Isolation and expression of a host response gene family encoding thaumatin-like proteins in incompatible oat-stem rust fungus interactions

Four cDNA clones (corresponding to tlp-1, -2, -3, and -4 genes) encoding thaumatin-like (TL), pathogenesis-related proteins were isolated from oat (Avena sativa) infected by an incompatible isolate Pga-1H of the oat stem rust fungus (Puccinia graminis f. sp. avenae). All four cDNA closes contained an open reading frame predicted to encode a 169-amino acid polypeptide with a signal peptide of 21 amino acids at the N-terminus, suggesting that these proteins are transported through a secretory pathway. The amino acid sequences revealed high homology among the four cDNA clones, 80 to 99% identity and 86 to 100% similarity. The tip genes and several TL protein genes of certain cereals are clustered into a small group that is phylogenetically separate from the major group of TL protein genes of several plant species. In plants infected with the incompatible isolate Pga- 1H, or an inappropriate isolate Pgt-8D of P. graminis f. sp. tritici, high levels of tip gene transcripts accumulated at 42 to 48 h AI and thereafter when hypersensitive host cell death occurred and hyphal growth was inhibited, whereas in plants infected with s compatible isolate Pga-6A, relatively lower amounts of transcripts were detected. Overall, transcript levels were higher with tip-1 than with the three other genes. Spray with a light mineral oil used as a spore carrier induced transient expression of tip-1, -2, and -3 genes at 16 to 30 h AI which obscured the initial induction of the tip genes in response to infection by the pathogens. In contrast, tip-4 was induced very little by oil spray, so that induction was clearly observed in response to either compatible, incompatible, or inappropriate isolates at 24 to 30 h AI. Wounding leaves by either slicing or puncturing them strongly induced tip-1 and tip-3, moderately induced tip- 2, but had no effect on tip-4. Taken together, the results showed that tip genes displayed differential responses to oil spray, mechanical wounding, and pathogen infection and that the expression of tip genes, especially tip- 1, in oat is associated with resistance reactions in response to infection by incompatible and inappropriate isolates of the stem rust fungi.