Production of the bioactive pigment elsinochrome A by a cultured mycobiont strain of the lichen Graphis elongata

Abstract

We report the production of the perylenequinone pigment elsinochrome A in aposymbiotic culture of the mycobiont of the crustose epiphytic lichen Graphis elongata Zenker (Lecanoromycetes), collected in Argentina (Buenos Aires). The substance was not detected in the lichenized thallus (using HPLC techniques) and is otherwise only known from one unrelated lichen and a few genera of non-lichenized, plant-pathogenic fungi in the class Dothideomycetes. The phylogenetic affinities of the lichen mycobiont and the cultured fungus were confirmed using DNA sequence data of the mitochondrial small subunit rDNA (mtSSU), which place the lichen fungus into the Allographa clade within Graphidaceae. The mycobiont pigment was purified and characterized by spectroscopic methods. This is the first case where a rare pigment, otherwise known from non-lichenized, plant-pathogenic fungi, is produced in aposymbiotic culture of a lichen mycobiont, while, at the same time, being absent from the lichen thallus itself. Based on this finding, we discuss the previously postulated hypothesis that lichen mycobionts maintain secondary metabolic pathways of non-lichenized ancestors in their genome, while gene expression and production of metabolites is suppressed in the lichenized state due to toxicity to the photobiont.