SummaryOat leaves produce phytoalexins, avenanthramides, in response to infection by pathogens or treatment with elicitors. The metabolism of avenanthramides was investigated using low molecular weight, partially deacetylated chitin as an elicitor. When oat leaf segments are floated on the elicitor solution, avenanthramides accumulate in the solution. The transfer of elicited oat leaves to solutions containing stable-isotope-labeled avenanthramides resulted in a rapid decrease in the labeled avenanthramides, suggesting the metabolism of avenanthramides. The rate of decrease was enhanced by elicitor treatment, and was dependent on the species of avenanthramides, with avenanthramide B decreasing most rapidly. The rates of biosynthesis and metabolism of avenanthramides A and B were measured using a model of isotope-labeling dynamics. Avenanthramide B was found to be more actively biosynthesized and metabolized than avenanthramide A. Radiolabeled avenanthramide B was incorporated into the cell wall fraction and 99% of incorporated activity was released by alkaline treatment. Gel filtration indicated that high-molecular-weight compounds derived from avenanthramide B were released by alkaline treatment. The decrease in stable-isotopelabeled avenanthramides was suppressed by catalase, salicylhydroxamic acid, and sodium ascorbate, suggesting the involvement of peroxidase in the metabolism. Consistent with this, peroxidase activity that accepts avenanthramide B as a substrate was induced in apoplastic fractions by elicitor treatment. The appearance of multiple basic isoperoxidases was observed by activity staining with 3-amino-9-ethylcarbazole coupled with isoelectric focusing of proteins from elicitor-treated leaves. These findings suggest that accumulated avenanthramides are further metabolized in apoplasts in oat leaves by inducible isoperoxidases.

O at phytoalexins, avenanthram ides, occur as constitutive com ponents in seeds. The am ounts of each av en an th ram id e w ere analyzed. The com position of av en an th ram id es in dry seeds was d ifferent from th at in elicitor-treated leaves. In seeds, av en an th ram id e C was most ab u n d an t w ith an am o u n t tw o tim es larger than th at of aven an th ram id e A o r B. O n the o th er h and , av en an th ram id e A was the m ajor com ponent in elicitor-treated leaves. The total am ount of av en an th ram id es in seeds increased 2.5 tim es during im bibition for 48 h although the com position did n o t change. The hydroxycinnam oyl-C oA :hydroxyanthranilate /V-hydroxycinnam oyltransferase (H H T, E C 2.3.1.-) activity, which is responsible for th e final co n d en sation step in th e av en an th ram id e biosynthesis, was detected in dry seeds. T he activity was localized in e n d o sp erm and scutellum , and slightly increased during 48-h im bibition. The enzym e was partially purified by anion exchange chrom atography from b o th dry seeds and elicitor-treated leaves. TTie activity was separated into two peaks by chrom atography, indicat ing th at H H T consists of at least tw o isoforms. The substrate specificities o f H H T isoform s from seeds w ere differen t from each other.

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