HISTOLOGICAL AND INITIAL MOLECULAR ANALYSIS OF UG99, THE SR31-BREAKING RACE OF THE WHEAT STEM RUST FUNGUS

The Sr31-virulent stem rust race Ug99 probably evolved from race UVPgt55 (Sr31-avimlent). To find differences between Ug99 and UVPgt55, both races were grown on two pairs of near-isogenic wheat lines with (Federation 4/Kavkaz and Thatchcr-Sr31) and without the Sr31 gene for resistance (cvs Federation and Thatcher). Five days post inoculation, a difference in growth of Ug99 and UVPgt55 became obvious on Federation 4/Kavkaz: while growth of UVPgt55 was inhibited, Ug99 grew well. In contrast, Thatcher-Sr31 inhibited the growth of both races, hinting at an additional resistance gene in this line not broken by Ug99. Furthermore, mycelia of both races were successfully grown in axenic culture to isolate genomic DNA of high purity. Inverse sequence-tagged repeat and random amplified polymorphic DNA analyses showed almost identical patterns in both races which clearly differed from the patterns of the Canadian stem rust race 32. Two SSHdifferential cDNA libraries were constructed, one representing mainly genes of Ug99 active in planta, the other genes of UVPgt55. Reciprocal hybridisations of cDNA macro-arrays corroborated the strong similarity between the two races, but some differentially expressed genes were found. Sixty-five clones of Ug99 and 91 clones of UVPgt55 were sequenced. Among these, five genes of UVPgt55 with a secretory signal peptide coding for proteins of unknown function were identified as potential avirulence genes.