Suppression of rust resistance in bread wheat (Triticum aestivum L.) by D-genome chromosomes

Several tests were done in bread wheat (Triticum aestivum L.) to demonstrate the occurrence of genes on D-genome chromosomes that suppress resistance to leaf rust (Puccinia recondita f. sp. tritici Rob. ex Desm.) and stem rust (Puccinia graminis f. sp. tritici Eriks. & Henn.). Ten rust-resistant wild tetraploid wheats (T. turgidum var. dicoccoides) were crossed with both durum (T. turgidum var. durum) and bread wheats. In all cases, resistance to leaf rust and stem rust was expressed in the hybrids with durum wheats but suppressed in the hybrids with bread wheats. Crosses were made between five diverse durum wheats and four diverse bread wheats. The pentaploid hybrid seedlings of 12 crosses were tested with leaf rust race 15 and in all cases the resistance of the durum parents was suppressed. Fourteen D-genome disomic chromosome substitution lines in the durum wheat 'Langdon' were tested with stem rust race 15B-1 and leaf rust race 15. Chromosomes 1B, 2B, and 7B were found to carry genes for resistance to stem rust but no suppressors were detected. Chromosomes 2B and 4B carried genes for resistance to leaf rust, and 1D and 3D carried suppressors. Crosses between seven D-genome monosomics of 'Chinese Spring' and three dicoccoides accessions showed that 'Chinese Spring' possesses genes on 1D, 2D, and 4D, which suppress the stem rust resistance of all three dicoccoides accessions. All three chromosomes must be present to suppress resistance, indicating that some form of complementary gene interaction is involved. In addition, 'Chinese Spring' carries a gene or genes on 3D that suppresses the leaf rust resistance of all three dicoccoides accessions, plus a gene or genes on 1D that suppresses the leaf rust resistance of only one of them. The data raise some interesting questions about the specificity of the suppressors. The high frequency of occurrence of suppressors in the bread wheat population suggests that they must have a selective advantage.