Epistatic interaction between rust resistance loci Ur-3 and Ur-5

Authors: Hurtado-Gonzales, Oscar; Pastor Corrales, Marcial

Submitted to: Bean Improvement Cooperative Annual Report
Publication Type: Trade Journal
Publication Acceptance Date: 2/25/2019
Publication Date: 4/30/2019
Citation: Hurtado-Gonzales, O.P., Pastor Corrales, M.A. 2019. Epistatic interaction between rust resistance loci Ur-3 and Ur-5. Bean Improvement Cooperative Annual Report. 62:23-24.

Interpretive Summary: Effective breeding for disease resistance to the common bean rust pathogen involves pyramiding rust resistance genes from various parents into a single genotype for a specific trait such as disease resistance. However, combining these genes into a single cultivar is often challenging specially when using phenotypic markers to confirm the presence of these genes during breeding selections. This process is not only labor intensive but it is also confounded by a phenomenon known as epistasis which is the interaction between genes in which one gene (epistatic) masks the presence of another gene (hypostatic). The use of tightly linked DNA markers can help circumvent gene pyramiding of these genes and is easily accomplished using highly accurate DNA markers that are tightly linked to the genes of interest. We reported the epistatic interaction between two rust resistance genes (Ur-3 and Ur-5) using a segregating population combined with rust phenotyping and molecular markers. The results of this study demonstrate that the DNA markers reported here will facilitate the development of common bean cultivars combining the Ur-3 and Ur-5 with other rust resistance genes.

Technical Abstract: Gene pyramiding involves the amalgamation of several desirable genes from various parents into a single genotype for a specific trait such as disease resistance. Because gene pyramiding broadens the genetic base of cultivars, it is a very important strategy for managing pathogens with high virulence diversity such as those that cause the rust, anthracnose, and angular leaf spot diseases of common bean. Most of the reported disease resistance in common bean are conferred by single and dominant genes. Combining these genes into a single cultivar is often challenging specially when using phenotypic markers to confirm the presence of these genes in the pyramiding process. One of the complications is epistasis or the interaction between genes in which one gene (epistatic) masks the presence of another gene (hypostatic). Epistasis is widespread in common bean and particularly among rust resistance genes. When using specific races of the rust pathogen to combine two genes or more genes, often certain genes hide the presence of other genes. For example, the Ur-3 and Ur-6 rust resistance genes are epistatic to the Ur-11 gene. The resistant reactions of the Ur-3 and Ur-6 genes are visualized as necrotic spots known as the hypersensitive reaction (HR). Conversely, the resistant reaction of Ur-11 is tiny rust pustule. When plants combining the Ur-3 and Ur-11 genes are inoculated with race 53 (31-1), to which both genes are resistant, these plants display only the HR (grade 2) type of reaction of Ur-3 and not the reaction of Ur-11. These results indicate that Ur-3 is epistatic to Ur-11 (Stavely 2000, Pastor-Corrales and Stavely 2002). Similarly, the combination of Ur-4 and Ur-5 loci results in epistasis in which Ur-5 masks the presence of Ur-4 (Valentini et al., 2015). Thus, epistatic interactions make difficult the pyramiding of genes using specific races of the rust pathogen. Successful pyramiding of these with races of the rust pathogen requires labor-intensive phenotyping with two or more races of the rust pathogen. On the other hand, the pyramiding of these genes is easily accomplished using highly accurate DNA markers that are tightly linked to the genes of interest. In the present study, we report the epistatic interaction between the Ur-3 and Ur-5 rust resistance genes using two recently developed KASP markers closely linked to these genes.