Stress induced carbazole phytoalexins in Glycosmis species

Abstract

Induced formation of a series of carbazole alkaloids was observed in leaves of Glycosmis parviflora and G. pentaphylla after wounding, UV-irradiation, and particularly after inoculation with the fungus Botrytis cinerea. Chemical variation between different provenances and even individuals of G. parviflora led to an accumulation of different derivatives from which three proved to be undescribed natural products. Their structures were identified by spectroscopic methods and named carbalexins A, B, and C. Bioautographic tests on TLC plates with Cladosporium herbarum exhibited strong antifungal activity for the new carbalexins as well as for the already known 2-hydroxy-3-methylcarbazole, but only weak effects for the pyranocarbazole glycoborinine. Detailed experiments with marked infection areas confirmed the restricted accumulation of carbazole derivatives which could not be detected in non-infected areas of the same leaf. Apart from carbazoles, in some individuals of G. parviflora an additional accumulation of the pyranoquinolones flindersine and methylflindersine was induced, which supports the already previously discussed biogenetic connections between carbazoles and prenylated quinolones.