Endolichenic fungi: a hidden source of bioactive metabolites

Lichens are synergetic organisms composed of a mycobiont in relationship with one or more photosynthetic partners (photobiont). These organisms grow on various substrates and sometimes under extreme ecological environments. A different group of non-obligate micro-fungi also resides inside the internal tissues of the lichen thalli without causing any harm to them. These microfungi are called as endolichenic fungi (ELF). Among various natural alternatives for the bioactive molecules, ELF are emerging as a source of metabolites of different chemical structural classes including alkaloids, quinones, furanones, pyrones, benzopyranoids, xanthones, terpenes, steroids, peptides and acyclic compounds. These compounds exhibited various biological activities, viz., anticancer, antiviral, antibacterial, antifungal and anti-Alzheimer's etc. This review aims to highlight about ninety-nine novel compounds out of the total 172 reported metabolites isolated during the period from 2008 to 2019 (up to March 2019) from ELF. An overview of the unexplored perspective of ELF as a source of novel natural bioactive products is provided. Some of the procedures for activation of the silent biosynthetic ‘gene clusters’ responsible for their production are also discussed.