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It has been already found that avenalumins (Avls) are accumulated in oat leaves responding to infection with incompatible races of the crown rust fungus4,5,13). These substances are benzoxazin and are considered to be phytoalexin to contribute to the expression of the cultivar-race specific resistance. A possible biosynthetic route for Avls, based on their chemical structures, is the phenylpropanoide pathway. Recently, however, we have shown that lipoxygenase (LOX) activity increases in leaves of six oat cultivars with an inoculation of incompatible race of Puccinia coronata12,15). This suggests that oat plants metabolize fatty acids to produce defensive substances against the rust infection as in the case of the rice-blast resistance, in which the production of antifungal unsaturated fatty acids is indicated in association with increased LOX activity2,3,8-11). In this study, therefore, we attempted to isolate possible new antifungal substances in the incompat ible oat-crown rust system. Oat cultivars Shokan-1, Pc 47, 48, 50, 51 and 61 (Avenae sativa L.) and races 226 and 203 of Puccinia coronata f. sp. avenae Fraser et Led. were used. Shokan-1, Pc 48, 50, 51 and 61 are highly resistant to race 226, responding with a vigorous hypersensitive reaction. Shokan-1 and Pc 47 are susceptible to races 203 and 226, respectively 14,15). Oat leaves (fresh weight 100g) were harvested 48hr after inoculation and stored at -80•Ž until use. Samples were dipped in 1,000ml of Et2O, kept at 5•Ž for 72hr, and homogenized with a Physcotron. The homogenate was filtered with Miracloth and the filtrate was shaken twice with 2% aqueous NaHCO3 and then with H2O, repeatedly. The Et2O phase was added to 0.1N HCl and then shaken 3 times with H2O. The Et2O was evaporated to dryness in vacuo and the residue was dissolved in H2O. The solution was partitioned with n-hexane and the aqueous phase was shaken with Et2O. The Et2O phase was evaporated to dryness in vacuo and the residue was dissolved in MeOH (neutral fraction: NEF). On the other hand, the aqueous phase containing NaHCO3 was adjusted to pH 2-3 with HCl and shaken twice with Et2O. The Et2O phase washed twice with H2O was evaporated to dryness in vacuo and the residue was dissolved in MeOH (acidic fraction: AEF). The neutral and acidic fractions were subjected to isolation of antifungal substances by preparative HPLC using an Asahipak ODP-50 column for NEF and a Finepak SIL C18T-5 column for AEF, respectively. The former column was first eluted with 30% CH3CN containing 1% HCOOH, and then the linear gradient was established with 30-100% CH,CN containing 1% HCOOH at 1ml/min for 20min, and finally washed with CH3CN. The latter column was eluted with 60% MeOH containing 0.1% AcOH at 1ml/min. Each fraction taken was dried in vacuo to remove HCOOH or AcOH prior to bioassay, and made to original volume with MeOH. Inhibitory activity was bioassayed by two methods and active fractions were collected. Each fraction (25ƒÊl) was added onto paper disks (Whatmann 3 MM; 0.5cm) and vacuumed overnight to remove organic solvents. The paper disks, on which uredospores were uniformly dispersed,

Isolation of Fatty Acid Metabolites in Oat Leaves Responding with Resistance to Puccinia coronata avenae.