Bacteria used in the biological control of plant-parasitic nematodes: populations, mechanisms of action, and future prospects

As a group of important natural enemies of nematode pests, nematophagous bacteria exhibit diverse modes of action: these include parasitizing; producing toxins, antibiotics, or enzymes; competing for nutrients; inducing systemic resistance of plants; and promoting plant health. They act synergistically on nematodes through the direct suppression of nematodes, promoting plant growth, and facilitating the rhizosphere colonization and activity of microbial antagonists. This review details the nematophagous bacteria known to date, including parasitic bacteria, opportunistic parasitic bacteria, rhizobacteria, Cry protein-forming bacteria, endophytic bacteria and symbiotic bacteria. We focus on recent research developments concerning their pathogenic mechanisms at the biochemical and molecular levels. Increased understanding of the molecular basis of the various pathogenic mechanisms of the nematophagous bacteria could potentially enhance their value as effective biological control agents. We also review a number of molecular biological approaches currently used in the study of bacterial pathogenesis in nematodes. We discuss their merits, limitations and potential uses.     

Biological control potential of plant growth‐promoting rhizobacteria suppression of Meloidogyne incognita on cotton and Heterodera glycines on soybean: A review

Over the past decade, we have seen an increasing market for biopesticides and an increase in number of microbial control studies directed towards plant‐parasitic nematodes. This literature survey provides an overview of research on biological control of two economically important plant‐parasitic nematodes, Meloidogyne incognita (Kofoid & White) Chitwood (southern root‐knot nematode) and Heterodera glycines Ichinohe (soybean cyst nematode) using spore‐forming plant growth‐promoting rhizobacteria (PGPR). In this review, the current biological control strategies for the management of those cotton and soybean nematodes, the mechanism of using BacillusPGPR for biological control of plant‐parasitic nematode including induced systemic resistance and antagonism and the future of biological control agents on management of plant‐parasitic nematodes are covered.     

A family of glycosyl hydrolase family 45 cellulases from the pine wood nematode Bursaphelenchus xylophilus

We have characterized a family of GHF45 cellulases from the pine wood nematode Bursaphelenchus xylophilus. The absence of such genes from other nematodes and their similarity to fungal genes suggests that they may have been acquired by horizontal gene transfer (HGT) from fungi. The cell wall degrading enzymes of other plant parasitic nematodes may have been acquired by HGT from bacteria. B. xylophilus is not directly related to other plant parasites and our data therefore suggest that horizontal transfer of cell wall degrading enzymes has played a key role in evolution of plant parasitism by nematodes on more than one occasion.     

Interactions between bacteria and plant-parasitic nematodes: now and then

Based on genome-to-genome analyses of gene sequences obtained from plant-parasitic, root-knot nematodes (Meloidogyne spp.), it seems likely that certain genes have been derived from bacteria by horizontal gene transfer. Strikingly, a common theme underpinning the function of these genes is their apparent direct relationship to the nematodes' parasitic lifestyle. Phylogenetic analyses implicate rhizobacteria as the predominant group of 'gene donor' bacteria. Root-knot nematodes and rhizobia occupy similar niches in the soil and in roots, and thus the opportunity for genetic exchange may be omnipresent. Further, both organisms establish intimate developmental interactions with host plants, and mounting evidence suggests that the mechanisms for these interactions are shared too. We propose that the origin of parasitism in Meloidogyne may have been facilitated by acquisition of genetic material from soil bacteria through horizontal transfer, and that such events represented key steps in speciation of plant-parasitic nematodes. To further understand the mechanisms of horizontal gene transfer, and also to provide experimental tools to manipulate this promising bio-control agent, we have initiated a genomic sequence of the bacterial hyper-parasite of plant parasitic nematodes, Pasteuria penetrans. Initial data have established that P. penetrans is closely related to Bacillus spp., to the extent that considerable genome synteny is apparent. Hence, Bacillus serves as a model for Pasteuria, and vice versa.     

Potential of Lecanicillium spp. for management of insects, nematodes and plant diseases

Fungi in the genus Lecanicillium (formerly classified as the single species Verticillium lecanii) are important pathogens of insects and some have been developed as commercial biopesticides. Some isolates are also active against phytoparasitic nematodes or fungi. Lecanicillium spp. use both mechanical forces and hydrolytic enzymes to directly penetrate the insect integument and the cell wall of the fungal plant pathogen. In addition to mycoparasitism of the plant pathogen, the mode of action is linked to colonization of host plant tissues, triggering an induced systemic resistance. Recently it was demonstrated that development of Lecanicillium hybrids through protoplast fusion may result in strains that inherit parental attributes, thereby allowing development of hybrid strains with broader host range and other increased benefits, such as increased viability. Such hybrids have demonstrated increased virulence against aphids, whiteflies and the soybean cyst nematode. Three naturally occurring species of Lecanicillium, L. attenuatum, L. longisporum, and an isolate that could not be linked to any presently described species based on rDNA sequences have been shown to have potential to control aphids as well as suppress the growth and spore production of Sphaerotheca fuliginea, the causal agent of cucumber powdery mildew. These results suggest that strains of Lecanicillium spp. may have potential for development as a single microbial control agent effective against several plant diseases, pest insects and plant parasitic nematodes due to its antagonistic, parasitic and disease resistance inducing characteristics. However, to our knowledge, no Lecanicillium spp. have been developed for control of phytopathogens or phytoparasitic nematodes.     

AMF-induced biocontrol against plant parasitic nematodes in Musa sp.: a systemic effect

Although mycorrhizal colonization provides a bioprotectional effect against a broad range of soil-borne pathogens, including plant parasitic nematodes, the commercial use of arbuscular mycorrhizal fungi (AMF) as biocontrol agents is still in its infancy. One of the main reasons is the poor understanding of the modes of action. Most AMF mode of action studies focused on AMF-bacterial/fungal pathogens. Only few studies so far examined AMF-plant parasitic nematode interactions. Therefore, the aim of the study was to determine whether the AMF Glomus intraradices was able to incite systemic resistance in banana plants towards Radopholus similis and Pratylenchus coffeae, two plant parasitic nematodes using a split-root compartmental set-up. The AMF reduced both nematode species by more than 50%, even when the AMF and the plant parasitic nematodes were spatially separated. The results obtained demonstrate for the first time that AMF have the ability to induce systemic resistance against plant parasitic nematodes in a root system.     

Parasitic nematodes, proteinases and transgenic plants.

Parasite proteinases have important functions in host-parasite interactions. Consequently, they have been investigated as targets for the control of both plant and animal parasites. Plant parasitic nematodes cause estimated annual losses to world agriculture of US$100 billion and, currently, their control often relies on highly toxic nematicides, with associated environmental risks. The potential of disrupting digestive proteinases for plant parasitic nematode control, via expression of proteinase inhibitors in transgenic plants, is summarized here by Catherine Lilley, Pauline Devlin, Peter Urwin and Howard Atkinson. They then consider whether the approach of expressing antinematode proteins in plants can be adapted for control of certain animal parasitic nematodes.     

Proteases from Bacillus: a new insight into the mechanism of action for rhizobacterial suppression of nematode populations

AIMS: The aim of this study was to investigate the role of proteases in Bacillus spp. of rhizobacteria in suppressing nematode populations and to understand their mechanism of action. METHODS AND RESULTS: Rhizobacteria with nematicidal activity were isolated from soil samples of five root knot nematode-infested farms. Among these strains, nematotoxicities of Bacillus strains were intensively analysed. Further assays of nematicidal toxins from Bacillus sp. strain RH219 indicated an extracellular cuticle-degrading protease Apr219 was an important pathogenic factor. The Apr219 shared high similarity with previously reported cuticle-degrading proteases from Brevibacillus laterosporus strain G4 and Bacillus sp. B16 (Bacillus nematocida). The cuticle-degrading protease genes were also amplified from four other nematicidal Bacillus strains isolated from the rhizosphere. In addition to Apr219, a neutral protease Npr219 from Bacillus sp. RH219 was also investigated for activity against nematodes. CONCLUSIONS: The wide distribution of cuticle-degrading proteases in Bacillus strains with nematicidal activity suggested that these enzymes likely play an important role in bacteria-nematode-plant-environment interactions and that they may serve as important nematicidal factors in balancing nematode populations in the soil. SIGNIFICANCE AND IMPACT OF THE STUDY: Increased understanding of the mechanism of action of Bacillus spp. against nematodes could potentially enhance the value of these species as effective nematicidal agents and develop new biological control strategies.     

Entomopathogenic nematodes,a potential microbial biopesticide: mass production and commercialisation status – a mini review

Parasitic nematodes have several important attributes that make them excellent candidates for biological control of soil insects. These nematodes can be produced by in vivo by baiting technique on insects and commercially by in vitro solid/liquid culturing. Numerous insect pests on many different crops are being controlled by these insect parasitic nematodes, including root weevils, flea beetles, mint root borer, colorado potato beetle, white grubs, caterpillars and plant parasitic root nematode, e.g. root-knot nematodes. Utilisation of entomopathogenic nematodes (EPN) has raised intense interest and has been a growing concern globally mainly because of its potential efficiency, exemption from registration and other impressive attributes for utilising against the control of soil dwelling pests. This review highlights the mass production, commercialisation and utilisation of EPN as microbial biopesticide in bio-intensive pest management programmes.     

RNA interference in plant parasitic nematodes: a summary of the current status

SUMMARYRNA interference (RNAi) has emerged as an invaluable gene-silencing tool for functional analysis in a wide variety of organisms, particularly the free-living model nematode Caenorhabditis elegans. An increasing number of studies have now described its application to plant parasitic nematodes. Genes expressed in a range of cell types are silenced when nematodes take up double stranded RNA (dsRNA) or short interfering RNAs (siRNAs) that elicit a systemic RNAi response. Despite many successful reports, there is still poor understanding of the range of factors that influence optimal gene silencing. Recent in vitro studies have highlighted significant variations in the RNAi phenotype that can occur with different dsRNA concentrations, construct size and duration of soaking. Discrepancies in methodology thwart efforts to reliably compare the efficacy of RNAi between different nematodes or target tissues. Nevertheless, RNAi has become an established experimental tool for plant parasitic nematodes and also offers the prospect of being developed into a novel control strategy when delivered from transgenic plants.     

Neurobiology of plant parasitic nematodes

The regulatory constraints imposed on use of chemical control agents in agriculture are rendering crops increasingly vulnerable to plant parasitic nematodes. Thus, it is important that new control strategies which meet requirements for low toxicity to non-target species, vertebrates and the environment are pursued. This would be greatly facilitated by an improved understanding of the physiology and pharmacology of these nematodes, but to date, these microscopic species of the Phylum Nematoda have attracted little attention in this regard. In this review, the current information available for neurotransmitters and neuromodulator in the plant parasitic nematodes is discussed in the context of the more extensive literature for other species in the phylum, most notably Caenorhabditis elegans and Ascaris suum. Areas of commonality and distinctiveness in terms of neurotransmitter profile and function between these species are highlighted with a view to improving understanding of to what extent, and with what level of confidence, this information may be extrapolated to the plant parasitic nematodes.     

Recent progress in the development of RNA interference for plant parasitic nematodes

RNA interference (RNAi), first described for Caenorhabditis elegans , has emerged as a powerful gene silencing tool for investigating gene function in a range of organisms. Recent studies have described its application to plant parasitic nematodes. Genes expressed in a range of cell types are silenced when preparasitic juvenile nematodes take up double-stranded (ds)RNA that elicits a systemic RNAi response. Important developments over the last year have shown that in planta expression of a dsRNA targeting a nematode gene can successfully induce silencing in parasitizing nematodes. When the targeted gene has an essential function, a resistance effect is observed paving the way for the potential use of RNAi technology to control plant parasitic nematodes.     

Effects of arbuscular mycorrhizal fungi and a non-pathogenic<Emphasis Type="Italic"> Fusarium oxysporum</Emphasis> on<Emphasis Type="Italic"> Meloidogyne incognita</Emphasis> infestation of tomato

Arbuscular mycorrhizal (AM) fungi and non-pathogenic strains of soil-borne pathogens have been shown to control plant parasitic nematodes. As AM fungi and non-pathogenic fungi improve plant health by different mechanisms, combination of two such partners with complementary mechanisms might increase overall control efficacy and, therefore, provide an environmentally safe alternative to nematicide application. Experiments were conducted to study possible interactions between the AM fungus Glomus coronatum and the non-pathogenic Fusarium oxysporum strain Fo162 in the control of Meloidogyne incognita on tomato. Pre-inoculation of tomato plants with G. coronatum or Fo162 stimulated plant growth and reduced M. incognita infestation. Combined application of the AM fungus and Fo162 enhanced mycorrhization of tomato roots but did not increase overall nematode control or plant growth. A higher number of nematodes per gall was found for mycorrhizal than non-mycorrhizal plants. In synergisms between biocontrol agents, differences in their antagonistic mechanisms seem to be less important than their effects on different growth stages of the pathogen.     

Role of allelochemicals in plant growth promoting rhizobacteria for biocontrol of phytopathogens

Soil borne fungal diseases pose serious constraints on agro-productivity. Biological control is non-hazardous strategy to control plant pathogens and improve crop productivity. PGPR (plant growth promoting rhizobacteria) have long been used as plant disease control agents. PGPR produced a wide range of secondary compounds that may act as signals—that is, allelochemicals that include metabolites, siderophores, antibiotics, volatile metabolites, enzymes and others. Their mode of action and molecular mechanisms provide a great awareness for their application for crop disease management. The present review highlights the role of PGPR strains, specifically referring to allelochemicals produced and molecular mechanisms. Further research to fine tune combinations of allelochemicals, plant-microbe–pathogen interaction will ultimately lead to better disease control.     

Response of Plant Parasitic and Free Living Soil Nematodes to Composted Animal Manure Soil Amendments

In an outside pot experiment, dry pig manure processed on pine sawdust litter and fermented for seven days by house fly larvae (fermented manure), and pine sawdust applied alone, and in combination with a spring application of inorganic nitrogen fertilizer were used to determine their effects on plant parasitic and free-living soil nematodes on sugar beets (cv. Antek). Non amended soil was used as a control. All treatments with fermented pig manure and sawdust with nitrogen fertilizer decreased number of plant parasitic nematodes and also root-fungal feeding nematodes compared to the untreated control. Sawdust applied alone had no effect on plant parasitic and root-fungal feeding nematode suppression. Free-living nematodes which were mainly bacteriovores and fungivores were significantly more abundant in soil amended with fermented pig manure, while the sawdust had no effect on these nematodes. The effect of all tested treatments on omnivores-predators was rather random, and in general, the number of these nematodes decreased after soil amendment applications compared to the untreated control.     

尖孢镰刀菌致病机理和化感作用研究进展

尖孢镰刀菌引起的枯萎病在生产中的防控相当困难。通过总结国内外相关文献,综述近年来有关尖孢镰刀菌致病机理和化感作用的研究进展。尖孢镰刀菌通过分泌毒素和细胞壁降解酶共同致病,谱系特异性区域的存在是其致病性强和宿主范围广的主要原因;在尖孢镰刀菌各专化型中已分离出大量致病相关基因;其他植物和拮抗微生物(木霉菌、丛枝菌根真菌、非致病性尖孢镰刀菌以及植物生长促生菌)可以分泌化感物质,作用于宿主植物和尖孢镰刀菌,直接抑制尖孢镰刀菌的生长或激活宿主植物的防御反应。未来有关尖孢镰刀菌致病机理研究应该在基因组测序基础上构建精细的遗传图谱;对化感作用的研究应当深入探讨分子机理,利用高通量测序等技术在转录组或蛋白组水平上明确宿主植物抗枯萎病相关基因,同时利用分子标记辅助育种来筛选新的抗枯萎病品种。     

RNA interference and plant parasitic nematodes

RNA interference (RNAi) has recently been demonstrated in plant parasitic nematodes. It is a potentially powerful investigative tool for the genome-wide identification of gene function that should help improve our understanding of plant parasitic nematodes. RNAi should help identify gene and, hence, protein targets for nematode control strategies. Prospects for novel resistance depend on the plant generating an effective form of double-stranded RNA in the absence of an endogenous target gene without detriment to itself. These RNA molecules must then become available to the nematode and be capable of ingestion via its feeding tube. If these requirements can be met, crop resistance could be achieved by a plant delivering a dsRNA that targets a nematode gene and induces a lethal or highly damaging RNAi effect on the parasite.     

Bacillus thuringiensis (Bt) toxin susceptibility and isolation of resistance mutants in the nematode Caenorhabditis elegans

The protein toxins produced by Bacillus thuringiensis (Bt) are the most widely used natural insecticides in agriculture. Despite successful and extensive use of these toxins in transgenic crops, little is known about toxicity and resistance pathways in target insects since these organisms are not ideal for molecular genetic studies. To address this limitation and to investigate the potential use of these toxins to control parasitic nematodes, we are studying Bt toxin action and resistance in Caenorhabditis elegans. We demonstrate for the first time that a single Bt toxin can target a nematode. When fed Bt toxin, C. elegans hermaphrodites undergo extensive damage to the gut, a decrease in fertility, and death, consistent with toxin effects in insects. We have screened for and isolated 10 recessive mutants that resist the toxin's effects on the intestine, on fertility, and on viability. These mutants define five genes, indicating that more components are required for Bt toxicity than previously known. We find that a second, unrelated nematicidal Bt toxin may utilize a different toxicity pathway. Our data indicate that C. elegans can be used to undertake detailed molecular genetic analysis of Bt toxin pathways and that Bt toxins hold promise as nematicides.     

施用农用化学品及生防制剂对土壤线虫群落动态影响的研究

1　引　　言在农业生态系统中 ,农业管理 (如化肥、农药等农用化学品的施用等 )对土壤线虫群落组成及多样性产生了显著影响[2～ 4,7] .线虫是农田土壤中的生物因子 ,对植物的生长可以产生直接或间接的影响 .植物寄生线虫对植物的影响主要包括 3种方式 :1)通过在植物的根部取食 ,直接影响植物的营养状况 ,造成植物营养不良 ;2 )通过其口针的穿刺过程 ,传播植物病毒 ,引起植物病毒病 ;3)通过线虫取食植物根部造成的伤口 ,引起与其它植物病原物的复合侵染 .非植物寄生线虫多数是自由生活的线虫 ,在有人为影响的农田中主要是腐生线虫参与土壤中…