Coumarins in helianthus tuberosus : characterization, induced accumulation and biosynthesis

Abstract

The biosynthesis of 7-hydroxylated coumarin phytoalexins has been examined in the tuber of Helianthus tuberosus. Ayapin and scopoletin where characterized by TLC, HPLC and GC-MS. Both compounds were excreted from sliced tuber tissues. They accumulated in response to treatment with chemical elicitors like CuCl 2 or sucrose, but not in response to MnCl 2. Scopoletin accumulation preceded ayapin build-up in the medium. Highest and earliest accumulation of both coumarins was measured after copper treatment. The formation of methylenedioxy bridges in several secondary metabolites is catalyzed by cyt P450 oxygenases. The changes in cyt P450 content, trans-cinnamate 4-hydroxylase and NADPH-cytochrome c reductase activities in microsomes from tuber tissues were, therefore, monitored for three days after wounding and elicitation. Cyt P450 content and NADPH-Cyt c reductase activity were not significantly modified in copper treated tissues, but increased three to five fold in response to sucrose. Copper elicitation resulted in a decrease in cinnamate-4-hydroxylase activity, while sucrose was a very strong and specific inducer of this Cyt P450 enzyme. No formation of ayapin was observed when radiolabelled scopoletin was incubated with NADPH and elicited plant microsomes, or fed to induced tuber tissues in vivo. No formation of methylenedioxy metabolite was obtained in vitro or in vivo from radiolabelled ferulate or feruloyl CoA.