Stem Rust Resistance in Wheat is Suppressed by a Subunit of the Mediator Complex

Stem rust is a devastating fungal disease of wheat that persistently undermines food security. Nearly 40 years ago, a gene was discovered that suppressed the expression of stem rust resistance (Sr) genes in the wheat cultivar Canthatch (CTH). When the suppressor gene is deleted or mutated, CTH expresses resistance to races of the stem rust fungus which includes the Ug99 race group that are otherwise virulent. Using doubled-haploid populations of crosses between CTH and EMS mutant lines in combination with sequenced flow-sorted chromosomes we mapped and identified the suppressor gene SuSr-D1 . Sequence analysis of nine independent CTH mutants showed nonsense mutations or deletions in Med15b.D, a subunit of the conserved Mediator complex that coordinates and activates transcription in eukaryotes. As suppression of resistance is common in wheat, the isolation of SuSr-D1 and understanding of this mechanism will facilitate the use of unutilised sources of resistance to promote sustainable agriculture and food security.