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

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 6 carbon. A previously unknown metabolite was produced for (-)maackiain by the broad host-range pathogen Colletotrichum gloeosporioides (Glomerella cingulata). This unknown was identified by LC-MS-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. gloeosporioides 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.