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Epigenetic regulation of Rhg1, a soybean cyst nematode resistance locus.

Ryan Zapotocny: University of Wisconsin-Madison, Department of Plant Pathology

<div>Soybean cyst nematode (SCN; <em>Heterodera glycines</em>) is a major pathogen of soybean. The primary source of resistance against SCN is a locus known as <em>Rhg1</em>. This ~30kb four-gene locus is tandemly repeated up to ten times in resistant varieties. We have previously reported that <em>Rhg1</em> transcript abundance and SCN resistance are roughly proportional to <em>Rhg1</em> copy number, and reported differential DNA methylation between resistant and susceptible haplotypes. One hypothesis is that the repetitive nature of the <em>Rhg1</em> locus constrains gene expression due to transcriptional gene silencing. To test this and other hypotheses, we have used <em>Mcr</em>BC analysis, treatment with cytosine analogs, and a single-nucleotide-resolution method known as Bisulfite Patch PCR. Our new <em>Mcr</em>BC analysis revealed shared <em>Rhg1</em> DNA methylation patterns in distinct soybean varieties carrying the same <em>Rhg1</em> haplotype. In a low-sensitivity assay, we did not detect extensive <em>Rhg1</em> demethylation after SCN infection. When roots were treated with the DNA methylation inhibitors zebularine and azacitidine, elevation of <em>Rhg1</em> gene expression was observed in high-copy <em>Rhg1</em> varieties. A trial run of the Bisulfite Patch PCR has revealed more precise data about <em>Rhg1</em> and additional experiments are in progress to obtain data on several soybean varieties at multiple timepoints during infection. By determining how this very important locus is regulated during SCN infection, we hope to assist efforts to improve SCN resistance.</div>