Selection of Bacterial Strains for Control of Root-Knot Disease Caused by Meloidogyne incognita

The role of Bacillus species in the management of plant-parasitic nematodes

Plant-parasitic nematodes (PPNs), including root-knot nematodes (Meloidogyne spp.), cyst nematodes (Heterodera and Globodera spp.), and other economically significant nematode species, pose severe threats to global agriculture. These nematodes employ diverse survival strategies, such as dormancy in cysts or robust infective juvenile stages. Consequently, their management is challenging. Traditional control methods, such as the use of chemical nematicides, are increasingly scrutinized because of environmental and health concerns. This review focuses on the specific mechanisms employed by Bacillus spp., including nematicidal compound production, systemic resistance induction, and cuticle degradation, to target root-knot and cyst nematodes. These mechanisms offer sustainable solutions for managing nematodes and promoting soil health by enhancing microbial diversity and nutrient cycling. An integrated approach leveraging Bacillus-based biocontrol is proposed to maximize efficacy and agricultural sustainability.

Bacillus velezensis A-27 as a potential biocontrol agent against Meloidogyne incognita and effects on rhizosphere communities of celery in field

Meloidogyne incognita, a highly destructive plant-parasitic nematode, poses a significant threat to crop production. The reliance on chemical nematicides for nematode control has been crucial; however, the banning of many effective nematicides due to their adverse effects has necessitated the exploration of alternative solutions. Rhizosphere biocontrol bacteria, particularly strains of Bacillus, have demonstrated promising results in managing plant-parasitic nematodes. In this study, strain A-27 was identified as Bacillus velezensis based on its morphological, physiological, and molecular characteristics, including 16 S rRNA and gyrA sequencing. Strain A-27 exhibited high larvicidal and ovicidal efficacy in vitro, with estimated LC50 values of 4.0570 × 108 CFU/mL for larvicidal efficacy and 3.6464 × 108 CFU/mL for ovicidal efficacy. In a pot experiment, B. velezensis A-27 significantly reduced the root gall index, achieving a control efficacy of 85.36%. Field experiments further indicated that A-27 reduced the root gall index with a control efficacy of 67.31%, while also decreasing the J2 population density of M. incognita and significantly enhancing the growth of celery plants. Additionally, high-throughput sequencing analysis revealed that B. velezensis A-27 significantly increased the relative abundances of Bacillus and Sphingomonas, while markedly reducing the relative abundances of Fusarium, Mortierella, and Cephaliophora in the celery rhizosphere. These findings suggest that B. velezensis A-27 has potential as an effective biocontrol agent against M. incognita, offering a promising alternative to chemical nematicides in sustainable agriculture.

The combination of chemical fertilizer affected the control efficacy against root-knot nematode and environmental behavior of abamectin in soil

Chemical fertilizer and pesticide are necessary in agriculture, which have been frequently used, sometimes even at the same time or in combination. To understand the interactions of them could be of significance for better use of these agrochemicals. In this study, the influence of chemical fertilizers (urea, potassium sulfate, ammonium sulfate and superphosphate) on the control efficacy and environmental behavior of abamectin was investigated, which could be applied in soil for controlling nematodes. In laboratory assays, ammonium sulfate at 1 and 2 g/L decreased the LC50 values of abamectin to Meloidogyne incognita from 0.17 mg/L to 0.081 and 0.043 mg/L, indicating it could increase the contact toxicity. In greenhouse trial, ammonium sulfate at 1000 mg/kg increased the control efficacy of abamectin by 1.37 times. Meanwhile, the combination of abamectin with ammonium sulfate could also promote the tomato seedling growth as well as the defense-related enzyme activity under M. incognita stress. The persistence and mobility of abamectin in soil were significantly elevated by ammonium sulfate, which could prolong and promote the control efficacy against nematodes. These results could provide reference for reasonable use of abamectin and fertilizers so as to increase the control efficacy and minimize the environmental risks.

Antagonistic Bacteria Bacillus velezensis VB7 Possess Nematicidal Action and Induce an Immune Response to Suppress the Infection of Root-Knot Nematode (RKN) in Tomato

Meloidogyne incognita, the root-knot nematode (RKN), a devastating plant parasitic nematode, causes considerable damage to agricultural crops worldwide. As a sedentary root parasite, it alters the root's physiology and influences the host's phytohormonal signaling to evade defense. The sustainable management of RKN remains a challenging task. Hence, we made an attempt to investigate the nematicide activity of Bacillus velezensis VB7 to trigger the innate immune response against the infection of RKN. In vitro assay, B. velezensis VB7 inhibited the hatchability of root-knot nematode eggs and juvenile mortality of M. incognita by 87.95% and 96.66%, respectively at 96 hrs. The application of B. velezensis VB7 challenged against RKN induced MAMP-triggered immunity via the expression of transcription factors/defense genes by several folds pertaining to WRKY, LOX, PAL, MYB, and PR in comparison to those RKN-inoculated and healthy control through RT-PCR. Additionally, Cytoscape analysis of defense genes indicated the coordinated expression of various other genes linked to immune response. Thus, the current study clearly demonstrated the effectiveness of B. velezensis VB7 as a potential nematicide and inducer of immune responses against RKN infestation in tomato.

Microbial landscapes of the rhizosphere soils and roots of Luffa cylindrica plant associated with Meloidogyne incognita

Introduction

The root-knot nematodes (RKN), especially Meloidogyne spp., are globally emerging harmful animals for many agricultural crops.

Methods

To explore microbial agents for biological control of these nematodes, the microbial communities of the rhizosphere soils and roots of sponge gourd (Luffa cylindrica) infected and non-infected by M. incognita nematodes, were investigated using culture-dependent and -independent methods.

Results

Thirty-two culturable bacterial and eight fungal species, along with 10,561 bacterial and 2,427 fungal operational taxonomic units (OTUs), were identified. Nine culturable bacterial species, 955 bacterial and 701 fungal OTUs were shared in both four groups. More culturable bacterial and fungal isolates were detected from the uninfected soils and roots than from the infected soils and roots (except no fungi detected from the uninfected roots), and among all samples, nine bacterial species (Arthrobacter sp., Bacillus sp., Burkholderia ambifaria, Enterobacteriaceae sp., Fictibacillus barbaricus, Microbacterium sp., Micrococcaceae sp., Rhizobiaceae sp., and Serratia sp.) were shared, with Arthrobacter sp. and Bacillus sp. being dominant. Pseudomonas nitroreducens was exclusively present in the infested soils, while Mammaliicoccus sciuri, Microbacterium azadirachtae, and Priestia sp., together with Mucor irregularis, Penicillium sp., P. commune, and Sordariomycetes sp. were found only in the uninfected soils. Cupriavidus metallidurans, Gordonia sp., Streptomyces viridobrunneus, and Terribacillus sp. were only in the uninfected roots while Aspergillus sp. only in infected roots. After M. incognita infestation, 319 bacterial OTUs (such as Chryseobacterium) and 171 fungal OTUs (such as Spizellomyces) were increased in rhizosphere soils, while 181 bacterial OTUs (such as Pasteuria) and 166 fungal OTUs (such as Exophiala) rose their abundance in plant roots. Meanwhile, much more decreased bacterial or fungal OTUs were identified from rhizosphere soils rather than from plant roots, exhibiting the protective effects of host plant on endophytes. Among the detected bacterial isolates, Streptomyces sp. TR27 was discovered to exhibit nematocidal activity, and B. amyloliquefaciens, Bacillus sp. P35, and M. azadirachtae to show repellent potentials for the second stage M. incognita juveniles, which can be used to develop RKN bio-control agents.

Discussion

These findings provided insights into the interactions among root-knot nematodes, host plants, and microorganisms, which will inspire explorations of novel nematicides.

Biological agents and their metabolites to control <i>Meloidogyne</i> spp. when growing vegetables (review)

Aim. Analysis of modern studies on the effectiveness of fungi and antagonist   bacteria   against   Meloidogyne   root‐knot   nematodes   on vegetable crops.

Materials and Methods. Studies of Russian and foreign scientists on the use of biological agents and their metabolites to control Meloidogyne spp. when growing vegetables have been carefully analysed.

Results.   The   harmfulness   of   gall   nematodes   on   vegetable   crops   is described.  Studies  on  the  most  pathogenic  species  of  Meloidogyne, including those common in Russia, are summarised. Information is given regarding  features  of  the  relationship  between  the  host  plant  and phytoparasites are highlighted. An analysis of the range of chemical and biological nematicides is presented. The problem of the lack of effective environmentally friendly products able to control root‐knot nematodes on vegetables, including  the  prospect of using  biological agents, has  been identified.   The   features   of   ongoing   research   on   the   study   of   the nematicidal activity of biological agents and their metabolites to control various   stages   of   development   of   Meloidogyne   species   have   been collected, analysed, systematised and described. The prospect of studying the mechanisms of action of microorganisms against root‐knot nematodes is substantiated in order to create new effective biological nematicides that allow the growth of high‐quality and healthy vegetable products.

Conclusion. Gall nematodes (Meloidogyne spp.) remain a current pest of soil‐grown  vegetables.  Scientists  are  actively  working  on  the  study  of nematophagous fungi and antagonist bacteria to create environmentally friendly  biological  nematicides.  With  proper  use,  biological  agents  and their metabolites can help protect plants from phytoparasites at the level of chemical nematicides and have an additional beneficial effect on the growth and development of vegetable crops.

 

Application Potential of Bacterial Volatile Organic Compounds in the Control of Root-Knot Nematodes

Plant-parasitic nematodes (PPNs) constitute the most damaging group of plant pathogens. Plant infections by root-knot nematodes (RKNs) alone could cause approximately 5% of global crop loss. Conventionally, chemical-based methods are used to control PPNs at the expense of the environment and human health. Accordingly, the development of eco-friendly and safer methods has been urged to supplement or replace chemical-based methods for the control of RKNs. Using microorganisms or their metabolites as biological control agents (BCAs) is a promising approach to controlling RKNs. Among the metabolites, volatile organic compounds (VOCs) have gained increasing attention because of their potential in the control of not only RKNs but also other plant pathogens, such as insects, fungi, and bacteria. This review discusses the biology of RKNs as well as the status of various control strategies. The discovery of VOCs emitted by bacteria from various environmental sources and their application potential as BCAs in controlling RKNs are specifically addressed.

Reproductive Toxicity of Furfural Acetone in Meloidogyne incognita and Caenorhabditis elegans

Furfural acetone (FAc) is a promising alternative to currently available nematicides, and it exhibits equivalent control efficiency on root-knot nematodes with avermectin in fields. However, its effect on the reproduction of root-knot nematode is poorly understood. In this study, the natural metabolite FAc was found to exhibit reproductive toxicity on Meloidogyne incognita and Caenorhabditis elegans. The number of germ cells of C. elegans was observed to decrease after exposure to FAc, with a reduction of 59.9% at a dose of 200 mg/L. FAc in various concentrations induced the germ-cell apoptosis of C. elegans, with an increase over six-fold in the number of apoptotic germ cells at 200 mg/L. These findings suggested that FAc decreased the brood size of nematode by inducing germ-cell apoptosis. Moreover, FAc-induced germ-cell apoptosis was suppressed by the mutation of gene hus-1, clk-2, cep-1, egl-1, ced-3, ced-4, or ced-9. The expression of genes spo-11, cep-1, and egl-1 in C. elegans was increased significantly after FAc treatment. Taken together, these results indicate that nematode exposure to FAc might inflict DNA damage through protein SPO-11, activate CEP-1 and EGL-1, and induce the core apoptosis pathway to cause germ-cell apoptosis, resulting in decreased brood size of C. elegans.

Genomic, Antimicrobial, and Aphicidal Traits of Bacillus velezensis ATR2, and Its Biocontrol Potential against Ginger Rhizome Rot Disease Caused by Bacillus pumilus

Ginger rhizome rot disease, caused by the pathogen Bacilluspumilus GR8, could result in severe rot of ginger rhizomes and heavily threaten ginger production. In this study, we identified and characterized a new Bacillus velezensis strain, designated ATR2. Genome analysis revealed B. velezensis ATR2 harbored a series of genes closely related to promoting plant growth and triggering plant immunity. Meanwhile, ten gene clusters involved in the biosynthesis of various secondary metabolites (surfactin, bacillomycin, fengycin, bacillibactin, bacilysin, difficidin, macrolactin, bacillaene, plantazolicin, and amylocyclicin) and two clusters encoding a putative lipopeptide and a putative phosphonate which might be explored as novel bioactive compounds were also present in the ATR2 genome. Moreover, B. velezensis ATR2 showed excellent antagonistic activities against multiple plant pathogenic bacteria, plant pathogenic fungi, human pathogenic bacteria, and human pathogenic fungus. B. velezensis ATR2 was also efficacious in control of aphids. The antagonistic compound from B. velezensis ATR2 against B.pumilus GR8 was purified and identified as bacillomycin D. In addition, B. velezensis ATR2 exhibited excellent biocontrol efficacy against ginger rhizome rot disease on ginger slices. These findings showed the potential of further applications of B. velezensis ATR2 as a biocontrol agent in agricultural diseases and pests management.