Induction of a complete secondary-product pathway in a cultured lichen fungus

Abstract

Experimental studies on the secondary metabolism characteristic of lichens have been impeded by the slow growth of the fungi and by the inconsistent results of many attempts to induce the pathways in the fungi isolated from their photosynthetic partners. In the present study, a lichen-specific secondary pathway was consistently induced in a lichen fungus ( Cladonia grayi) grown in the absence of the alga. The depside (4- O-demethylsphaerophorin) and two depsidones (grayanic and 4- O-demethylgrayanic acids) found in the natural lichen began to accumulate a few days after the transfer of lightly fragmented mycelia from liquid to solid medium. Induction was enhanced on drier substrates and was correlated with the proliferation of aerial hyphae, where the major product (grayanic acid) accumulated in extracellular patches visible by fluorescence microscopy. The time course was analyzed by quantitative high-performance liquid chromatography of extracts from small cultures grown on nylon filters. Induction was rapid in view of the slow growth of the fungus, and secondary productivity was comparable to that of some nonlichen fungi. These results confirm that the alga is not needed for catalysis in lichen depside and depsidone biosynthesis and suggest instead that the characteristic secondary metabolism of natural lichen is linked to their aerial habit of growth.