Oxidation of the phytoalexin maackiain to 6,6a-dihydroxy-maackiain by Colletotrichum gloeosporioides

Article abstract of DOI:10.1016/S0031-9422(97)00082-4

Phytoalexins are low molecular weight antibiotic compounds produced by plants in response to infection by microbes. These antimicrobial compounds are thought to provide resistance to microbial invasion and colonization. (- )Maackiain and its pterocarpan relatives can be oxidized at a number of sites, including at the 6a carbon. A previously unknown metabolite was produced from (-)maackiain by the broad host-range pathogen Colletotrichum gloeosporioides (Glomerella cingulata). This unknown was identified by LCMS- MS and NMR spectroscopy to be 6,6a-di-OH-maackiain (3,6,6a-trihydroxy-8,9- methylenedioxy-pterocarpan). It is produced by isolates that represent all four races and pathotypes of C. gloeosporiodes isolated from the tropical forage legume Stylosanthes spp. We present evidence that the primary metabolite (-)6a-OH-maackiain is subsequently hydroxylated at carbon 6, a step resulting in a compound that is increased in polarity and decreased in toxicity relative to the parent compound and (-)6a-OH-maackiain. This further oxidation may be required for efficient excretion or carbon source scavenging.