that of PI 462312 in the region spanning the Rpp3 locus. Objectives of this study were to evaluate the reactions of the 52 PIs to a panel of P. pachyrhizi isolates originating from the continental United States and to identify accessions with reaction patterns notably different from those of PI 462312 (Rpp3) and Hyuuga (Rpp3 + Rpp5). Seedlings of the PIs, PI 462312, Hyuuga and a susceptible check were inoculated with 13 purified isolates, and their infection types were determined after two weeks. Based on reactions to the panel of domestic isolates and to key exotic isolates used in a previous study, 32 PIs had reaction patterns similar to that of PI 462312 and eight had patterns more similar to that of Hyuuga. Twelve PIs had reaction patterns that differed more extensively from those of both PI 462312 and Hyuuga. While most of the PIs with reaction patterns similar to that of PI 462312 also had a similar or identical Rpp3 region haplotype, the haplotypes of PI 578457A and PI 416873B were only 33% and 67% similar, respectively. The Rpp3-region haplotypes of the eight accessions with Hyuuga-like reaction patterns and the 12 PIs with unusual reaction patterns were all identical or nearly identical to that of PI 462312. Differences in levels of sporulation on PIs that produced RB lesions were also observed, suggesting that some of the PIs might carry a different allele from PI 462312 and Hyuuga at the Rpp3 locus. This study provides further information about PIs that appear to have unique alleles at the Rpp3 locus and others that appear to have the Rpp3 gene or a similar, potentially superior gene. This information will be useful in breeding rust-resistant soybean cultivars for the southern United States.