Biotechnology of Natural Products by Wilfried Schwab Bernd Markus Lange & Matthias Wüst

Author:Wilfried Schwab, Bernd Markus Lange & Matthias Wüst
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

7.3 Taxol Production by Endophytes

Endophytes are considered asymptomatic bacterial and fungal microorganisms that for at least a part of their life cycle inhabit the intercellular spaces in plant tissues [68, 170]. However, the original definition referred to any species living together, such as microorganisms found within a plant [9]. Like many biological terms, this one is constantly evolving as our knowledge of plant microbiomes and host interactions increases [211]. Recent evidence for bacterial endophytes within the cytoplasm and the periplasm suggests that the specification of intercellular spaces should be updated, at least for bacterial endophytes [149, 186, 206]. It has been predicted that at least one species is likely to be present within each of the 350,699 [150] different plant species on Earth [181]. Endophytic fungi cause symptomless infections but are closely related to biotrophic and necrotrophic pathogens. Indeed, some species appear to have swapped roles multiple times during their evolutionary history [40]. Interestingly, infected plants acquire selectable advantages resulting from endophyte colonization, including improved growth and increased resilience to abiotic and biotic stress [33, 40, 161].

Over the last few decades, several genera of endophytic fungi have been shown to synthesize a plethora of valuable natural products [133]. The pool of secondary metabolites produced by endophytic fungi is vast and includes phenolic acids such as tyrosol and p-coumaric acids [102], quinones [217], hundreds of terpenoids [174], plant hormones such as gibberellins and indoleacetic acid [94, 204], as well as antimicrobial compounds such as Hsp90 inhibitors [126]. Endophytes have also been proposed to synthesize the potent cytotoxic compound taxol [177]. The finding that Taxomyces adreanae isolated from the phloem of T. brevifolia was capable of taxol production caused an explosion of subsequent publications reporting similar capabilities in other endophytic microorganisms [177]. Approximately 200 different fungi representing diverse orders are thought to produce taxol [54, 69]. Reported productivity has ranged from as little as 0.001 ng mL−1 [111] to ~800 ng mL−1 [118] in various isolates, but so far these results have not been independently reproduced [54]. Therefore, it is still unclear whether some of these species can actually produce taxanes, or whether the detected compounds were more likely artefacts of laboratory culture methods, or misidentified by the analytical and immunological methods deployed in the original studies [54, 71, 176]. The most intriguing aspect is how so many fungi could acquire the independent ability to synthesize this complex diterpenoid when at least 19 enzymatic steps are required, and which other compounds they can produce [34].

In many of the early publications claiming taxol production from isolated endophytes, the compound was detected using a competitive inhibition enzyme immunoassay . Without the necessary positive and negative controls, this method lacks stringency when only ng levels of a target compound are present [71]. Indeed, approximately 10 times the concentration of taxanes was detected in the tobacco (Nicotiana tabacum) plants used as a negative control than endophytes, which did not produce taxol but had carryover from their plant hosts. The authors proposed that the signal detected in

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