Nematicidal activity of verrucarin A and roridin A isolated from Myrothecium verrucaria against Meloidogyne incognita

Metarhizium carneum Formulations: A Promising New Biological Control to Be Incorporated in the Integrated Management of Meloidogyne enterolobii on Tomato Plants

The increase in the populations of root-knot nematode Meloidogyne enterolobii in various vegetables such as tomatoes grown under greenhouse conditions as well as increasing restrictions on the use of certain chemical nematicides have led to the search for new, effective management strategies, preferably ones that are sustainable biological alternatives. In this work, two formulations of the nematophagous fungus Metarhizium carneum, one concentrated suspension and one wettable powder, were evaluated under greenhouse conditions to reduce the M. enterolobii infestation in tomato plants. In addition, the effectiveness of the liquid formulation of M. carneum was compared with two biological and three chemical commercial nematicides. The results show that the two M. carneum formulations reduced the M. enterolobii population density by 78 and 66% in relation to the control treatment. In comparison, the liquid formulation of M. carneum and Purpureocillium lilacinum treatments reduced nematode population density by 72 and 43%, respectively, while for metam sodium preplanting applications followed by M. carneum applications during the tomato growth stage, the reduction was 96%. The alternate use of some chemical compounds plus the application of M. carneum as a biocontrol is a good starting strategy for managing M. enterolobii populations. These results confirm that M. carneum is a serious candidate for the short-term commercialization of an environmentally friendly biological nematicide.

Transcriptomic and physiological analysis of the effect of octanoic acid on Meloidogyne incognita

Root knot nematodes are the most devastating root pathogens, causing severe damage and serious economic losses to agriculture worldwide. Octanoic acid has been reported as one of the nematicides, and its mode of action is not fully understood. The main objective of this study was to elucidate the effect of octanoic acid on Meloidogyne incognita by transcriptomic analysis combined with physiological and biochemical assays. In the toxicity assays with octanoic acid, the threshold concentration with nematicidal activity and the maximum concentration to which nematodes could respond were 0.03 μL/mL and 0.08 μL/mL respectively. Microscopic observation combined with protein and carbohydrates assays confirmed that the structure of the second-stage juveniles (J2s) was severely disrupted after 72 h of immersion in octanoic acid. Transcriptome analysis has shown that octanoic acid can interfere with the nematode energy metabolism, lifespan and signaling. Although the effects are multifaceted, the findings strongly point to the cuticle, lysosomes, and extracellular regions and spaces as the primary targets for octanoic acid. In addition, nematodes can withstand the negative effects of low concentration of octanoic acid to some extent by up-regulating the defense enzyme system and heterologous metabolic pathways. These findings will help us to explore the nematicidal mechanism of octanoic acid and provide important target genes for the development of new nematicides in the future.

Natural nematicidal metabolites and advances in their biocontrol capacity on plant parasitic nematodes

Covering: 2010 to 2021Natural nematicidal metabolites are important sources of nematode control. This review covers the isolation and structural determination of nematicidal metabolites from 2010 to 2021. We summarise chemical structures, bioactivity, metabolic regulation and biosynthesis of potential nematocides, and structure-activity relationship and application potentiality of natural metabolites in plant parasitic nematodes' biocontrol. In doing so, we aim to provide a comprehensive overview of the potential roles that natural metabolites can play in anti-nematode strategies.

High killing rate of nematode and promotion of rice growth by synthetic volatiles from Bacillus strains due to enhanced oxidative stress response

The plant parasitic nematode Aphelenchoides besseyi is a major pest that poses serious threats to different vegetables and crop plants. In the present study, volatiles isolated from Bacillus spp. were utilized as green biocontrol agents to overcome nematodes. In in vitro experiment, Bacillus spp. GBSC56, SYST2, and FZB42 showed the strongest nematicidal activity with killing rates of 80.78%, 75.69%, and 60.45%, respectively, as compared with control. The selected synthetic volatile organic compounds (VOCs), namely albuterol, benzaldehyde (BDH), 1,2-benzisothiazol-3(2H)-one (1,2-HIT), dimethyl disulfide (DMDS), 2-undecanone (2-UD), and 1,3-propanediole (1,3-PD), exhibited strong nematicidal activity, with A. besseyi killing rate of 85.58%, 82.65%, 81.75%, 80.36%, 84.45%, and 82.36%, respectively, at 400 μg/mL. Microscopic analysis proved that the rapid mortality was due to the production of reactive oxygen species (ROS). Molecular docking attributed this ROS production to the nematicidal effect of synthetic VOCs on NADH DEHYDROGENASE SUBUNIT 2, which is known to play a critical role in the suppression of ROS in nematode models. In a greenhouse experiment, the Bacillus strains GBSC56, SYST2, and FZB42 and their synthetic VOCs significantly improved the physiological parameters in terms of growth promotion traits. In addition, selected genes related to growth promotion and defense genes showed a significant upregulation of their expression in rice seedlings treated with those synthetic VOCs. Overall, these findings revealed that the selected Bacillus strains and their synthetic VOCs possess high potential against A. besseyi. Moreover, this study also sheds new light on the mechanisms by which specific Bacillus nematicidal VOCs influence important genes involved in rice plant growth promotion and could effectively be used to suppress plant parasitic nematodes.

Utilization of Fishery-Processing By-Product Squid Pens for Scale-Up Production of Phenazines via Microbial Conversion and Its Novel Potential Antinematode Effect

Fishery by-products (FBPs) have been increasingly investigated for the extraction and production of a vast array of active molecules. The aim of this study was to produce phenazine compounds from FBPs via microbial fermentation and assess their novel antinematode effect. Among various FBPs, squid pen powder (SPP) was discovered as the most suitable substrate for phenazine production by Pseudomonas aeruginosa TUN03 fermentation. Various small-scale experiments conducted in flasks for phenazine production indicated that the most suitable was the newly designed liquid medium which included 1% SPP, 0.05% MgSO4, and 0.1% Ca3(PO4)2 (initial pH 7). Phenazines were further studied for scale-up bioproduction in a 14 L bioreactor system resulting in a high yield (22.73 µg/mL) in a much shorter cultivation time (12 h). In the fermented culture broth, hemi-pyocyanin (HPC) was detected as a major phenazine compound with an area percentage of 11.28% in the crude sample. In the bioactivity tests, crude phenazines and HPC demonstrate novel potential nematicidal activity against black pepper nematodes, inhibiting both juveniles (J2) nematodes and egg hatching. The results of this work suggest a novel use of SPP for cost-effective bioproduction of HPC, a novel potential nematodes inhibitor. Moreover, the combination of MgSO4 and Ca3(PO4)2 was also found to be a novel salt composition that significantly enhanced phenazine yield by P. aeruginosa fermentation in this work.

Isolation and characterisation of nematicidal compound, leolorin C, from Leonotis leonurus acetone leaf extract

ETHNOPHARMACOLOGICAL RELEVANCE

Leonotis leonurus (L.) R.Br. (Lamiaceae) is a perennial shrub native to South Africa used to treat various diseases including digestive tract problems, intestinal worms and constipation.

AIM OF THE STUDY

The aim was to isolate and characterise nematicidal compounds from leaves of L. leonurus.

MATERIALS AND METHODS

Bioassay-guided fractionation was carried out using the free-living nematode Caenorhabditis elegans as a model organism. Structural elucidation of the purified compound was carried out using NMR spectroscopic analyses and UPLC-QTOF-MS. The fractions and the isolated compound were tested for nematicidal activity on motility of plant-parasitic Meloidogyne incognita juveniles (J2s) and J2 hatch inhibition. Further screening was done to determine the minimum inhibitory concentration (MIC) of the fractions against bacterial phytopathogens and cytotoxicity against Vero kidney cells.

RESULTS

Leoleorin C isolated from L. leonurus had moderate activity against C. elegans juveniles (34%) but was not active against J2 motility and J2 hatch of M. incognita. Thus, activity against the free-living C. elegans did not correspond with efficacy against plant-parasitic nematodes. Leoleorin C was not active against the tested bacterial phytopathogens, but some activity was observed in the bioautography assay against Clavibacter michiganensis subsp. michiganensis, the organism causing bacterial canker in tomatoes. The plant extract, fractions and leolorin C were relatively non-toxic to Vero cells with LC50 values greater than 0.01 mg/mL.

CONCLUSION

The crude extract of L. leonurus and fractions may be useful in developing complementary treatments for controlling nematodes and phytopathogens. This study does not support the use of free-living nematodes as a model to isolate anti-parasitic compounds from plants.

Bacillus nematocida B16 Enhanced the Rhizosphere Colonization of Pochonia chlamydosporia ZK7 and Controlled the Efficacy of the Root-Knot Nematode Meloidogyne incognita

Pochonia chlamydosporia is widely applied in many countries as a biocontrol fungus against parasitic nematodes in plants. In a field experiment, the combined use of Bacillus nematocida B16 increased the biocontrol efficiency of P. chlamydosporia ZK7 against Meloidogyne incognita. Further study indicated that the colonization of P. chlamydosporia ZK7 in the rhizosphere soil and the roots of tomatoes was significantly higher in the combined use group than in the control group. Gas chromatography was conducted to determine the effects of signaling substances. Five compounds, hexanal, (E)-2-hexenal, furfural, benzaldehyde, and 2-nonanone, were found to be highly altered in the volatile compounds produced in the soil under the combined application. The changes in benzaldehyde and 2-nonanone were the main factors that resulted in an increase in the colonization of fungi P. chlamydosporia ZK7 in the tomato roots. Furfural was the main volatile substance that affected the colonization of fungi P. chlamydosporia ZK7 in the soil. The combined use of B. nematocida B16 and P. chlamydosporia ZK7 altered the volatile ranges and resulted in increased colonization of biocontrol fungi and improved biocontrol efficiency against nematodes. This combined model could be used to promote the ability of biocontrol fungi to control root-knot nematodes.

Volatile Organic Compounds from Bacillus aryabhattai MCCC 1K02966 with Multiple Modes against Meloidogyne incognita

Plant-parasitic nematodes cause severe losses to crop production and economies all over the world. Bacillus aryabhattai MCCC 1K02966, a deep-sea bacterium, was obtained from the Southwest Indian Ocean and showed nematicidal and fumigant activities against Meloidogyne incognita in vitro. The nematicidal volatile organic compounds (VOCs) from the fermentation broth of B. aryabhattai MCCC 1K02966 were investigated further using solid-phase microextraction gas chromatography-mass spectrometry. Four VOCs, namely, pentane, 1-butanol, methyl thioacetate, and dimethyl disulfide, were identified in the fermentation broth. Among these VOCs, methyl thioacetate exhibited multiple nematicidal activities, including contact nematicidal, fumigant, and repellent activities against M. incognita. Methyl thioacetate showed a significant contact nematicidal activity with 87.90% mortality at 0.01 mg/mL by 72 h, fumigant activity in mortality 91.10% at 1 mg/mL by 48 h, and repellent activity at 0.01-10 mg/mL. In addition, methyl thioacetate exhibited 80-100% egg-hatching inhibition on the 7th day over the range of 0.5 mg/mL to 5 mg/mL. These results showed that methyl thioacetate from MCCC 1K02966 control M. incognita with multiple nematicidal modes and can be used as a potential biological control agent.

Nematicidal Activity of Cyclopiazonic Acid Derived From Penicillium commune Against Root-Knot Nematodes and Optimization of the Culture Fermentation Process

Among 200 fungal strains isolated from the soil, only one culture filtrate of Aspergillus flavus JCK-4087 showed strong nematicidal activity against Meloidogyne incognita. The nematicidal metabolite isolated from the culture filtrate of JCK-4087 was identified as cyclopiazonic acid (CPA). Because JCK-4087 also produced aflatoxins, six strains of Penicillium commune, which have been reported to be CPA producers, were obtained from the bank and then tested for their CPA productivity. CPA was isolated from the culture filtrate of P. commune KACC 45973. CPA killed the second-stage juveniles of M. incognita, M. hapla, and M. arearia with EC50-3 days 4.50, 18.82, and 60.51 μg mL-1, respectively. CPA also significantly inhibited egg hatch of M. incognita and M. hapla after a total of 28 days of treatment with the concentrations > 25 μg mL-1. The enhancement of CPA production by P. commune KACC 45973 was explored using an optimized medium based on Plackett-Burman design (PBD) and central composite design (CCD). The highest CPA production (381.48 μg mL-1) was obtained from the optimized medium, exhibiting an increase of 7.88 times when compared with that from potato dextrose broth culture. Application of the wettable power-type formulation of the ethyl acetate extract of the culture filtrate of KACC 45973 reduced gall formation and nematode populations in tomato roots and soils under greenhouse conditions. These results suggest that CPA produced by P. commune KACC 45973 can be used as either a biochemical nematicide or a lead molecule for developing chemical nematicides to control root-knot nematodes.

Chemical Nematicides: Recent Research Progress and Outlook

Plant-parasitic nematodes have caused huge economic losses to agriculture worldwide and seriously threaten the sustainable development of modern agriculture. Chemical nematicides are still the most effective means to manage nematodes. However, the long-term use of organophosphorus and carbamate nematicides has led to a lack of field control efficacy and increased nematode resistance. To meet the huge market demand and slow the growth of resistance, new nematicides are needed to enter the market. The rational design and synthesis of new chemical scaffolds to screen for new nematicides is still a difficult task. We reviewed the latest research progress of nematicidal compounds in the past decade, discussed the structure-activity relationship and mechanism of action, and recommended some nematicidal active fragments. It is hoped that this review can update the recent progress on nematicide discoveries and provide new ideas for the design and mechanism of action studies of nematicides.

New tricks for old dogs: Two new macrocyclic trichothecene epimers and absolute configuration of 16-hydroxyverrucarin B

Two new compounds, 3'-epi-16-hydroxyverrucarin A and 3'-epiverrucarin X, have been isolated and identified, and the characterization data of a series of known trichothecenes have been refined. The interesting structure and potent biological activities of macrocyclic trichothecenes have been of interest to the scientific community for several decades. However, some of the characterization data for the older analogues of this class are not well documented, either because of a lack of absolute configuration or a lack of clarity in the NMR data, largely due to technological limitations at the time they were discovered. NMR techniques, application of Mosher's esters analysis, and electronic circular dichroism were used here both to refine the characterization of known trichothecenes, as well as to uncover new structures. These studies demonstrate strategies that can be used to interrogate the characterization data of well-known secondary metabolites, thereby gaining greater insight into methods that can be used to refine previous literature.

Nematicidal activity of 5-iodoindole against root-knot nematodes

Multi-drug resistance in nematodes is a serious problem as lately several resistant phenotypes have emerged following the intermittent usage of synthetic nematicides. Contemporary research continues to focus on developing and/or repurposing small molecule inhibitors that are eco-friendly. Here, we describe the repurposing of the indole derivative, 5-iodoindole, as a nematicide for the root-knot nematode, Meloidogyne incognita. 5-Iodoindole effectively killed juveniles and freshly hatched juveniles by inducing multiple vacuole formation. Notably, at higher dosage (50 μg/mL), 5-iodoindole induced rapid juvenile death within 6 h. Microscopic analysis confirmed that the rapid death was due to the generation of reactive oxygen species (ROS). Computational docking attributed this ROS production to the antagonistic effect of 5-iodoindole on glutathione S-transferase (GST), which is known to play a critical role in the suppression of ROS in nematode models. Furthermore, 5-iodoindole also effectively reduced the gall formations and eggs masses of M. incognita on Solanum lycopersicum roots in pot experiments, and importantly it did not harm the physiological properties of the plant. Overall, the study provides valuable insights on the use of 5-iodoindole as an alternate measure to control root-knot nematodes. Overall, our findings suggest the efficacy of 5-iodoindole should be studied under field conditions.

Utilization of Nematode Destroying Fungi for Management of Plant-Parasitic Nematodes-A Review

Nematode destroying fungi are potential biocontrol agent for management of plant-parasitic nematodes. They inhibit nematode population through trapping devices or by means of enzymes and metabolic products. They regulate nematode behavior by interfering plant-nematode recognition, and promote plant growth. For more effective biocontrol, thorough understanding of the biology of nematode destroying fungi, targeted nematode pest and the soil ecology and environmental condition in the field is necessary. This review highlights different types of nematode destroying fungi, their mode of action as well as commercial products based on reports published in this area of research.