Research review paperEndophytes as in vitro production platforms of high value plant secondary metabolites
Introduction
Fungi have been known to be a rich repository of medicinally important compounds since the discovery of penicillin. Today, the range of drugs derived from fungi stretch from antibiotics to immune-suppressants to anti-cholesterol drugs (statins). While plants still remain the major source of drugs or their lead molecules, with every new bioactive molecule reported from a plant source, there follows reports of endangered status or even extinction of a medicinally important plant due to over-harvesting. Hence, the focus turned toward fungi namely the “endophytes”, which reside within these medicinally important plants and thus may have acquired their medicinal abilities. The landmark in this area of endophyte bioprospecting was undoubtedly the discovery of Taxomyces andreanae (Stierle et al., 1993), the first endophyte reported to produce taxol, the billion dollar anti-cancer lead molecule (Fig. 1a). The number of patents and publications that followed this discovery triggered high hopes of a sustainable alternative production route using endophytic fungi for almost all the major life-saving drugs, without having to harvest the trees to meet the ever-increasing market demands. Indeed the initial discovery was followed by a plethora of different endophytes reported to produce various other bioactive molecules including camptothecin, vincristine, vinblastine, huperzine, podophyllotoxin, diosgenin and azadirachtin (Zhao et al., 2011b) (Fig. 1b–h). However, despite numerous reports documenting the secondary metabolites from endophytes in the last two decades (Table 1), there have been no major breakthroughs in terms of commercial exploitation of any endophyte as a source of bioactive molecules. Reports related to inconsistent production of many of these biomolecules in the axenic culture of endophytes upon isolation from the host plant have raised doubts on commercial feasibility of endophytes as sustainable production platforms. In this review, literature evidence available in support as well as in opposition of the presence of host-independent biosynthetic machinery in endophytic fungi has been presented. Recent advances in the bioprocessing of endophytes post the taxol boom have also been discussed. This review aims at highlighting the need for greater research on the complex ecological interactions between endophytes and their host plant as well as with other endophytic microbiota in the host. A deeper understanding of these interactions, when combined with rational bioprocess optimization, will enable the provision of a culture environment which is conducive to sustainable and maximum production of high value secondary metabolites, during in vitro cultivation of potential endophytes.
Section snippets
What are endophytes? — definition, discovery and classification
The term endophyte was coined by Heinrich Anton de Bary in 1884 to originally define any organism occurring within plant tissues (Hyde and Soytong, 2008). Broadly this definition includes bacteria (Kobayashi and Palumbo, 2000), fungi (Stone et al., 2000), algae (Trémouillaux-Guiller et al., 2002), insects (Tooker and Hanks, 2004) and other vascular plants (Marler et al., 1999). However, majority of the endophyte research has focused on endophytic fungi (Arnold and Lewis, 2005), almost making
Host–endophyte relationship — role in host plant evolution, defense and secondary metabolite production
All the plant species, including non-vascular plants, ferns, conifers and angiosperms, are believed to be in symbiotic relationship with endophytic fungi (Rodriguez et al., 2009). Many plants harbor as many as hundreds of endophytes within apparently healthy tissues (Stone et al., 2000). The co-evolution of fungi and land plants may have begun much earlier than the early Devonian period. It has been hypothesized that the early mutualistic symbiosis between an alga and fungal partners catalyzed
Limitations of producing bioactive secondary metabolites from plants
Several statistics point at how indispensable plants are as secondary metabolite sources. Twenty five percent of the drugs prescribed worldwide originate from plants, with plant-based drugs accounting for 11 % of the 252 basic and essential drugs recognized by the WHO (Dubey et al., 2012). At least 120 distinct plant-based active compounds are being used as important drugs in one or more countries (Taylor, 2000). Natural products, especially plant-derived, have contributed to 47% of the
Strategies for sustainable and enhanced production of secondary metabolites in endophytes
In order to realize and completely tap the secondary metabolite arsenal of endophytic fungi on an industrial scale, yield and productivity enhancement strategies at several levels are required. A combination of genetic/metabolic and bioprocess engineering may be used to sustain and enhance production of high value secondary metabolites from endophytic fungi. While the potential of endophytes as sources of high value phytochemicals and novel natural products is enormous, the yields are
Strategies to unlock the cryptic/silenced gene clusters in endophytes
While the application of bioprocess optimization strategies on endophytic fungi in axenic culture has led to increased product yields in many cases, most often genomic instability or silencing of the biosynthetic genes in axenic culture hinders attempts of bioprocess development using newly isolated strains. Hence, the challenges associated with stabilizing the productive capabilities of the axenic culture of endophytes can be addressed via the use of strategies like co-cultivation and
Conclusions
The vast amount of knowledge that is already available and the newer pieces of the puzzle that are emerging in endophyte research only make it obvious that endophytic fungi will be one of the major sources of secondary metabolites in the near future. In addition, considering the various yield enhancement strategies available that can be tailored to endophytic fungi, the future potential of endophytic fungi research for the production of high value natural products looks extremely promising.
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