Detoxification of the phytoalexins maackiain and medicarpin by fungal pathogens of alfalfa

Nine fungal pathogens of alfalfa (Medicago sativa) (lucerne) were assayed for their ability to metabolize the pterocarpanoid phytoalexins (-)maackiain and (-)medicarpin. All of the alfalfa fungal isolates were able to metabolize both (-)maackiain and (-)medicarpin. Six different initial reaction products were observed, and often a single isolate produced multiple metabolic products. All the products have been previously described, except for those produced by Cercospora medicaginis. This fungus hydroxylated the pterocarpans at the 1a carbon to form a 1a[R]OH-dienone, in which the hydroxyl group is trans to the bridgehead protons, and is the 1a epimer of the previously described cis form of the compound. (-)Maackiain and both of the la hydroxylated epimers were tested for toxicity on isolates of Nectria haematococca mating population I and Saccharomyces cerevisiae that do not degrade maackiain or its 1aOH-dienone products. Both epimers were less toxic than maackiain, and the trans epimer was less toxic than the cis form.