|Van De Mortel, Martijn|
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2009
Publication Date: 2/27/2009
Citation: Meyer, J.D., Silva, D.C., Yang, C., Van De Mortel, M., Pedley, K.F., Hill, J.H., Shoemaker, R.C., Abdelnoor, R., Whitham, S.A., Graham, M.A. 2009. Identification and Analyses of Candidate Genes for Rpp4 Mediated Resistance to Asian Soybean Rust in Soybean (Glycine max). Plant Physiology. 150:295-307. Interpretive Summary: Asian Soybean Rust is a threat to soybean production in many areas of the world and has recently made its way to the United States. Resistance is very rare and thus far, only five sources of resistance have been identified (Rpp1-Rpp5). Yield losses due to ASR as high as 80% have been reported. Therefore, identifying the resistance genes and closely linked markers is essential in improving U.S. commercial cultivars. Previous work has identified a genomic region responsible for the Rpp4 mediated resistance. By sequencing this region, we identified three candidate disease resistance genes in a susceptible cultivar and five candidate genes of the same family in the resistant cultivar. These genes are very large and are most similar to downy mildew resistance genes from lettuce. To determine if the candidate genes were responsible for resistance, molecular methods (gene silencing) were used to turn off the genes in the resistant genotype. The results confirmed that one of the five candidate genes in the resistant parent was responsible for resistance. Molecular markers were used to further reduce the number of candidate genes to three. Of these, only one gene was highly expressed in the resistant parent making it the candidate for Rpp4 mediated resistance. Our findings have identified a definitive source for Rpp4 mediated resistance to ASR. The markers we developed can be used by breeders to incorporate resistance to ASR into commercial cultivars. Development of resistant cultivars will reduce yield losses caused by ASR and will benefit growers and consumers.
Technical Abstract: Asian Soybean Rust (ASR) is a formidable threat to soybean production in many areas of the world including the United States. Only five sources of resistance have been identified (Rpp1, Rpp2, Rpp3, Rpp4, and Rpp5). Rpp4 was previously identified in the resistant genotype PI459025B and it was mapped within 2 cM of Satt288 on soybean chromosome 18 (linkage group G). Using SSR markers, we developed a BAC contig for the Rpp4 locus in the susceptible cultivar Williams82 (Wm82). Sequencing within this region identified three Rpp4 candidate disease resistance genes (Rpp4C1 - Rpp4C3 (Wm82)) belonging to a family of coiled coil- nucleotide binding site-leucine rich repeat (CC-NBS-LRR) disease resistance genes with high similarity to the RCG2 disease resistance family from lettuce. Constructs developed from the Wm82 Rpp4 candidate genes were used for virus-induced gene silencing experiments to silence resistance in PI459025B, confirming that homologous genes confer resistance. Using primers developed from conserved sequences in the Wm82 Rpp4 candidate genes, we have identified five Rpp4 candidate genes (Rpp4C1 - Rpp4C5 (PI459025B)) from the resistant genotype. Additional markers developed from the Rpp4 contig further defined the region containing Rpp4 to include Rpp4C2 (PI459025B), and likely Rpp4C4 (PI459025B) and Rpp4C5 (PI459025B). Sequencing of RT-PCR products revealed that Rpp4C4 (PI459025B) is highly expressed in the resistant genotype while expression of the other candidate genes is largely undetectable. T hese data support Rpp4C4 (PI459025B) as the single candidate gene for Rpp4-mediated resistance to ASR.