Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2008
Publication Date: 5/15/2009
Citation: Chakroaborty, N., Curley, J., Frederick, R.D., Hyten, D.L., Nelson, R.L. Hartman, G.L., Diers, B.W. 2009. Mapping and Confirmation of a New Allele at Rpp1 from Soybean PI 504538A Conferring RB Lesion Type Resistance to Soybean Rust. Crop Science. 49:783-790. Interpretive Summary: Soybean rust (SBR) is a destructive disease of soybeans caused by the fungus Phakopsora pachyrhizi that has recently been detected in the U.S. Four resistance genes, Rpp1-4, have been described in soybean that recognize specific isolates of P. pachyrhizi which result in either an immune reaction or reddish-brown (RB) lesions rather than susceptible tan lesions. In previous germplasm evaluations, the soybean accession PI 594538A was found to produce an RB resistance reaction to the P. pachyrhizi isolate ZM01-1 from Zimbabwe. A population derived from a cross of PI 594538A with the susceptible soybean cultivar Loda was screened by inoculating with isolate ZM01-1. The RB resistance in PI 594538A mapped as a dominant gene on linkage group G very close to Rpp1. A second mapping population inoculated with ZM01-1 revealed that the RB resistance was the result of a new allele at Rpp1. This new Rpp1 allele was named Rpp?1b. Since Rpp1?b provides unique resistance against P. pachyrhizi isolates that are not recognized by other SBR resistance genes, incorporation of Rpp?1b into soybean lines will expand resistance to these P. pachyrhizi isolates.
Technical Abstract: Soybean rust (SBR), caused by Phakopsora pachyrhizi Syd., is a destructive soybean [Glycine max (L.) Merr] disease and identification of new resistance genes is essential for effective, long-term rust management. Our research objectives were to map and confirm the location of resistance gene(s) in the PI 594538A. The mapping was done in a population of 98 F3:4 lines from a cross between PI 594538A (reddish-brown (RB) lesions), and the susceptible cultivar Loda (tan lesions). The lines were inoculated with the P. pachyrhizi isolate ZM01-1 from Zimbabwe. The RB resistance in PI 594538A mapped on linkage group G as a single dominant gene. It is likely an allele of Rpp1 or a new closely linked gene because it mapped within 1 cM of the Rpp1 region and ZM01-1 produced RB lesions on PI 594538A and tan lesions on PI 200492, the original source of Rpp1, and L85-2378, a ‘Williams 82’ isoline carrying Rpp1. The RB reaction was associated with reduced lesion numbers (P < 0.0001) and sporulation (P = 0.0002) and explained 78% and 19% of the phenotypic variation for these traits. The mapping of a new Rpp1 allele was confirmed in a second population inoculated with ZM01-1. This new allele has been designed Rpp?1b and might be useful for increasing the diversity of rust resistance in soybean cultivars.