Degradation of the Pterocarpan Phytoalexin (—)-Maackiain by Ascochyta rabiei

Abstract

Ten strains of Ascochyta rabiei pathogenic to chickpea ( Cicer arietinum L.) were shown to be potent degraders of the chickpea pterocarpan phytoalexin ( - )-maackiain (1) ([6aR: 11 aR]-3- hydroxy-8,9-methylenedioxypterocarpan). In degradative studies with mycelial preparations and crude protein extracts seven catabolites could be isolated and structurally elucidated by spectroscopic techniques. The main routes of maackiain degradation are reduction to a 2´-hydroxyisoflavan (2) and oxidation to an 1 a-hydroxy-pterocarp-1,4-diene-3-one (3) with subsequent reductions of the early catabolites in ring A. A catabolic sequence for maackiain is compared with the degradation pattern of pterocarpan phytoalexins observed with other fungi.