A single-nucleotide insertion in Rxp confers durable resistance to bacterial pustule in soybean

The soybean Rxp gene, encoding a bHLH transcription factor and an ACT-like domain, has an rxp allele producing a truncated protein that confers resistance to pustule-causing Xanthomonas axonopodis pv. glycines.

In soybean, bacterial pustules caused by Xanthomonas axonopodis pv. glycines lead to premature defoliation and decreased yield in warm, wet climates. In the USA, approximately 70 years ago, bacterial pustules were eliminated by introducing a recessive resistance allele, rxp, of the Rxp gene, representing the first example of successful soybean breeding for durable disease resistance in North America. In this study, we isolated this historical Rxp gene from resistant soybean varieties using positional cloning. The 1.06 Mb region where Rxp was reported to reside was narrowed down to an 11.1 kb region containing a single gene, Glyma.17g090500. The resistance allele, rxp, contains a T insertion. A complementation test of the Rxp allele in resistant plants confirmed the identification of the Rxp gene. The product of the susceptible wild-type allele, Rxp, is presumed to be a basic helix–loop–helix (bHLH) transcription factor with an aspartate kinase, chorismate mutase, and TyrA (ACT)–like domain. This gene was mainly expressed in extended leaves, and its homologs were identified to be distributed in angiosperms. A total of six alleles were obtained: four from spontaneous variation, including the wild-type and three mutant alleles that encoded truncated proteins, and two from ethyl methanesulfonate mutants, including an allele that encoded a truncated protein and a missense allele. By evaluating the resistance of these six alleles, we found that the loss of function of RXP decreased the bacterial pustule lesions. This study provides important insights into the soybean rxp allele, which confers durable resistance to bacterial pustules.