Endophytes: The unmapped repository for natural products

Endophytes are the microorganisms present within the living tissues of plants. It has been suggested that every plant harbours at least one to two endophytes. However, very few plants have been characterized for their endophytic population, in comparison to their known diversity. A search in PubMed using the keyword ‘endophyte’ shows that there has been a considerable increase in the number of publications focusing on endophytes, i.e., 32 in the 2000s to ~500 in 2017 and roughly 200 in the first few months of 2018. The trend suggests increasing interest in endophytes as sources of novel metabolites. The pointers in early studies had shown the presence of novel natural compounds produced by endophytes. Indeed, the therapeutic molecules in many plants have been proposed to be produced by endophytes and not the host plants themselves. These molecules have the potential to serve as added resources in the desperate search for bio-active compounds which can combat various diseases and syndromes prevalent today and which are fast-losing effective therapeutics. Their presence in plants growing in diverse habitats adds to their potential for chemodiversity. Modern omics technologies, involving next-generation sequencing, metagenomics and metatranscriptomics, have shown a promise in better understanding of plant-endophyte relationships and can play a significant role in establishing the biosynthetic potential of endophytes. Therefore, bioprospecting for endophytes constitutes an attractive area of research. Thus, the current chapter provides a comprehensive account of these microorganisms as they correlate to various habitats, their role in ‘benefit-sharing’ with their hosts and the recent technologies which have unveiled their involvement in various aspects of their host plants’ lives.