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Two oat genes encoding hydroxycinnamoyl-CoA:hydroxyanthranilate N-hydroxycinnamoyltransferase (HHT) and S-adenosyl-L-methionine:trans-caffeoyl-CoA 3-O-methyltransferase (CCoAOMT), both of which are possibly involved in the biosynthesis of oat avenanthramide phytoalexins, were cloned and their expression profiles in response to biological stress were studied. Four distinct cDNAs of oat HHT (AsHHT1-4) were isolated with the degenerative polymerase chain reaction method. The enzymatic activity of AsHHT1 expressed in E. coli was found using hydroxyanthranilate and hydroxycinnamoyl-CoAs as cosubstrates. Cloned oat CCoAOMT (AsCCoAOMT) encoded a polypeptide of 130 amino acid residues with 77.7 to 80.8% identities to the CCoAOMT sequences from other plant species. The accumulation of AsHHT1 and AsCCoAOMT transcripts increased concomitantly with phytoalexin accumulation by the treatment of victorin, a specific elicitor in oat lines carrying the Pc-2/Vb gene. Pharmacological approaches indicated the involvement of Ca^sup 2+^, NO, and protein kinases in the signaling pathways of AsHHT1 and AsCCoAOMT mRNA induction. When oat leaves were inoculated with Puccinia coronata, the mRNA expression of AsHHT1 and AsCCOAOMT increased in both incompatible and compatible interactions but more rapidly in incompatible interaction. Interestingly, however, significant phytoalexin accumulation was observed only in incompatible interaction during the experimental period, suggesting that phytoalexin accumulation may be inhibited in one or more posttranscriptional processes in the compatible interaction.
The disease-resistance response of plant cells is composed of a multitude of biochemical events, including the structural reinforcement of cell wall, hypersensitive response (programmed cell death), the development of systemic acquired resistance, and the accumulation of newly produced antifungal chemicals called phytoalexins (Dixon and Paiva 1995; Grayer and Kokubun 2001). We have studied specific resistance mechanisms in the cultivar-race interactions between oats (Avenu sutivu L.) and the crown rust fungus (Puccinia coronata f. sp. avenue). In the crown rust-oat system, single gene-mediated rusl resistance in the P. coronata lines is highly correlated with the production of phytoalexin and the cessation of fungal growth in the primary leaves (Mayama et al. 1981, 1982, 1995a).
Victoria-type resistance to certain races of crown rust is conferred by a single dominant gene of oats, Pc-2 (Luke et al. 1966). The Pc-2 oat lines, however, are susceptible to Cochiobolus victoriae because of sensitivity to the fungus-produced host-specific toxin victorin. Victorin sensitivity is also controlled by a single dominant gene, Vb, in the host plant...