Genetic mapping of resistance to Aphanomyces root rot in alfalfa

Authors: Samac, Deborah - Debby; Dornbusch, Melinda - Mindy; BUCCIARELLI, BRUNA - University Of Minnesota; Miller, Susan; Yu, Long-Xi

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 9/4/2018
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Aphanomyces root rot (ARR), caused by the oomycete Aphanomyces euteiches, is one of the most important yield-limiting factors in production of legumes. In Europe, it is the main limiting factor for pea production while in the U.S. it is one of the most important diseases of alfalfa and pea. Alfalfa varieties with resistance to ARR became widely available in the 1990s; however, failure of resistant varieties identified a second race of the pathogen. Recent reports of failure of varieties with resistance to both race 1 and race 2 suggest that additional pathogenic races are present in alfalfa production fields. A total of 53 soil samples from alfalfa production fields in Minnesota and 40 soil samples from New York were assayed for the presence of the two races of A. euteiches. In both states, race 2 isolates were most common. None of the 44 individual strains could overcome the resistance in the check variety WAPH5 that has resistance to race 1 and race 2. Zoospores were found to be attracted primarily to the root hair zone of alfalfa seedlings and penetrated root cells within one hour of inoculation. Cells of resistant plants reacted to penetration with a rapid and localized hypersensitive response characterized by cell death and browning of penetrated cells. Cells of the pathogen were limited to only epidermal cells or a small number of cortical cells in resistant seedlings. Highly upregulated genes in resistant plants at 24 hours after infection include genes involved in the phenylpropanoid biosynthetic pathway leading to phytoalexin (medicarpin) production, chitinase, and beta-1,3-glucanase. The production of phenylpropanoid compounds and suberization of the stele was seen microscopically in resistant plants at 24 hours after inoculation. Similar responses were seen for resistance to race 1 and race 2 strains. To gain a better understanding of the genes involved in resistance, an F1 mapping population was used for genotyping-by-sequencing followed by genotype calling. SNP markers significantly associated with resistance to strain race 1 were identified on chromosome 1. Using the genome sequence for cultivated alfalfa at the diploid level, three candidate NBS-LRR genes were identified within 38 kbp of the most significant marker (P=2.76 x 10-23). Significant SNP markers associated with resistance to strain MER-4 (race 2) were located on chromosome 2 (P=1.7 x 10-9). These results indicate that a small number of genes are involved in the hypersensitive resistance mechanism in the variety 53V52.