A novel in vitro chemotaxis bioassay to assess the response of Meloidogyne incognita towards various test compounds

Involvement of G-protein alpha subunit in soybean cyst nematode chemotaxis

The soybean cyst nematode (SCN; Heterodera glycines Ichinohe) is a significant agricultural pest that causes extensive damage to soybean production worldwide. Second-stage juveniles (J2s) of the SCN migrate through the soil and infest the roots of host plants in response to certain chemical substances secreted from the host roots. Therefore, controlling SCN chemotaxis could be an effective strategy for its management. In the present study, we identified the Hg-gpa-3d gene, which encodes the G protein alpha subunit, as a key regulator of SCN chemotaxis. Gene silencing of Hg-gpa-3d reduced the attraction of SCN J2s to host roots, as well as to nitrate ions, a chemoattractant recognized through a mechanism different from that of host recognition. However, silencing of Hg-gpa-3d did not affect avoidance behavior towards unpleasant temperatures or stylet protrusion. These results suggest that Hg-gpa-3d is a crucial gene in the regulation of SCN chemotaxis and provide new insights into the chemotactic mechanisms of the SCN.

Induced knockdown of Mg-odr-1 and Mg-odr-3 perturbed the host seeking behavior of Meloidogyne graminicola in rice

Root-knot nematode Meloidogyne graminicola is one of the most destructive plant parasites in upland as well as direct seeded rice. As an integral part of nematode biology, host finding behavior involves perceiving and responding to different chemical cues originating from the rhizosphere. A sustainable management tactic may include retardation of nematode chemoreception that would impair them to detect and discriminate the host stimuli. Deciphering the molecular basis of nematode chemoreception is vital to identify chokepoints for chemical or genetic interventions. However, compared to the well-characterized chemoreception mechanism in model nematode Caenorhabditis elegans, plant nematode chemoreception is yet underexplored. Herein, the full-length cDNA sequences of two chemotaxis-related genes (Mg-odr-1 and Mg-odr-3) were cloned from M. graminicola. Both the genes were markedly upregulated in the early developmental stages of M. graminicola suggesting their involvement in host finding processes. RNAi-induced independent knockdown of Mg-odr-1 and Mg-odr-3 caused behavioral aberration in second-stage juveniles of M. graminicola which in turn perturbed the nematodes' host finding ability and parasitic success inside rice roots. Additionally, nematodes' chemotactic response to different host root exudates, volatile and nonvolatile compounds was affected. Our results demonstrating the role of specific chemosensory genes in modulating M. graminicola host seeking behavior can enrich the existing knowledge of plant nematode chemoreception mechanism, and these genes can be targeted for novel nematicide development or in planta RNAi screens.

Metabolic variations in root tissues and rhizosphere soils of weak host plants potently lead to distinct host status and chemotaxis regulation of Meloidogyne incognita in intercropping

Root-knot nematodes (RKNs) inflict extensive damage to global agricultural production. Intercropping has been identified as a viable agricultural tool for combating RKNs, but the mechanisms by which intercropped plants modulate RKN parasitism are still not well understood. Here, we focus on the cucumber-amaranth intercropping system. We used a range of approaches, including the attraction assay, in vitro RNA interference (RNAi), untargeted metabolomics, and hairy root transformation, to unveil the mechanisms by which weak host plants regulate Meloidogyne incognita chemotaxis towards host plants and control infection. Amaranth roots showed a direct repellence to M. incognita through disrupting its chemotaxis. The in vitro RNAi assay demonstrated that the Mi-flp-1 and Mi-flp-18 genes (encoding FMRFamide-like peptides) regulated M. incognita chemotaxis towards cucumber and controlled infection. Moreover, M. incognita infection stimulated cucumber and amaranth to accumulate distinct metabolites in both root tissues and rhizosphere soils. In particular, naringenin and salicin, enriched specifically in amaranth rhizosphere soils, inhibited the expression of Mi-flp-1 and Mi-flp-18. In addition, overexpression of genes involved in the biosynthesis of pantothenic acid and phloretin, both of which were enriched specifically in amaranth root tissues, delayed M. incognita development in cucumber hairy roots. Together, our results reveal that both the distinct host status and disruption of chemotaxis contribute to M. incognita inhibition in intercropping.

Novel Genes and Key Signaling Molecules Involved in the Repulsive Response of Meloidogyne incognita against Biocontrol Bacteria

The ability of the model organism, Caenorhabditis elegans, to distinguish and escape from pathogenic bacteria has been extensively studied; however, studies on the repulsive response of Meloidogyne incognita are still in their infancy. We have recently demonstrated that biocontrol bacteria induce a repulsive response in M. incognita via two classical signaling pathways. The present study aimed to identify the novel genes and signaling molecules of M. incognita that potentially contribute to its defense reaction. Analysis of the transcriptome data of M. incognita with and without a repulsive response against Bacillus nematocida B16 obtained 15 candidate genes, of which the novel genes Minc3s01748g26034 and Minc3s02548g30585 were found to regulate the aversive behavior of M. incognita, and their functions were further validated. To further confirm the neuronal localization of the two novel genes in M. incognita, in situ hybridization was conducted using the digoxin-labeled probes of ten tag genes, and preferentially profiled the localization of amphid sensory neurons of M. incognita. Analysis of the overviewed neuronal map suggested that Minc3s01748g26034 and Minc3s02548g30585 functioned in ASK/ASI and CEPD/V neurons, respectively. During their interactions, the volatile compounds 3-methyl-butyric acid and 2-methyl-butyric acid produced by the biocontrol bacteria were predicted as the primary signaling molecules that promoted the repulsive behavior of M. incognita against biocontrol bacteria. The findings provided novel insights into the mechanisms underlying the repulsive response of M. incognita that are different from the canonical molecular pathways previously found in C. elegans and can aid in developing novel strategies for controlling root-knot nematodes.

Toxicity of fungal-derived volatile organic compounds against root-knot nematodes

BACKGROUND

Root-knot nematodes (RKNs), including Meloidogyne species, are among the most destructive plant-parasites worldwide. Recent evidence suggests that entomopathogenic fungi (EPF) can antagonize RKNs. Such antagonistic effects are likely mediated by toxic metabolites, including volatile organic compounds (VOCs), produced by the fungi. However, how widespread these effects are across EPF species, and which VOCs mediate negative interactions between EPF and RKNs needs to be further elucidated.

RESULTS

First, we evaluated the nematicidal effect of VOCs emitted by 46 EPF isolates against Meloidogyne incognita and found variable toxicity depending on the isolate. Second, we measured the nematicidal effect of highly toxic isolates, including species in the genus Talaromyces, Aspergillus, Clonostachys, and Purpureocillium and, third, we analyzed the nematicidal effect of major VOCs, including 2-methyl-1-propanol, 3-methyl-1-butanol, isopropyl alcohol and 2-methyl-3-pentanone. The mortality of M. incognita juveniles (J2s) was generally high (50%) either via airborne or in-solution contact with VOCs. Moreover, the tested VOCs significantly inhibited egg hatching, and repelled J2s away from the VOCs.

CONCLUSION

This study not only provides insights into the ecological function of VOCs in the rhizosphere, but also provides new approaches for developing environmentally friendly control methods of RKNs in agroecosystems. © 2023 Society of Chemical Industry.

Involvement of cyclic nucleotide-gated channels in soybean cyst nematode chemotaxis and thermotaxis

The soybean cyst nematode (SCN) is one of the most damaging pests affecting soybean production. SCN displays important host recognition behaviors, such as hatching and infection, by recognizing several compounds produced by the host. Therefore, controlling SCN behaviors such as chemotaxis and thermotaxis is an attractive pest control strategy. In this study, we found that cyclic nucleotide-gated channels (CNG channels) regulate SCN chemotaxis and thermotaxis and Hg-tax-2, a gene encoding a CNG channel, is an important regulator of SCN behavior. Gene silencing of Hg-tax-2 and treatment with a CNG channel inhibitor reduced the attraction of second-stage juveniles to nitrate, an attractant with a different recognition mechanism from the host-derived chemoattractant(s), and to host soybean roots, as well as their avoidance behavior toward high temperatures. Co-treatment of ds Hg-tax-2 with the CNG channel inhibitor indicated that Hg-tax-2 is a major regulator of SCN chemotaxis and thermotaxis. These results suggest new avenues for research on control of SCN.

Dual enzyme-powered chemotactic cross β amyloid based functional nanomotors

Nanomotor chassis constructed from biological precursors and powered by biocatalytic transformations can offer important applications in the future, specifically in emergent biomedical techniques. Herein, cross β amyloid peptide-based nanomotors (amylobots) were prepared from short amyloid peptides. Owing to their remarkable binding capabilities, these soft constructs are able to host dedicated enzymes to catalyze orthogonal substrates for motility and navigation. Urease helps in powering the self-diffusiophoretic motion, while cytochrome C helps in providing navigation control. Supported by the simulation model, the design principle demonstrates the utilization of two distinct transport behaviours for two different types of enzymes, firstly enhanced diffusivity of urease with increasing fuel (urea) concentration and secondly, chemotactic motility of cytochrome C towards its substrate (pyrogallol). Dual catalytic engines allow the amylobots to be utilized for enhanced catalysis in organic solvent and can thus complement the technological applications of enzymes.

Stringent in-silico identification of putative G-protein-coupled receptors (GPCRs) of the entomopathogenic nematode Heterorhabditis bacteriophora

The infective juveniles (IJs) of entomopathogenic nematode (EPN) Heterorhabditis bacteriophora find and infect their host insects in heterogeneous soil ecosystems by sensing a universal host cue (CO2) or insect/plant-derived odorants, which bind to various sensory receptors, including G protein-coupled receptors (GPCRs). Nematode chemosensory GPCRs (NemChRs) bind to a diverse set of ligands, including odor molecules. However, there is a lack of information on the NemChRs in EPNs. Here we identified 21 GPCRs in the H. bacteriophora genome sequence in a triphasic manner, combining various transmembrane detectors and GPCR predictors based on different algorithms, and considering inherent properties of GPCRs. The pipeline was validated by reciprocal BLAST, InterProscan, GPCR-CA, and NCBI CDD search. Functional classification of predicted GPCRs using Pfam revealed the presence of four NemChRs. Additionally, GPCRs were classified into various families based on the reciprocal BLAST approach into a frizzled type, a secretin type, and 19 rhodopsin types of GPCRs. Gi/o is the most abundant kind of G-protein, having a coupling specificity to all the fetched GPCRs. As the 21 GPCRs identified are expected to play a crucial role in the host-seeking behavior, these might be targeted to develop novel insect-pest management strategies by tweaking EPN IJ behavior, or to design novel anthelminthic drugs. Our new and stringent GPCR detection pipeline may also be used to identify GPCRs from the genome sequence of other organisms.

Repulsive response of Meloidogyne incognita induced by biocontrol bacteria and its effect on interspecific interactions

The aversive behavior of Caenorhabditis elegans is an important strategy that increases their survival under pathogen infection, and the molecular mechanisms underlying this behavior have been described. However, whether this defensive response occurs in plant-parasitic nematodes (PPNs), which have quite different life cycles and genomic sequences from the model nematode, against biocontrol microbes and affects interspecific interactions in ecological environments remains unclear. Here, we showed that Meloidogyne incognita, one of the most common PPNs, engaged in lawn-leaving behavior in response to biocontrol bacteria such as Bacillus nematocida B16 and B. thuringiensis Bt79. Genomic analysis revealed that the key genes responsible for the aversive behavior of C. elegans, such as serotonin-and TGF-β-related genes in canonical signaling pathways, were homologous to those of M. incognita, and the similarity between these sequences ranged from 30% to 67%. Knockdown of the homologous genes impaired avoidance of M. incognita to varying degrees. Calcium ion imaging showed that the repulsive response requires the involvement of the multiple amphid neurons of M. incognita. In situ hybridization specifically localized Mi-tph-1 of the serotonin pathway to ADF/NSM neurons and Mi-dbl-1 of the TGF-β pathway to AVA neurons. Our data suggested that the repulsive response induced by different biocontrol bacteria strongly suppresses the invasion of tomato host plants by M. incognita. Overall, our study is the first to clarify the pathogen-induced repulsive response of M. incognita and elucidate its underlying molecular mechanisms. Our findings provide new insights into interspecific interactions among biocontrol bacteria, PPNs, and host plants.

Toward Chemical Ecology of Plant-Parasitic Nematodes: Kairomones, Pheromones, and Other Behaviorally Active Chemical Compounds

An overview of natural chemical compounds involved in plant-parasitic nematode (PPN) behavior is presented and classified following a system accepted by chemoecologists. Kairomonal and other egg-hatching stimulants, as well as attractants for juveniles, are presented. Sex, aggregation, egg-hatching, and putative diapause PPN pheromones are analyzed and grouped into clusters of primers and releasers. The role of over 500 chemical compounds, both organic and inorganic, involved in PPN behavior is reviewed, with the most widely analyzed and least studied fields of PPN chemical ecology indicated. Knowledge on PPN chemical ecology facilitates environmentally friendly integrated pest management. This could be achieved by disrupting biointeractions between nematodes and their host plants and/or between nematodes. Data on biologically active chemicals reveals targets for resistant plant selection, including through application of gene silencing techniques.

Negative regulation of root-knot nematode parasitic behavior by root-derived volatiles of wild relatives of Cucumis metuliferus CM3

Root-knot nematodes (RKN; Meloidogyne spp.) cause a significant decrease in the yield of cucumber crops every year. Cucumis metuliferus is an important wild germplasm that has resistance to RKN in which plant root volatiles are thought to play a role. However, the underlying molecular mechanism is unclear. To investigate it, we used the resistant C. metuliferus line CM3 and the susceptible cucumber line Xintaimici (XTMC). CM3 roots repelled Meloidogyne incognita second-stage larvae (J2s), while the roots of XTMC plants attracted the larvae. CM3 and XTMC were found to contain similar amounts of root volatiles, but many volatiles, including nine hydrocarbons, three alcohols, two aldehydes, two ketones, one ester, and one phenol, were only detected in CM3 roots. It was found that one of these, (methoxymethyl)-benzene, could repel M. incognita, while creosol and (Z)-2-penten-1-ol could attract M. incognita. Interestingly, creosol and (Z)-2-penten-1-ol effectively killed M. incognita at high concentrations. Furthermore, we found that a mixture of CM3 root volatiles increased cucumber resistance to M. incognita. The results provide insights into the interaction between the host and plant-parasitic nematodes in the soil, with some compounds possibly acting as nematode biofumigation, which can be used to manage 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.

Faba bean root exudates alter pea root colonization by the oomycete Aphanomyces euteiches at early stages of infection

Aphanomyces euteiches is an oomycete pathogen that causes the pea root rot. We investigated the potential role of early belowground defense in pea (susceptible plant) and faba bean (tolerant plant) at three days after inoculation. Pea and faba bean were inoculated with A. euteiches zoospores. Root colonization was examined. Root exudates from pea and faba bean were harvested and their impact on A. euteiches development were assessed by using in vitro assays. A. euteiches root colonization and the influence of the oomycete inoculation on specialized metabolites patterns and arabinogalactan protein (AGP) concentration of root exudates were also determined. In faba bean root, A. euteiches colonization was very low as compared with that of pea. Whereas infected pea root exudates have a positive chemotaxis index (CI) on zoospores, faba bean exudate CI was negative suggesting a repellent effect. While furanoacetylenic compounds were only detected in faba bean exudates, AGP concentration was specifically increased in pea.This work showed that early in the course of infection, host susceptibility to A. euteiches is involved via a plant-species specific root exudation opening new perspectives in pea root rot disease management.

Aromatic compounds that attract Meloidogyne species second-stage juveniles in soil

BACKGROUND

Nematode attractants could serve in nematode control strategies by combining with chemical or biological nematicides or by interrupting the nematodes' host-finding process. The attractiveness of some benzenoid aromatic compounds, mainly benzoic acids, alcohols, aldehydes and phenols, to second-stage juveniles (J2) of four Meloidogyne species (M. hapla, M. incognita, M. javanica and M. marylandi) was evaluated by using trap tubes and balls filled with washed dune sand buried in nematode-inoculated sand in Petri dishes.

RESULTS

Two-methoxybenzaldehyde, 2-methoxycinnamaldehyde, 2-hydroxybenzoic acid (salicylic acid), 2-hydroxy-3-methoxybenzaldehyde (o-vanillin), 3-methoxybenzoic acid, 4-methoxybenzoic acid and trans-cinnamic acid effectively attracted J2 of all or most of the four Meloidogyne species to trap tubes in a one-compound assay. When nematodes were exposed to three different compounds simultaneously in the three-compound assay, J2 of all Meloidogyne species were attracted mainly to 2-methoxycinnamaldehyde, salicylic acid and 4-methoxybenzoic acid. Exceptions were M. hapla J2, which were not attracted to salicylic acid. In the soil column assay, M. javanica and M. incognita J2 were attracted upward to 4-methoxybenzoic acid in a trap ball located 4 or 8 cm above the inoculation point, whereas salicylic acid and 3-methoxybenzoic acid demonstrated slight, if any attraction.

CONCLUSION

Although some of the tested compounds exist in root exudates, it is not clear whether they are involved in the nematode host-searching process in nature. The attractants found in the study have potential for use in Meloidogyne species control, probably as a nematode trap constituent or as compounds that disrupt the nematodes' host-finding process. © 2021 Society of Chemical Industry.

A Novel In Vitro Tool to Study Cyst Nematode Chemotaxis

This study presents a novel three-dimensional (3D) tool "3D in vitro choice" for chemotaxis assays with cyst nematodes. The original 3D in vitro choice was customized through digital printing. Freshly hatched second stage juveniles (J2s) of the cyst nematode Globodera pallida were used as the nematode model to illustrate chemo-orientation behavior in the 3D system. The efficiency and reliability of the 3D in vitro choice were validated with 2% Phytagel as navigation medium, in three biological assays and using tomato root exudates or potato root border cells and their associated mucilage as a positive attractant as compared with water. For each biological assay, J2s were hatched from the same population of a single generation glasshouse-cultured cysts. This novel easy to use and low-cost 3d-device could be a useful replacement to Petri dishes assays in nematode behavioral studies due to the ease of deposition of nematodes and test substances, coupled with its distinctive zones that allow for precision in choice making by the nematodes.

Homologs of Caenorhabditis elegans Chemosensory Genes Have Roles in Behavior and Chemotaxis in the Root-Knot Nematode Meloidogyne incognita

Nematode chemosensation is a vital component of their host-seeking behavior. The globally important phytonematode Meloidogyne incognita perceives and responds (via sensory organs such as amphids and phasmids) differentially to various chemical cues emanating from the rhizosphere during the course of host finding. However, compared with the free-living worm Caenorhabditis elegans, the molecular intricacies behind the plant nematode chemotaxis are a yet-unexploited territory. In the present study, four putative chemosensory genes of M. incognita, namely, Mi-odr-1, Mi-odr-3, Mi-tax-2, and Mi-tax-4 were molecularly characterized. Mi-odr-1 mRNA was found to be expressed in the cell bodies of amphidial neurons and phasmids of M. incognita. Mi-odr-1, Mi-odr-3, Mi-tax-2, and Mi-tax-4 transcripts were highly expressed in early life stages of M. incognita, consistent with a role of these genes in host recognition. Functional characterization of Mi-odr-1, Mi-odr-3, Mi-tax-2, and Mi-tax-4 via RNA interference revealed behavioral defects in M. incognita and perturbed attraction to host roots in Pluronic gel medium. Knockdown of Mi-odr-1, Mi-odr-3, Mi-tax-2, and Mi-tax-4 resulted in defective chemotaxis of M. incognita to various volatile compounds (alcohol, ketone, aromatic compound, ester, thiazole, pyrazine), nonvolatiles of plant origin (carbohydrate, phytohormone, organic acid, amino acid, phenolic), and host root exudates in an agar-Pluronic gel-based assay plate. In addition, ascaroside-mediated signaling was impeded by downregulation of chemosensory genes. This new information that behavioral response in M. incognita is modulated by specific olfactory genes can be extended to understand chemotaxis in other nematodes.

Impacts of Root Metabolites on Soil Nematodes

Plant parasitic nematodes cause significant crop damage globally. Currently, many nematicides have been banned or are being phased out in Europe and other parts of the world because of environmental and human health concerns. Therefore, we need to focus on sustainable and alternative methods of nematode control to protect crops. Plant roots contain and release a wide range of bioactive secondary metabolites, many of which are known defense compounds. Hence, profound understanding of the root mediated interactions between plants and plant parasitic nematodes may contribute to efficient control and management of pest nematodes. In this review, we have compiled literature that documents effects of root metabolites on plant parasitic nematodes. These chemical compounds act as either nematode attractants, repellents, hatching stimulants or inhibitors. We have summarized the few studies that describe how root metabolites regulate the expression of nematode genes. As non-herbivorous nematodes contribute to decomposition, nutrient mineralization, microbial community structuring and control of herbivorous insect larvae, we also review the impact of plant metabolites on these non-target organisms.