Isolation and Expression of a Host Response Gene Family Encoding Thaumatin-Like Proteins in Incompatible Oat-Stem Rust Fungus Interactions. Kuo-Chih Lin. Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108, U.S.A. William R. Bushnell (1,2), Les J. Szabo (1,2), and Alan G. Smith3 (1) Department of Plant Pathology, University of Minnesota; (2) Cereal Rust Laboratory, Agricultural Research Service, U.S. Department of Agriculture; and (3) Department of Horticultural Science, University of Minnesota, St. Paul, MN 55108, U.S.A. MPMI 9:511-522. Accepted 3 April 1996. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1996.

Four cDNA clones (corresponding to tlp-I, -2, -3, and -4 genes) encoding thaumatin-like (TL), pathogenesis-related proteins were isolated from oat {Avena saliva) infected by an incompatible isolate Pga-IH of the oat stem rust fungus (Puccinia graminis f. sp. avenae). All four cDNA clones contained an open reading frame predicted to encode a 169-aminn acid polypcptidc 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 tlp 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-IH, or an inappropriate isolate Pgt-SD 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 hy-phal growth was inhibited, whereas in plants infected with a compatible isolate Pga-6A, relatively lower amounts of transcripts were detected. Overall, transcript levels were higher with tlp-1 than with the three other genes. Spray with a light mineral oil used as a spore carrier induced transient expression of tlp-1, -2, and -3 genes at 16 to 30 h AI which obscured the initial induction of the tlp genes in response to infection by the pathogens. In contrast, tlp-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 tlp-1 and tlp-3, moderately induced tlp-2, but had no effect on tlp-4. Taken together, the results showed that tlp genes displayed differential responses to oil spray, mechanical wounding, and pathogen infection and that the expression of tlp genes, especially tlp-I, in oat is associated with resistance reactions in response to infection by incompatible and inappropriate isolates of the stem rust fungi.