Stem rust is an important disease of wheat that can be controlled using resistance genes. The gene SuSr-D1 identified in cultivar 'Canthatch' suppresses stem rust resistance. SuSr-D1 mutants are resistant to several races of stem rust that are virulent on wild-type plants. Here we identify SuSr-D1 by sequencing flow-sorted chromosomes, mutagenesis, and map-based cloning. The gene encodes Med15, a subunit of the Mediator Complex, a conserved protein complex in eukaryotes that regulates expression of protein-coding genes. Nonsense mutations in Med15b.D result in expression of stem rust resistance. Time-course RNAseq analysis show a significant reduction or complete loss of differential gene expression at 24 h post inoculation in med15b.D mutants, suggesting that transcriptional reprogramming at this time point is not required for immunity to stem rust. Suppression is a common phenomenon and this study provides novel insight into suppression of rust resistance in wheat.

Agriculture and Agri Food Canada Morden Research and Development Centre 101 Route 100 Morden MB R6M 1Y5 Canada

CIMMYT ICRAF House United Nations Avenue Gigiri Village Market Nairobi 00621 Kenya

CSIRO Agriculture and Food GPO Box 1700 Canberra ACT 2601 Australia

Department of Plant Sciences and Plant Pathology Montana State University Bozeman MT 59717 USA

Institute of Experimental Botany Czech Academy of Sciences Centre of the Region Haná for Biotechnological and Agricultural Research Šlechtitelů 31 779 00 Olomouc Czech Republic

John Innes Centre Norwich Research Park Norwich NR4 7UH UK

Plant Breeding Institute Cobbitty University of Sydney Private Bag 4011 Narellan NSW 2567 Australia

Research School of Biology The Australian National University Acton ACT 2601 Australia

The Sainsbury Laboratory University of East Anglia Norwich Research Park Norwich NR4 7UK UK

USDA ARS Cereal Disease Laboratory University of Minnesota St Paul MN 55108 USA

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Wheat Pm55 alleles exhibit distinct interactions with an inhibitor to cause different powdery mildew resistance

Capturing Wheat Phenotypes at the Genome Level

Chromosome analysis and sorting