Current Biology
Volume 25, Issue 19, 5 October 2015, Pages 2570-2576
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An Endophyte Constructs Fungicide-Containing Extracellular Barriers for Its Host Plant

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Highlights

  • The cancer drug Taxol is a fungicide against wood-decaying fungi (WDF) in yew trees

  • Inside yew, a fungal endophyte produces Taxol and migrates to pathogen entry points

  • The endophyte sequesters Taxol in hydrophobic bodies (HBs); WDF induce their release

  • The HBs can coalesce to form extracellular barriers laced with fungicidal Taxol

Summary

Surface cracks create sites for pathogen invasion. Yew trees (Taxus) hyperbranch from long-lived buds that lie underneath the bark [1], resulting in persistent bark cracking and deep air pockets, potentially allowing pathogens to enter the nutrient-rich vascular system (vertical phloem and inter-connected radial medullary rays [MR]). Yew is famous as the source of the anti-cancer diterpenoid drug Taxol. A mystery has been why both the tree and its resident non-pathogenic fungi (endophytes) synthesize Taxol, apparently redundantly [2, 3, 4, 5, 6, 7]. These endophytes, as well as pure Taxol, suppress fungal pathogens including wood-decaying fungi (WDF) [8, 9, 10, 11]. Here we show that a Taxol-producing fungal endophyte, Paraconiothyrium SSM001 [12], migrates to pathogen entry points including branch cracks. The fungus sequesters Taxol in intracellular hydrophobic bodies that are induced by WDF for release by exocytosis, after which the bodies can coalesce to form remarkable extracellular barriers, laced with the fungicide. We propose that microbial construction of fungicide-releasing hydrophobic barriers might be a novel plant defense mechanism. We further propose that the endophyte might be evolutionarily analogous to animal immune cells, in that it might expand plant immunity by acting as an autonomous, anti-pathogen sentinel that monitors the vascular system.

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