Induced Mutations of a Genetic Suppressor of Resistance to Wheat Stem Rust

A genetic suppressor of resistance to stem rust, Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn., of wheat (Triticum aestivum L.) is on the long arm of chromosome 7D. Mutation or inactivation of the suppressor gene would allow expression of the suppressed resistance genes already present in hexaploid wheat or of resistance genes transferred to wheat from diploid or tetraploid relatives. Our objective was to induce a mutation of the suppressor gene and describe its inheritance. Seed treatment of the hexaploid wheat cultivar Canthatch (CTH) with ethyl methanesulfonate produced 15 stem rust-resistant mutants. The mutant lines were tested for reaction to 13 races of stem rust, and crossed with CTH and 7D aneuploids of CTH and ‘Chinese Spring’ (CS). Parental, F₁, F₂, and F₃ seedlings were tested for reaction to race CRL-QSH of stem rust. Similarity among the 15 mutants in reactions to 13 races of stem rust indicated that each of the mutants had a similar genetic alteration resulting in an inactivated or deleted suppressor gene. The suppressor gene mutations showed incomplete dominance and monogenic inheritance in crosses with CTH. One mutant was ditelosomic 7DS and lacked the suppressor gene. Allelism tests indicated that the mutated gene(s) in the other 14 mutants were at or near the same locus on 7DL. Crosses between the mutants and CS monosomic 7D or CS ditelosomic 7DS indicated that CTH may have three recessive genes for resistance that were inhibited by the suppressor. The mutant lines may permit the use of the suppressed genes for stem rust resistance in hexaploid wheat and related species to breed hexaploid wheat cultivars with improved resistance. Joint contribution of USDA-ARS and the North Dakota Agric. Exp. Stn., North Dakota State Univ., Fargo, ND 58105. Published with the approval of the Director, North Dakota Agric. Exp. Stn. as Journal Series no. 1973.