Resistance to Phakopsora pachyrhizi in soybean PI 587905 maps to the Rpp1 locus and exhibits variable dominance associated with plant ontogeny

Authors: Smith, James - Rusty; Ray, Jeffery - Jeff; MOREL, WILFRIDO - Centro De Investigacion; Frederick, Reid; Walker, David

Submitted to: Journal of Crop Improvement
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2015
Publication Date: 9/15/2015
Publication URL: https://handle.nal.usda.gov/10113/4086113
Citation: Smith, J.R., Ray, J.D., Morel, W., Frederick, R.D., Walker, D.R. 2015. Resistance to Phakopsora pachyrhizi in soybean PI 587905 maps to the Rpp1 locus and exhibits variable dominance associated with plant ontogeny. Journal of Crop Improvement. 29:581-601.

Interpretive Summary: Soybean rust is a fungal disease that can cause significant yield loss to soybeans worldwide. The disease entered the United States in 2004 and caused great concern that the U.S. soybean crop would be damaged. USDA scientists quickly set about testing many of the soybean strains in the USDA soybean collection in hopes of identifying soybeans with resistance to this fungus. In the process, soybean PI 587905 was identified as having resistance to the fungus. PI 587905 was used in the current study to create a large family of new soybean lines; some with resistance to the fungus and some without resistance to the fungus. The families were thoroughly tested in the presence of the fungus in both greenhouse and field studies, and it was determined that PI 587905 has a single gene for resistance to soybean rust. Furthermore, a genetic map was created, and it was determined that the single resistance gene was located at a place in the soybean genome called Rpp1. Finally, after careful review of all the scientific data, both from this study and from other studies, it was determined that PI 587905 carries a unique version of the gene at Rpp1. This information is significant because it means that PI 587905 may have resistance to specific isolates of soybean rust to which other soybeans have no resistance. As a result of this study, soybean breeders will know that PI 587905 is a very unique and useful soybean for developing resistant soybean cultivars in the future.

Technical Abstract: Soybean rust, caused by Phakopsora pachyrhizi Sydow, results in significant yield loss worldwide. Soybean accession PI 587905, previously identified as having resistance to P. pachyrhizi, was used to create two independent populations (F2 plants and F2-derived F3 lines) segregating for resistance. Both populations were derived from the cross DS-880 × PI 587905. Seedlings of the F2-derived F3 (F2:3) lines were inoculated with a natural field population of P. pachyrhizi in a greenhouse, and adult F1 and F2 plants were assayed in a field in southeastern Paraguay in order to determine and map the inheritance of resistance in PI 587905. Ninety-five F2:3 lines, 147 F2 plants, and seven F1 plants were evaluated for lesion type using the following phenotypic classes: immune reaction (IR), reddish-brown (RB) lesions, or tan (TAN) lesions, either localized or universally distributed. Based on lesion type, each F2:3 line was rated as either resistant, susceptible, or segregating, and this classification was then used to infer the phenotype and genotype of each corresponding F2 plant. Both the F2 and the F2-inferred segregation ratios fit the 1:2:1 (resistant:heterozygous:susceptible) ratio expected for segregation of a single gene (P>0.05). In the field assay, RB lesions occurred on all of the F1 plants and on 50% of the F2 plants but were not observed on either parent, indicating incomplete dominance. Molecular markers flanking the Rpp1 through Rpp5 P. pachyrhizi resistance loci were used to map the resistance gene in PI 587905 to the Rpp1 locus. This information will be useful to plant breeders for developing resistant cultivars.