Rhizosphere Interactions and the Exploitation of Microbial Agents for the Biological Control of Plant-Parasitic Nematodes

Understanding the interaction between an obligate hyperparasitic bacterium, Pasteuria penetrans and its obligate plant-parasitic nematode host, Meloidogyne spp

Pasteuria penetrans is an endospore-forming bacterium, which is a hyperparasite of root-knot nematodes Meloidogyne spp. that are economically important pests of a wide range of crops. The life cycle of the bacterium and nematode are described with emphasis on the bacterium's potential as a biocontrol agent. Two aspects that currently prohibit the commercial development of the bacterium as a biocontrol agent are the inability to culture it outside its host and its host specificity. Vegetative growth of the bacterium is possible in vitro; however, getting the vegetative stages of the bacterium to enter sporogenesis has been problematic. Insights from genomic survey sequences regarding the role of cation concentration and the phosphorylation of Spo0F have proved useful in inducing vegetative bacteria to sporulate. Similarly, genomic data have also proved useful in understanding the attachment of endospores to the cuticle of infective nematode juveniles, and a Velcro-like model of spore attachment is proposed that involves collagen-like fibres on the surface of the endospore interacting with mucins on the nematode cuticle. Ecological studies of the interactions between Daphnia and Pasteuria ramosa are examined and similarities are drawn between the co-evolution of virulence in the Daphnia system and that of plant-parasitic nematodes.

Nematicidal cardenolides from Nerium indicum Mill

Three nematicidal cardenolides were obtained from the AcOEt extract of Nerium indicum Mill. by bioassay-guided fractionation. They include a new compound, 3beta-O-(beta-D-diginosyl)-14,15alpha-dihydroxy-5alpha-card-20(22)-enolide (1), and two known compounds, uzarigenin (2) and cardenolide N-1 (3). The median lethal concentrations (LC(50)) of compounds 1-3 against the nematodes Bursaphelenchus xylophilus, Panagrellus redivivus, and Caenorhabditis elegans at 72 h were 103.3, 49.0, and 45.4 mg l(-1), 257.0, 62.7, and 177.8 mg l(-1), and 242.9, 29.1, and 41.7 mg l(-1), respectively. This is the first report about the nematicidal activity of cardenolides.

Nematicidal coumarins from Heracleum candicans Wall

The root extract of Heracleum candicans Wall. exhibited antagonistic activities against nematodes Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle and Panagrellus redivivus (Linn.) Goodey. Through bioassay-guided fractionations, three coumarins were obtained from the extract of H. candicans and determined to be 8-geranyloxypsoralen (1), imperatorin (2), and heraclenin (3) based on spectra data. All three compounds possessed nematicidal activities against the two tested nematodes. The median lethal concentrations (LC(50)) of compounds 1-3 at 72 h were 188.3, 161.7, and 114.7 mg L(-1) respectively against B. xylophilus and were 117.5, 179.0, and 148.7 mg L(-1) respectively against P. redivivus. This is the first report about species in the Umbelliferae family that possesses nematicidal activity.

Persistence of Shiga toxin-producing Escherichia coli O26 in various manure-amended soil types

AIMS

To evaluate the behaviour of Shiga toxin-producing Escherichia coli (STEC) O26 strains inoculated in manure-amended soils under in vitro conditions.

METHODS AND RESULTS

Four green fluorescent protein (GFP)-labelled STEC O26 strains were inoculated in duplicate (at 10(6) CFU g(-1)) in three different manure-amended soil types, including two loam soils (A and B) and one clay loam soil (C), and two incubation temperatures (4 and 20 degrees C) were tested. STEC counts and soil physical parameters were periodically monitored. STEC O26 cells were able to persist during extended periods in soil even in the presence of low moisture levels, i.e. less than 0 x 08 g H2O g(-1) dry soil. At 4 and 20 degrees C, STEC could be detected in soil A for 288 and 196 days, respectively, and in soils B and C for at least 365 days postinoculation at both temperatures. The ambient temperature (i.e. 20 degrees C) was significantly associated with the highest STEC count decline in all soils tested.

CONCLUSIONS

The temperature and soil properties appear to be contributory factors affecting the long-term survival of STEC O26 in manure-amended soils.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides useful information regarding the ecology of STEC O26 in manure-amended soils and may have implications for land and waste management.

Bacterial endophytes: recent developments and applications

Endophytic bacteria have been found in virtually every plant studied, where they colonize the internal tissues of their host plant and can form a range of different relationships including symbiotic, mutualistic, commensalistic and trophobiotic. Most endophytes appear to originate from the rhizosphere or phyllosphere; however, some may be transmitted through the seed. Endophytic bacteria can promote plant growth and yield and can act as biocontrol agents. Endophytes can also be beneficial to their host by producing a range of natural products that could be harnessed for potential use in medicine, agriculture or industry. In addition, it has been shown that they have the potential to remove soil contaminants by enhancing phytoremediation and may play a role in soil fertility through phosphate solubilization and nitrogen fixation. There is increasing interest in developing the potential biotechnological applications of endophytes for improving phytoremediation and the sustainable production of nonfood crops for biomass and biofuel production.

Paralysis of nematodes: shifts in the transcriptome of the nematode-trapping fungus Monacrosporium haptotylum during infection of Caenorhabditis elegans

The transcriptional response in the parasitic fungus Monacrosporium haptotylum and its nematode host Caenorhabditis elegans were analysed during infection using cDNA microarrays. The array contained 2684 fungal and 372 worm gene reporters. Dramatic shifts occurred in the transcriptome of M. haptotylum during the different stages of the infection. An initial transcriptional response was recorded after 1 h of infection when the traps adhered to the cuticle, but before immobilization of the captured nematodes. Among the differentially expressed genes were two serine protease genes (spr1 and spr2), and several homologues to genes known to be regulated in other pathogenic fungi. After 4 h, when approximately 40% of the nematodes were paralysed, we identified an upregulated cluster of 372 fungal genes which were not regulated during the other phases of the infection. This cohort contained a large proportion (79%) of genes that appear to be specific for M. haptotylum and closely related species. These genes were of two different classes: those translating into presumably functional peptides and those with no apparent protein coding potential (non-coding RNAs). Among the infection-induced C. elegans genes were those encoding antimicrobial peptides, protease inhibitors and lectins.

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.

A degradation product of the salicylic acid pathway triggers oxidative stress resulting in down-regulation of Bacillus subtilis biofilm formation on Arabidopsis thaliana roots

Bacillus subtilis, a plant growth promoting rhizobacteria (PGPR), induces growth response and protection against pathogenic organisms through colonization and biofilm formation on the Arabidopsis thaliana root surface. In the current investigation, we utilized various Arabidopsis defense pathway mutants in a series of studies and showed that the plants recognize B. subtilis by a chemical-dependent cascade, which is independent of the salicylic acid (SA), jasmonic acid (JA), or ethylene pathways. These experiments revealed the importance of root surface chemistry in colonization and biofilm formation by B. subtilis. It was found that B. subtilis FB17 could not form biofilms on the roots of NahG, a transgenic Arabidopsis line for salicylate hydroxylase that produces catechol as the degradation product of SA. These findings suggest that catechol may play a direct role in inhibiting B. subtilis FB17 biofilm formation on the NahG root surface, possibly through induction of reactive oxygen species (ROS) in the roots. Using both in vitro microtitre plate and in planta assays we confirmed that catechol inhibited biofilm formation, but not the planktonic growth, of B. subtilis. Inhibition of biofilm formation was shown to be the result of a physiological response by B. subtilis to the presence of catechol, which resulted in the down-regulation of transcription of the yqxM-sipW-tasA and epsA-O operons, both of which are required for biofilm formation by B. subtilis. These data indicate that the suppression of biofilm formation on NahG plants was strongly influenced by the root-derived catechol production through ROS-mediated down-regulation of B. subtilis biofilm genes.

Isolation of maize soil and rhizosphere bacteria with antagonistic activity against Aspergillus flavus and Fusarium verticillioides

Bacterial isolates from Mississippi maize field soil and maize rhizosphere samples were evaluated for their potential as biological control agents against Aspergillus flavus and Fusarium verticillioides. Isolated strains were screened for antagonistic activities in liquid coculture against A. flavus and on agar media against A. flavus and F. verticillioides. We identified 221 strains that inhibited growth of both fungi. These bacteria were further differentiated by their production of extracellular enzymes that hydrolyzed chitin and yeast cell walls and by production of antifungal metabolites. Based on molecular and nutritional identification of the bacterial strains, the most prevalent genera isolated from rhizosphere samples were Burkholderia and Pseudomonas, and the most prevalent genera isolated from nonrhizosphere soil were Pseudomonas and Bacillus. Less prevalent genera included Stenotrophomonas, Agrobacterium, Variovorax, Wautersia, and several genera of coryneform and enteric bacteria. In quantitative coculture assays, strains of P. chlororaphis and P. fluorescens consistently inhibited growth of A. flavus and F. verticillioides in different media. These results demonstrate the potential for developing individual biocontrol agents for simultaneous control of the mycotoxigenic A. flavus and F. verticillioides.

Extracellular enzymes and the pathogenesis of nematophagous fungi

Nematophagous fungi are an important group of soil microorganisms that can suppress the populations of plant-parasitic nematodes. The pathogenic mechanisms of nematophagous fungi are diverse: They can be parasitical-mechanical through producing specialized capturing devices, or toxin-dependent. During infections, a variety of virulence factors may be involved against nematodes by nematophagous fungi. In this review, we present up-to-date information on the modes of infection by nematophagous fungi. The roles of extracellular hydrolytic enzymes and other virulence factors involved in infection against nematodes were summarized. The biochemical properties and peptide sequences of a special group of enzymes, the serine proteases, were compared, and their implications in infections were discussed. We also discussed the impact of emerging new techniques on our understanding of this unique group of fungi.

Soil microorganisms control plant ectoparasitic nematodes in natural coastal foredunes

Belowground herbivores can exert important controls on the composition of natural plant communities. Until now, relatively few studies have investigated which factors may control the abundance of belowground herbivores. In Dutch coastal foredunes, the root-feeding nematode Tylenchorhynchus ventralis is capable of reducing the performance of the dominant grass Ammophila arenaria (Marram grass). However, field surveys show that populations of this nematode usually are controlled to nondamaging densities, but the control mechanism is unknown. In the present study, we first established that T. ventralis populations are top-down controlled by soil biota. Then, selective removal of soil fauna suggested that soil microorganisms play an important role in controlling T. ventralis. This result was confirmed by an experiment where selective inoculation of microarthropods, nematodes and microbes together with T. ventralis into sterilized dune soil resulted in nematode control when microbes were present. Adding nematodes had some effect, whereas microarthropods did not have a significant effect on T. ventralis. Our results have important implications for the appreciation of herbivore controls in natural soils. Soil food web models assume that herbivorous nematodes are controlled by predaceous invertebrates, whereas many biological control studies focus on managing nematode abundance by soil microorganisms. We propose that soil microorganisms play a more important role than do carnivorous soil invertebrates in the top-down control of herbivorous ectoparasitic nematodes in natural ecosystems. This is opposite to many studies on factors controlling root-feeding insects, which are supposed to be controlled by carnivorous invertebrates, parasitoids, or entomopathogenic nematodes. Our conclusion is that the ectoparasitic nematode T. ventralis is potentially able to limit productivity of the dune grass A. arenaria but that soil organisms, mostly microorganisms, usually prevent the development of growth-reducing population densities.

The MAD1 adhesin of Metarhizium anisopliae links adhesion with blastospore production and virulence to insects, and the MAD2 adhesin enables attachment to plants

Metarhizium anisopliae is a fungus of considerable metabolic and ecological versatility, being a potent insect pathogen that can also colonize plant roots. The mechanistic details of these interactions are unresolved. We provide evidence that M. anisopliae adheres to insects and plants using two different proteins, MAD1 and MAD2, that are differentially induced in insect hemolymph and plant root exudates, respectively, and produce regional localization of adhesive conidial surfaces. Expression of Mad1 in Saccharomyces cerevisiae allowed this yeast to adhere to insect cuticle. Expression of Mad2 caused yeast cells to adhere to a plant surface. Our study demonstrated that as well as allowing adhesion to insects, MAD1 at the surface of M. anisopliae conidia or blastospores is required to orientate the cytoskeleton and stimulate the expression of genes involved in the cell cycle. Consequently, the disruption of Mad1 in M. anisopliae delayed germination, suppressed blastospore formation, and greatly reduced virulence to caterpillars. The disruption of Mad2 blocked the adhesion of M. anisopliae to plant epidermis but had no effects on fungal differentiation and entomopathogenicity. Thus, regulation, localization, and specificity control the functional distinction between Mad1 and Mad2 and enable M. anisopliae cells to adapt their adhesive properties to different habitats.

Isolation of bacterial antagonists of Aspergillus flavus from almonds

Bacteria were isolated from California almond orchard samples to evaluate their potential antifungal activity against aflatoxin-producing Aspergillus flavus. Fungal populations from the same samples were examined to determine the incidence of aflatoxigenic Aspergillus species. Antagonistic activities of the isolated bacterial strains were screened against a nonaflatoxigenic nor mutant of A. flavus, which accumulates the pigmented aflatoxin precursor norsolorinic acid (NOR) under conditions conducive to aflatoxin production. Using solid and liquid media in coculture assays, 171 bacteria isolated from almond flowers, immature nut fruits, and mature nut fruits showed inhibition of A. flavus growth and/or inhibition of NOR accumulation. Bacterial isolates were further characterized for production of extracellular enzymes capable of hydrolyzing chitin or yeast cell walls. Molecular and physiological identification of the bacterial strains indicated that the predominant genera isolated were Bacillus, Pseudomonas, Ralstonia, and Burkholderia, as well as several plant-associated enteric and nonenteric bacteria. A set of 20 isolates was selected for further study based on their species identification, antifungal phenotypes, and extracellular enzyme production. Quantitative assays using these isolates in liquid coculture with a wild-type, aflatoxin-producing A. flavus strain showed that a number of strains completely inhibited fungal growth in three different media. These results indicate the potential for development of bacterial antagonists as biological control agents against aflatoxigenic aspergilli on almonds.

In vitro influence of bacterial mixtures on Fusarium verticillioides growth and fumonisin B1 production: effect of seeds treatment on maize root colonization

AIMS

Enterobacter cloacae, Microbacterium oleovorans, Pseudomonas solanacearum and Bacillus subtilis were investigated in order to evaluate: (i) the inoculum size of two bacterial mixtures on Fusarium verticillioides growth and fumonisin B1 production in vitro at different water activities and (ii) the efficacy of a seed treatment with the best bacterial mixture on F. verticillioides root colonization in greenhouse studies.

METHODS AND RESULTS

The influence of bacterial mixtures (1 = E. cloacae and M. oleovorans and 2 = P. solanacearum and B. subtilis) to antagonize 13 F. verticillioides strains at different inoculum concentrations (10(8), 10(9) and 10(10) cells ml(-1)) and water activities (0.937, 0.955 and 0.982 aW) were examined. Antibiosis, growth rate and fumonisin B1 production were determined. Bacterial mixture 1 proved to exert the most effective control. Seed treatment with mixture 1 at 10(8) cells ml(-1) had the best inhibitory effect on F. verticillioides root colonization.

CONCLUSIONS

These results suggest that the combination E. cloacae and M. oleovorans has the potential for the biological control of F. verticillioides as a maize seed inoculant.

SIGNIFICANCE AND IMPACT OF THE STUDY

The application of this knowledge contributes to prevent the vertical transmission of F. verticillioides.

Extracellular protease of Pseudomonas fluorescens CHA0, a biocontrol factor with activity against the root-knot nematode Meloidogyne incognita

In Pseudomonas fluorescens CHA0, mutation of the GacA-controlled aprA gene (encoding the major extracellular protease) or the gacA regulatory gene resulted in reduced biocontrol activity against the root-knot nematode Meloidogyne incognita during tomato and soybean infection. Culture supernatants of strain CHA0 inhibited egg hatching and induced mortality of M. incognita juveniles more strongly than did supernatants of aprA and gacA mutants, suggesting that AprA protease contributes to biocontrol.

Biocontrol of Bacillus subtilis against Fusarium verticillioides in vitro and at the maize root level

Bacillus species as a group offer several advantages over other bacteria for protection against root pathogens because of their ability to form endospores, and because of the broad-spectrum activity of their antibiotics. The objectives of this work were to determine the ability of strains of Bacillus to inhibit Fusarium verticillioides growth and fumonisin B(1) accumulation in vitro, and to evaluate the ability of the best bacterium for preventing rhizosphere and endorhizosphere colonization by F. verticillioides. Bacterial populations from the maize rhizoplane were obtained, and the capacity of ten Bacillus strains to inhibit fungal growth and fumonisin B(1) accumulation in vitro was assayed. According to these results, B. subtilis CE1 was selected as the best antagonist for testing maize root colonization of F. verticillioides. Bacillus subtilis CE1 at 10(8) and 10(7) CFU ml(-1) inocula was able to reduce rhizoplane and endorhizosphere colonization of F. verticillioides in greenhouse trials. The strain B. subtilis CE1 could be a potential biological control agent against F. verticillioides at the root level.

Screening procedures for selecting rhizobacteria with biocontrol effects upon Fusarium verticillioides growth and fumonisin B1 production

Screening is a critical step in the discovery of microbial agents that can exert biological control of Fusarium verticillioides at the root level. The objectives of this research were to determine the utility of a niche overlap index to realise the first screening of maize rhizobacterial isolates during different water activities. Studies were conducted to evaluate various methods for second screening with different modes of action. The antifungal activity of bacterial isolates through antibiosis assay was checked and the influence of different isolates on Fusarium verticilliodes growth and fumonisin B(1) was studied. Eleven competitive rhizobacterial isolates (Arthrobacter globiformis RC1, Azotobacter armeniacus RC2, A. armeniacus RC3, A. globiformis RC4, A. globiformis RC5, A. armeniacus RC6, Pseudomonas solanacearum RC7, Bacillus subtilis RC8, B. subtilis RC9, P. solanacearum RC10, B. subtilis RC11) were selected for the studies which followed. All bacteria were able to utilise the widest range of carbon sources and showed the highest niche overlap indices at the water activities tested. All bacterial antagonists reduced fumonisin B(1) production at all levels tested. Isolates belonging to Pseudomonas and Bacillus genera significantly inhibited fumonisin B(1) production, which ranged between 70 and 100%. Also, A. armeniacus RC2 caused important fumonisin B(1) reduction. The results of the present work suggest that A. armeniacus RC2, A. armeniacus RC3, B. subtilis RC8, B. subtilis RC9, B. subtilis RC11, P. solanacearum RC7, and P. solanacearum RC10 could have practical value in the control of F. verticillioides root colonisation. This paper is part of an on-going study to determine their application at the field level.

Comparison of sandy soils suppressive or conducive to ectoparasitic nematode damage on sugarcane

ABSTRACT Two South African sandy soils, one suppressive and the other conducive to ectoparasitic nematode damage on monoculture sugarcane, were compared. Analysis of field transects indicated that the suppressive soil displayed a comparatively higher population of the weak ectoparasite Helicotylenchus dihystera, whose predominance among ectoparasitic nematodes is known to limit yield loss caused by more virulent phytonematodes. Soil type was identical at both sites (entisols), but the suppressive soil had a higher organic matter content and a lower pH, which correlated with H. dihystera population data. In contrast, microclimatic differences between the two field sites were unlikely to be responsible for the suppressive or conducive status of the soils, as shown in a greenhouse experiment. The two soils exhibited a bacterial community of the same size but with different genetic structures, as indicated by automated ribosomal intergenic spacer analysis (RISA). The number of culturable fluorescent pseudomonads was higher for the conducive soil, probably because extensive root damage caused by ectoparasitic nematodes favored proliferation of these bacteria. This study shows that apparently small differences in soil composition between fields located in the same climatic area and managed similarly can translate into contrasted nematode communities, ectoparasitic nematode damage levels, and sugarcane yields.

Detection and quantification of Plectosphaerella cucumerina, a potential biological control agent of potato cyst nematodes, by using conventional PCR, real-time PCR, selective media, and baiting

Potato cyst nematodes (PCN) are serious pests in commercial potato production, causing yield losses valued at approximately $300 million in the European Community. The nematophagous fungus Plectosphaerella cucumerina has demonstrated its potential as a biological control agent against PCN populations by reducing field populations by up to 60% in trials. The use of biological control agents in the field requires the development of specific techniques to monitor the release, population size, spread or decline, and pathogenicity against its host. A range of methods have therefore been developed to monitor P. cucumerina. A species-specific PCR primer set (PcCF1-PcCR1) was designed that was able to detect the presence of P. cucumerina in soil, root, and nematode samples. PCR was combined with a bait method to identify P. cucumerina from infected nematode eggs, confirming the parasitic ability of the fungus. A selective medium was adapted to isolate the fungus from root and soil samples and was used to quantify the fungus from field sites. A second P. cucumerina-specific primer set (PcRTF1-PcRTR1) and a Taqman probe (PcRTP1) were designed for real-time PCR quantification of the fungus and provided a very sensitive means of detecting the fungus from soil. PCR, bait, and culture methods were combined to investigate the presence and abundance of P. cucumerina from two field sites in the United Kingdom where PCN populations were naturally declining. All methods enabled differences in the activity of P. cucumerina to be detected, and the results demonstrated the importance of using a combination of methods to investigate population size and activity of fungi.

Perspectives on plant and soil nematology

During my career in Plant Pathology/Nematology, many major advancements have occurred in the study of nematodes-even with their being largely soilborne and thus often overlooked. These biotrophic organisms include the most widespread and important group of plant pathogens-the root-knot nematodes Meloidogyne species-which attack most major crops, as well as thousands of non-crop plant species. Landmark achievements that catalyzed research on these organisms included the discovery of effective nematicides, ectoparasitic forms, elucidation of disease complexes, nematodes as virus vectors, development of host resistance, and new technologies for research. Evolving research thrusts involve interfacing traditional and molecular systematics/diagnostics, adoption of the Caenorhabditis elegans-molecular genetics resource for general nematological research, focus on genetics of parasitism, use of molecular tools in developing host resistance, ecological and quantitative facets, and soil-biology-ecology based integrated management. Educational and international programs are encountering many changes and challenges, as is support for nematology in general.

Bacteria associated with cysts of the soybean cyst nematode (Heterodera glycines)

The soybean cyst nematode (SCN), Heterodera glycines, causes economically significant damage to soybeans (Glycine max) in many parts of the world. The cysts of this nematode can remain quiescent in soils for many years as a reservoir of infection for future crops. To investigate bacterial communities associated with SCN cysts, cysts were obtained from eight SCN-infested farms in southern Ontario, Canada, and analyzed by culture-dependent and -independent means. Confocal laser scanning microscopy observations of cyst contents revealed a microbial flora located on the cyst exterior, within a polymer plug region and within the cyst. Microscopic counts using 5-(4,6-dichlorotriazine-2-yl)aminofluorescein staining and in situ hybridization (EUB 338) indicated that the cysts contained (2.6 +/- 0.5) x 10(5) bacteria (mean +/- standard deviation) with various cellular morphologies. Filamentous fungi were also observed. Live-dead staining indicated that the majority of cyst bacteria were viable. The probe Nile red also bound to the interior polymer, indicating that it is lipid rich in nature. Bacterial community profiles determined by denaturing gradient gel electrophoresis analysis were simple in composition. Bands shared by all eight samples included the actinobacterium genera Actinomadura and STREPTOMYCES: A collection of 290 bacteria were obtained by plating macerated surface-sterilized cysts onto nutrient broth yeast extract agar or on actinomycete medium. These were clustered into groups of siblings by repetitive extragenic palindromic PCR fingerprinting, and representative isolates were tentatively identified on the basis of 16S rRNA gene sequence. Thirty phylotypes were detected, with the collection dominated by Lysobacter and Variovorax spp. This study has revealed the cysts of this important plant pathogen to be rich in a variety of bacteria, some of which could presumably play a role in the ecology of SCN or have potential as biocontrol agents.

Cloning of and genetic variation in protease VCP1 from the nematophagous fungus Pochonia chlamydosporia

The fungus Pochonia chlamydosporia is a biocontrol agent with commercial potential for root knot and cyst nematodes. It produces an alkaline serine protease, VCP1, during infection of nematode eggs. The gene encoding VCP1 was sequenced and the sequences of cDNAs from six isolates from different nematode hosts were compared. The gene encoding VCP1 was similar to PR1 from Metarhizium anisopliae with similar regulatory elements. Comparison of translated cDNA sequences revealed two amino acid polymorphisms at positions 65 and 99, indicating a difference between isolates from cyst and root nematodes. The positions and nature of the polymorphisms indicated that the two forms of VCP1 might have different properties and this was tested with five chromogenic polypeptide substrates. Enzyme assays revealed the two forms differed in their abilities to utilise Succ-Ala-Ala-Pro-Phe-pNa and Succ-Ala-Val-Pro-Phe-pNa, suggesting different amino acid affinities at the S3 binding region. This indicates host related genetic variation in VCP1 between isolates of P. chlamydosporia isolated from different nematode hosts, which might contribute to host preference. Such differences may be important in future exploitation of P. chlamydosporia as a nematode biocontrol agent.

Colonization of plant roots by egg-parasitic and nematode-trapping fungi

•  The ability of the nematode-trapping fungus Arthrobotrys oligospora and the nematode egg parasite Verticillium chlamydosporium to colonize barley (Hordeum vulgare) and tomato (Lycopersicum esculentum) roots was examined, together with capability of the fungi to induce cell wall modifications in root cells. •  Chemotropism was studied using an agar plate technique. Root colonization was investigated with light microscopy and scanning electron microscopy, while compounds involved in fungus-plant interactions were studied histochemically. •  Only A. oligospora responded chemotropically to roots. Colonization of barley and tomato by both fungi involved appressoria to facilitate epidermis penetration. V. chlamydosporium colonized tomato root epidermis and produced chlamydospores. Papillae, appositions and lignitubers ensheathing hyphae on tomato were also found. Phenolics (including lignin), protein deposits and callose were present in papillae in both hosts. Both fungi were still present in epidermal cells 3 months after inoculation. •  Nematophagous fungi colonized endophytically monocotyledon and dicotyledon plant roots. Arthrobotrys oligospora seemed to be more aggressive than V. chlamydosporium on barley roots. Both fungi induced cell wall modifications, but these did not prevent growth. The response of root cells to colonization by nematophagous fungi may have profound implications in the performance of these organisms as biocontrol agents of plant parasitic nematodes.