Bacterial volatiles represent an important source for new natural nematicidal compounds that may be developed as novel nematicidal agents to control plant-parasitic nematodes. In this study, two marine bacteria,Pseudoalteromonas marinastrain H-42 andVibrio atlanticusstrain S-16, were isolated from seawater and the bay scallop (Argopecten irradians), respectively, collected from the subtidal beds at Qingdao, China.In vitrobioassays indicated that culture supernatants of the two bacteria displayed strong nematicidal activities with corrected mortalities of more than 93% after 24 h exposure against the pine wood nematode (PWN),Bursaphelenchus xylophilus. Maximal nematicidal activity was exhibited by culture supernatants of the two marine bacteria incubated at 15-20°C for 3 days in Zobell 2216E medium prepared with 100% seawater and initial pH 6.0-7.0, inoculating 19-h-old cultures. Through analysis using solid-phase microextraction-gas chromatography/mass spectrometry, the volatiles produced by the two bacteria were shown to contain mainly dimethyl disulphide, benzaldehyde, dimethyl trisulphide,tert-butylamine, acetone and dimethylamine, N-(diisopropylphosphino)methyl-. Results from nematicidal assay using pure commercial compounds instead of the candidate volatiles demonstrated that the four volatile compounds, dimethyl trisulphide (LC90 = 0.060 mmol l−1), benzaldehyde (LC90 = 0.309 mmol l−1), dimethyl disulphide (LC90 = 0.162 mmol l−1) andtert-butylamine (LC90 = 0.366 mmol l−1), showed strong nematicidal activities against PWN. This is the first report on the nematicidal activity of volatile organic compounds (VOC) from marine microorganisms. This study indicates that the nematicidal VOC produced by marine bacteria are potential substitutes for current chemical control options of pine wilt disease caused by PWN which greatly threatens global forest resources.
The ethanol extract of the Ficus carica L. leaves was tested to show strong nematicidal activity against pine wood nematode (PWN), Bursaphelenchus xylophilus, causing 90.93% corrected mortality within 72 h at 1.0 mg/mL. From the ethyl acetate soluble fraction of the F. carica L. leaves extract, the main nematicidal constituents were obtained by bioassay-guided isolation and identified as linear furocoumarins bergapten (1) and psoralen (2) by mass and NMR spectral data analysis. Bergapten and psoralen had significant nematicidal activity against PWN with the LC50 values of 97.08 aKSnd 115.03 μ g/mL within 72 h, respectively. The two furocoumarins could inhibit the activities of amylase, cellulase and acetylcholinesterase (AchE) from PWN. The morphologies of PWNs changed much after they were treated by bergapten and psoralen. The physiological effects of bergapten and psoralen on PWN might provide helpful clues to elucidate their nematicidal mechanisms.
The ethanol extracts from the roots of Angelica pubescens Maxim. f. biserrata Shan et Yuan was toxic against the pine wood nematode Bursaphelenchus xylophilus. The ethyl acetate-soluble fraction derived from this extract increased its potency with a mortality of 95.25% in 72 hr at 1.0 mg/mL. Four nematotoxic coumarins were obtained from the ethyl acetate extract by bioassay-guided isolation. These were identified as osthole 1, columbianadin 2, bergapten 3 and xanthotoxin 4 by mass and nuclear magnetic resonance spectral data analysis. The LC 50 values against B. xylophilus in 72 hr were 489.17, 406.74, 430.08, and 435.66 μ M, respectively. These compounds also altered the smooth morphology of the B. xylophilus exoskeleton to a rough and pitted appearance as visualized by electron microscopy. The coumarins 1-4 possessed significant acetylcholinesterase inhibitory activities but had negligible effects on amylase and cellulase. This research provides additional clues to the nematotoxic mechanism of coumarins against the pine wood nematode B. xylophilus. This work will assist in the development of coumarin nematicides with enhanced activity using molecular modifications of the core coumarin structure.
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