Plant-associated Fungi: Methods for Taxonomy, Diversity, and Bioactive Secondary Metabolite Bioprospecting.

Plants harbor a large reservoir of fungal diversity, encompassing endophytic, epiphytic, phytopathogenic, and rhizosphere-associated fungi. Despite this diversity, relatively few fungal species have been characterized as sources of bioactive secondary metabolites. The role of secondary metabolites is still not fully understood; however, it is suggested that these metabolites play important roles in defense mechanisms and fungal interactions with other organisms. Hence, fungal secondary metabolites have potential biotechnological applications as prototype molecules for the development of therapeutic drugs. In this chapter, we describe the main methods used for routine fungi isolation, production of crude fungal extracts, and chemical characterization of bioactive compounds. In addition, explicative notes about the steps described are provided to explore the diversity of the endophytic, phytopathogenic, epiphytic, and rhizosphere fungi and to evaluate the biotechnological potential of each group.

Biological activities of ophiobolin K and 6-epi-ophiobolin K produced by the endophytic fungus Aspergillus calidoustus.

Endophytic fungi represent ubiquitous microbial organisms able to live in the tissues of different plants around the world and represent a prolific source of bioactive metabolites. In the present study, the endophytic fungus Aspergillus calidoustus was isolated from the medicinal plant Acanthospermum australe (Asteraceae), and identified using molecular, physiological and morphological methods. A methylene chloride crude extract of A. calidoustus has been produced and subjected to antifungal bioassay-directed fractionation which resulted in the isolation of the two bioactive compounds: ophiobolin K and 6-epi-ophiobolin K. These pure compounds displayed antifungal activity against fungal plant pathogens, protozoal activity against Trypanosoma cruzi, and cytotoxic activity against human tumoral cell lines. The results show that A. calidoustus was able to produce the antifungal and cytotoxic metabolites ophiobolin K and 6-epi-ophiobolin K, which may help the fungus to colonise and occupy the substratum as well as survive in natural environments.