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. 2017 Jan;10(1):175-188.
doi: 10.1111/1751-7915.12467. Epub 2016 Dec 19.

Bioactive secondary metabolites with multiple activities from a fungal endophyte

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Bioactive secondary metabolites with multiple activities from a fungal endophyte

Catherine W Bogner et al. Microb Biotechnol. 2017 Jan.

Abstract

In order to replace particularly biohazardous nematocides, there is a strong drive to finding natural product-based alternatives with the aim of containing nematode pests in agriculture. The metabolites produced by the fungal endophyte Fusarium oxysporum 162 when cultivated on rice media were isolated and their structures elucidated. Eleven compounds were obtained, of which six were isolated from a Fusarium spp. for the first time. The three most potent nematode-antagonistic compounds, 4-hydroxybenzoic acid, indole-3-acetic acid (IAA) and gibepyrone D had LC50 values of 104, 117 and 134 μg ml-1 , respectively, after 72 h. IAA is a well-known phytohormone that plays a role in triggering plant resistance, thus suggesting a dual activity, either directly, by killing or compromising nematodes, or indirectly, by inducing defence mechanisms against pathogens (nematodes) in plants. Such compounds may serve as important leads in the development of novel, environmental friendly, nematocides.

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Figures

Figure 1
Figure 1
Flow chart illustrating the process of extraction and fractionation of bioactive compounds produced by endophytic Fusarium oxysporum 162 on solid rice media. All compounds (1‐11) are highlighted in grey. Numbers in parentheses are dry weights (mg) of fractions.
Figure 2
Figure 2
Structures of compounds 1‐11 isolated from endophytic Fusarium oxysporum 162, grown on solid rice media.
Figure 3
Figure 3
Mortality rates of M. incognita J2 larvae after 24, 48 and 72 h contact with four Fo162 metabolites: 4‐hydroxybenzoic acid (5), indole‐3‐acetic acid (3), gibepyrone D (1) and methyl 2‐(4‐hydroxyphenyl)acetate (7) and the nematicides carbofuran (P1) and aldicarb (P2) as positive controls. (A) Dose‐dependent mortality test in 20, 50 and 100 μg ml−1. (B) Dose‐dependent mortality test in 150, 200 and 250 μg ml−1. (C) Comparison between the most potent compound 5 and the two positive controls. A compound was considered lethal when it caused significantly ( 0.05) high percentage of nematode death compared with the negative control (1% methanol). Data are expressed as the means ± standard errors of five replicates. Significance was tested according to Holm–Sidak multiple comparisons versus control group using Sigma plot 12.5. Means followed by asterisks (*** = P < 0.001; ** = P < 0.01) are significantly different from the mean percentage of dead nematodes in the negative control.
Figure 4
Figure 4
Bright field microscopic images of M. incognita morphology from bioassay treatment with 4‐hydroxybenzoic acid (5). Untreated nematode and treated nematode are indicated in figures A and B respectively. Visible vacuole‐like droplets can be seen inside the body (middle region) of the nematode (B).
Figure 5
Figure 5
Raman spectra of nematodes treated with compounds. Figures A1–A4 show bright field (BF) images of M. incognita treated with crude extract, compound (3) indole‐3‐acetic acid, compound (5) 4‐hydroxybenzoic acid and untreated respectively. Figures B1–B4 are the Raman images of the BF images. Images were generated by integrating the intensities of the C–H stretching vibrations which are characteristic for organic molecules. Figures C1–C4 were reconstructed from figures B1–B4 using spectral decomposition algorithm as described in the in‐house literature (methods section) while Figures D1–D4 represent the associated Raman spectra information.
Figure 6
Figure 6
Relative composition of detected lipids in M. incognita after treatment with the crude extract.
Figure 7
Figure 7
Diagram showing the tripartite interactions among plant, endophyte and nematode. The endophyte (Fo162) can induce nematode resistance through direct and indirect mechanisms. In the plant, defence responses towards the nematode and vigour can be increased by the endophyte‐produced 4‐hydroxybenzoic acid (4‐HBA) and indole‐3‐acetic (IAA). Additionally, the phytohormone (IAA) and the salicylic acid isomer (4‐HBA) have a dual function and can, together with other metabolites produced by the endophyte, be involved in the killing of the nematode, M. incognita. GP‐D, gibepyrone D; MPHA, methyl 2‐(4‐hydroxyphenyl)acetate; IAAME, indole‐3‐acetic acid methyl ester; MHB, methyl 4‐hydroxybenzoate; GP‐G, gibepyrone G.

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