Differential induction of chorismate mutase isoforms by elicitors in oat leaves

Avenanthramides, a series of substituted cinnamic acid amides with anthranilate, are phytoalexins in oats (Avena sativa L.). The precursors of avenanthramides, cinnamate and anthranilate, are biosynthesized via the shikimate pathway that branches at chorismate. Chorismate mutase (CM, EC 5.4.99.5) is the first enzyme on the branch that provides the cinnamate part of avenanthramides. The induction of CM was investigated in primary oat leaves using oligo-N-acetylchitooligosaccharides as elicitors. The CM activity started to increase 6 h after elicitation, and reached a maximum by 9 h, being around twice as large as that in control leaves. Among the oligo-N-acetylchitooligosaccharides tested, tetra-, penta-, and hexasaccharides effectively induced the CM activity in a dose-dependent manner. The activity was separated into two major peaks on anion exchange chromatography with Mono Q, indicating that at least two CM isoforms are present in oat leaves. A comparison of elution profiles of CM activity in intact and elicitor-treated leaves revealed that only one CM isoform is responsive to the elicitor. Two CM isoforms in oat leaves were partially purified and characterized. Both CM isoforms were insensitive to l-phenylalanine, l-tyrosine, l-tryptophan, and caffeate. The fractionation of oat cells indicated that both CM isoforms localized in plastids.