Host-pathogen studies of wheat leaf rust resistance in Triticum turgidum

English: Wheat and the wheat leaf rust pathogen Puccinia tritieina co-evolved for several millennia. The frequency by which host resistance genes are overcome by the pathogen has lead to a constant search for new genes, in particular in wild species related to wheat, to enlarge the resistance gene pool. For this reason four subspecies of T. turgidum that was known to have adult-plant resistance to leaf rust were crossed with the bread wheat cultivar SST55 and studied in terms of expression, inheritance and molecular markers. Seedling plants of T. turgidum ssp. dieoceoides (Kom. ex Aschers. and Graebn.) Theil. var. kotehianum, T. turgidum ssp. pyramidales, T. turgidum ssp. durum (Desf.) Husn var. libyeum and T. turgidum ssp. abyssinieum were inoculated with different races of P. tritieina, P. graminis f. sp. tritiei and P. striiformis f. sp. tntlei. A differential interaction was observed between accessions and pathotypes, indicating that the plants had race-specific resistance. All showed adult-plant resistance with leaf rust infection types ranging between ";" (flecking) and ";1eN" (flecks and small pustules associated with chlorosis and/or necrosis). Despite of low pollen viability, sterility and recessiveness of resistance genes in three of the accessions, F1, F2 and F3'Swere produced. T. turgidum ssp. durum v. libyeum was the only accession that produced resistant F1's, indicating a dominant gene or genes. Mendelian ratios in the F2 progeny suggested that resistance in this accession resulted from two major genes. The F2 of the T. turgidum ssp. abyssinicum and T. turgidum ssp. pyramidales crosses did not segregate according to Mendelian ratios and two minor genes conferred resistance of T. turgidum ssp. dicoccoides v. kotchianum. The molecular AFLP technique was used to to follow the introgression of resistance from tetraploid T. turgidum ssp. dicoccoides v. kotchianum and T. turgidum ssp. durum v. libycum to the hexaploid bread wheat cultivar SST55. In total nine fragments were solely introgressed from T. turgidum ssp. dicoccoides v. kotchianum and four from T. turgidum ssp. duturn v. libycum and were present in all resistant accessions. These fragments have potential to be developed into molecular markers. Histology tests done on accession T. turgidum ssp. durum v. libycum confirmed a post-haustoria I expression of resistance. Resistance in this accession was strongly associated with a hypersensitive response. Necrosis started at the aborted sub-stomatal vesicle stage and continued through to colony formation. All colonies were enveloped in necrotic leaf tissue with the parent having a higher hypersensitivity index than the F1. In some cases it seemed as if necrosis was systemic and carried along the veins. Although hypersensitive resistance as observed in this study is not considered durable it should not be overlooked in the search for new resistance genes. The challenge to the breeder is to use these genes in genetic backgrounds where it will be protected against pathogenic adaptation in the leaf rust fungus.