BACKGROUND Root‐knot nematodes (RKNs) are harmful plant‐parasitic nematodes that cause serious damage to plant hosts. In the long‐term practice of RKN management, bacterial nematicides have attracted increasing attention as an effective biocontrol means. Here we determined the active substances against Meloidogyne incognita from a nematicidal bacterium, developed a biocontrol agent (BCA) based on optimized culture processes. The effects of the BCA on RKN control and plant growth‐promotion were evaluated in tomato pot trials. RESULTS Pseudomonas simiae strain MB751 exhibiting significant nematicidal activity against M. incognita second‐stage juveniles (J2) with approximately 80% mortality (with culture supernatant, 96% volume percentage) was isolated from a vineyard. A set of purification and identification experiments was performed to determine the main nematicidal component in MB751. A cyclic dipeptide Cyclo(L‐Pro‐L‐Leu) was identified with a lethal concentration necessary to kill 50% of the population (LC50) of 65.3 μg mL−1 against M. incognita J2. Following optimization trials on culture medium/fermentation conditions, such as the single factor test, Plackett–Burman test, steepest ascent, and response surface methodology experiments, the MB751 fermentation broth was then prepared as a BCA via a cold‐air drying process. The BCA and was evaluated in tomato pot experiments for effectiveness in suppressing M. incognita. Significant effects on M. incognita suppression and plant‐growth promotion as well as induced systemic resistance to M. incognita of tomato, were observed. CONCLUSION The cyclic dipeptide‐producing bacterium P. simiae MB751 exhibited high nematicidal activity and performance. Further development of this BCA should be pursued for the management of M. incognita in agriculture. © 2021 Society of Chemical Industry.
The increasing prevalence of crop-threatening root-knot nematodes (RKNs) has stimulated extensive research to discover effective nematicides. A highly focused strategy for accomplishing this is the development of biocontrol agents by a variety of soilborne microorganisms, as different bacterial metabolites have demonstrated promising nematicidal activities. In this study, we characterized the nematicidal and suppressive activity of a bacterial isolate against the agriculturally important RKN Meloidogyne incognita and the model nematode Caenorhabditis elegans, and the main M. incognita-toxic metabolite of the strain. After a preliminary screening of 22 bacterial isolates with a corrected mortality (CM) of whole-cell culture greater than 50% against C. elegans from different RKN-incident soils in China, a total of 14 isolates with CM of the supernatant of culture suspension (SCS) higher than 50% against both M. incognita and C. elegans were rescreened. An isolate with the highest CM of 86.1% and 95.0% for M. incognita and C. elegans, respectively, was further identified as the species Brevundimonas bullata via morphological examination, physiological and biochemical assays and alignment analysis of 16S rRNA gene sequences. The SCS of this strain, namely, B. bullata MB756, exhibited synchronous M. incognita killing activity along with significant detrimental effects on the growth, brood size, and locomotion of C. elegans. The effects of heat treatment, pH, inoculations, and protease K proteolysis on the CM of MB756 SCS were evaluated. A major M. incognita-toxic substance in the MB756 SCS was assayed and identified using thin-layer chromatography, column chromatography and high-performance liquid chromatography with a mass spectrometer, and it was preliminarily identified as 2-ethylhexan-1-ol, with a molecular formula of C8H18O and a molecular weight of 130.3 Da.
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