Destruction of lichen chemical defenses by a fungal pathogen

Abstract

Lichen secondary metabolites are known to inhibit various animal consumers and pathogenic microorganisms. Nevertheless, many obligate fungal pathogens have evolved a tolerance to these inhibitory lichen compounds. We recently discovered a new lichen pathogen in the fungal genus Fusarium that is not only tolerant of lichen compounds but also able to degrade many of these compounds. This organism was discovered in field investigations of a different lichenicolous fungus, Marchandiomyces corallinus, which was found growing on lichens (Lasallia papulosa and L. pensylvanica) that normally inhibit its growth. Subsequent experiments established that M. corallinus is found on Losallia species only when Fusarium is also present. We hypothesized that Fusarium altered the inhibitory chemistry of Lasallia spp. and permitted colonization by M. corallinus. A laboratory experiment to test this hypothesis demonstrated that sterilized tissues of Lasallia papulosa exposed to Fusarium for 30 d are readily degraded by M. corallinus; control tissues left in sterile water for 30 d continue to inhibit growth of M. corallinus. High performance liquid chromatography (HPLC) established that the lichen compound lecanoric acid, one of several lichen compounds that inhibit growth of M. corallinus, is degraded by extracellular enzymes produced by this newly discovered Fusarium. Taken together, our results demonstrate that enzymatic degradation of lichen compounds permits colonization of lichens by fungi that would otherwise be chemically excluded.