Interactions in the tomato rhizosphere of two Pseudomonas biocontrol strains with the phytopathogenic fungus Fusarium oxysporum f. sp. radicis-lycopersici

Microbial interactions in the mycorrhizosphere and their significance for sustainable agriculture

In sustainable, low-input cropping systems the natural roles of microorganisms in maintaining soil fertility and biocontrol of plant pathogens may be more important than in conventional agriculture where their significance has been marginalised by high inputs of agrochemicals. Better understanding of the interactions between arbuscular mycorrhizal fungi and other microorganisms is necessary for the development of sustainable management of soil fertility and crop production. Many studies of the influence of mycorrhizal colonisation on associated bacterial communities have been conducted, however, the mechanisms of interaction are still poorly understood. Novel approaches including PCR-based methods, stable isotope profiling, and molecular markers have begun to shed light on the activity, identity and spatiotemporal location of bacteria in the mycorrhizosphere. This paper reviews current knowledge concerning the interactions between arbuscular mycorrhizal fungi and other microorganisms, particularly bacteria, and discusses the implications these interactions may have in sustainable agriculture.

Colonization process of olive tissues by Verticillium dahliae and its in planta interaction with the biocontrol root endophyte Pseudomonas fluorescens PICF7

The colonization process of Olea europaea by the defoliating pathotype of Verticillium dahliae, and the in planta interaction with the endophytic, biocontrol strain Pseudomonas fluorescens PICF7 were determined. Differential fluorescent protein tagging was used for the simultaneous visualization of P. fluorescens PICF7 and V. dahliae in olive tissues. Olive plants were bacterized with PICF7 and then transferred to V. dahliae-infested soil. Monitoring olive colonization events by V. dahliae and its interaction with PICF7 was conducted using a non-gnotobiotic system, confocal laser scanner microscopy and tissue vibratoming sections. A yellow fluorescently tagged V. dahliae derivative (VDAT-36I) was obtained by Agrobacterium tumefaciens-mediated transformation. Isolate VDAT-36I quickly colonized olive root surface, successfully invaded root cortex and vascular tissues via macro- and micro-breakages, and progressed to the aerial parts of the plant through xylem vessel cells. Strain PICF7 used root hairs as preferred penetration site, and once established on/in root tissues, hindered pathogen colonization. For the first time using this approach, the entire colonization process of a woody plant by V. dahliae is reported. Early and localized root surface and root endophytic colonization by P. fluorescens PICF7 is needed to impair full progress of verticillium wilt epidemics in olive.

BENEFICIAL RHIZOSPHERE PSEUDOMONADS

Among the many bacteria present on and around the root, Pseudomonas bacteria are (among) the best root colonizers and therefore very suitable to apply for beneficial purposes. In this chapter, we discuss the possibilities to use such bacteria for the following purposes: fertilization of the plant, stimulation of plant growth and yield, reduction of plant stress, and reduction of plant diseases. This research was supported by numerous grants, especially from the Dutch Organization for scientific research (NWO), EET, the European Commission and INTAS.

Antagonistic effects of Pseudomonas jessenii against Pythium aphanidermatum: morphological, ultrastructural and cytochemical aspects

Pseudomonas jessenii isolate EC-S101, an antagonistic rhizobacterium, induces morphological abnormalities such as topical swelling and excessive lateral branching in phytopathogenic Peronosporomycetes Pythium aphanidermatum hyphae as a result of radial growth inhibition in a dual culture assay. Rhodamine-phalloidin staining revealed that these abnormalities were associated with disorganization of actin cytoskeleton. Both the morphological forms of actin, filaments and plaques, were affected progressively. At early stage of interaction (in less affected hyphae), the filaments were either eliminated or disarrayed. At advance stage of interaction (in severely affected hyphae), even the plaques population was decreased or disappeared. The effects of P. jessenii on actin architecture of Py. aphanidermatum were comparable to latrunculin B, a known actin assembly inhibitor. In addition, at early stage of interaction, the quantities of nuclei, lipid bodies and mitochondria became higher than those in control. At advance stage of interaction, the quantities of these organelles were almost similar, higher and lower, respectively, compare to those in control. Scanning electron microscopy exhibited cell wall disruption and accumulation of extracellular material associated with severely affected hyphae. Ultrastructural observations of the affected hyphae displayed additional features of considerable thickening of cell wall, enlargement of vacuoles, sinking of redundant lipid bodies into the enlarged vacuoles and wall appositions. We conclude that in addition to interference in morphogenesis and growth of Py. aphanidermatum, P. jessenii suppresses the pathogen through sub-cellular disorganization, specifically the actin architecture. This is the first report on disruption of cytoskeleton in a eukaryotic phytopathogen by an antagonistic rhizobacterium.

A novel gene, phcA from Pseudomonas syringae induces programmed cell death in the filamentous fungus Neurospora crassa

The phytopathogen Pseudomonas syringae competes with other epiphytic organisms, such as filamentous fungi, for resources. Here we characterize a gene in P. syringae pv. syringae B728a and P. syringae pv. tomato DC3000, termed phcA, that has homology to a filamentous fungal gene called het-c. phcA is conserved in many P. syringae strains, but is absent in one of the major clades, which includes the P. syringae pathovar phaseolicola. In the filamentous fungus Neurospora crassa, HET-C regulates a conserved programmed cell death pathway called heterokaryon incompatibility (HI). Ectopic expression of phcA in N. crassa induced HI and cell death that was dependent on the presence of a functional het-c pin-c haplotype. Further, by co-immunoprecipitation experiments, a heterocomplex between N. crassa HET-C1 and PhcA was associated with phcA-induced HI. P. syringae was able to attach and extensively colonize N. crassa hyphae, while an Escherichia coli control showed no association with the fungus. We further show that the P. syringae is able to use N. crassa as a sole nutrient source. Our results suggest that P. syringae has the potential to utilize phcA to acquire nutrients from fungi in nutrient-limited environments like the phyllosphere by the novel mechanism of HI induction.

Bacterial mycophagy: definition and diagnosis of a unique bacterial-fungal interaction

This review analyses the phenomenon of bacterial mycophagy, which we define as a set of phenotypic behaviours that enable bacteria to obtain nutrients from living fungi and thus allow the conversion of fungal into bacterial biomass. We recognize three types of bacterial strategies to derive nutrition from fungi: necrotrophy, extracellular biotrophy and endocellular biotrophy. Each is characterized by a set of uniquely sequential and differently overlapping interactions with the fungal target. We offer a detailed analysis of the nature of these interactions, as well as a comprehensive overview of methodologies for assessing and quantifying their individual contributions to the mycophagy phenotype. Furthermore, we discuss future prospects for the study and exploitation of bacterial mycophagy, including the need for appropriate tools to detect bacterial mycophagy in situ in order to be able to understand, predict and possibly manipulate the way in which mycophagous bacteria affect fungal activity, turnover, and community structure in soils and other ecosystems.

Detection of plant-modulated alterations in antifungal gene expression in Pseudomonas fluorescens CHA0 on roots by flow cytometry

The biocontrol activity of the root-colonizing Pseudomonas fluorescens strain CHA0 is largely determined by the production of antifungal metabolites, especially 2,4-diacetylphloroglucinol. The expression of these metabolites depends on abiotic and biotic environmental factors, in particular, elements present in the rhizosphere. In this study, we have developed a new method for the in situ analysis of antifungal gene expression using flow cytometry combined with green fluorescent protein (GFP)-based reporter fusions to the phlA and prnA genes essential for the production of the antifungal compounds 2,4-diacetylphloroglucinol and pyrrolnitrin, respectively, in strain CHA0. Expression of phlA-gfp and prnA-gfp in CHA0 cells harvested from the rhizosphere of a set of plant species as well as from the roots of healthy, leaf pathogen-attacked, and physically stressed plants were analyzed using a FACSCalibur. After subtraction of background fluorescence emitted by plant-derived particles and CHA0 cells not carrying the gfp reporters, the average gene expression per bacterial cell could be calculated. Levels of phlA and prnA expression varied significantly in the rhizospheres of different plant species. Physical stress and leaf pathogen infection lowered phlA expression levels in the rhizosphere of cucumber. Our results demonstrate that the newly developed approach is suitable to monitor differences in levels of antifungal gene expression in response to various plant-derived factors. An advantage of the method is that it allows quantification of bacterial gene expression in rhizosphere populations at a single-cell level. To our best knowledge, this is the first study using flow cytometry for the in situ analysis of biocontrol gene expression in a plant-beneficial bacterium in the rhizosphere.

Collimonas fungivorans, an unpredicted in vitro but efficient in vivo biocontrol agent for the suppression of tomato foot and root rot

Although bacteria from the genus Collimonas have demonstrated in vitro antifungal activity against many different fungi, they appeared inactive against the plant-pathogenic fungus Fusarium oxysporum f.sp. radicis-lycopersici (Forl), the causal agent of tomato foot and root rot (TFRR). Visualization studies using fluorescently labelled organisms showed that bacterial cells attached extensively to the fungal hyphae under nutrient-poor conditions but not in glucose-rich Armstrong medium. Collimonas fungivorans was shown to be as efficient in colonizing tomato root tips as the excellent colonizer Pseudomonas fluorescens strain WCS365. Furthermore, it appeared to colonize the same sites on the root as did the phytopathogenic fungus. Under greenhouse conditions in potting soil, C. fungivorans performed as well in biocontrol of TFRR as the well-established biocontrol strains P. fluorescens WCS365 and Pseudomonas chlororaphis PCL1391. Moreover, under biocontrol conditions, C. fungivorans did not attach to Forl hyphae colonizing plant roots. Based on these observations, we hypothesize that C. fungivorans mainly controls TFRR through a mechanism of competition for nutrients and niches rather than through its reported mycophagous properties, for which attachment of the bacteria to the fungal hyphae is assumed to be important.

Characterization of the cultivable microbiota of sprouts and their potential for application as protective cultures

The microbiota of ten seeds and ready-to-eat sprouts produced thereof was characterized by bacteriological culture and denaturing gradient gel electrophoresis (DGGE) of amplified DNA fragments of the 16S rRNA gene. The predominant bacterial biota of hydroponically grown sprouts mainly consisted of enterobacteria, pseudomonades and lactic acid bacteria (LAB). For adzuki, alfalfa, mung bean, radish, sesame and wheat, the ratio of these bacterial groups changed strongly in the course of germination, whereas for broccoli, red cabbage, rye and green pea the ratio remained unchanged. Within the pseudomonades, Pseudomonas gesardii and Pseudomonas putida have been isolated and strains of the potentially pathogenic species Enterobacter cancerogenes and Pantoea agglomerans were found as part of the main microbiota on hydroponically grown sprouts. In addition to the microbiota of the whole seedlings, the microbiota of root, hypocotyl and seed leafs were examined for alfalfa, radish and mung bean sprouts. The highest and lowest total counts for aerobic bacteria were found on seed leafs and hypocotyls, respectively. On the other hand, the highest numbers for LAB on sprouts were found on the hypocotyl. When sprouting occurred under the agricultural conditions, e.g. in soil, the dominating microbiota changed from enterobacteria to pseudomonades for mung beans and alfalfa sprouts. No pathogenic enterobacteria have been isolated from these sprout types. Within the pseudomonades group, Pseudomonas jessenii and Pseudomonas brassicacearum were found as dominating species on all seedling parts from soil samples. In practical experiments, a strain of P. jessenii was found to exhibit a potential for use as protective culture, as it suppresses the growth of pathogenic enterobacteria on ready-to-eat sprouts.

Selection of bacteria able to control Fusarium oxysporum f. sp. radicis-lycopersici in stonewool substrate

AIMS

Tomato foot and root rot (TFRR), caused by Fusariumoxysporum f. sp. radicis-lycopersici (Forl), is an economically important disease of tomato. The aim of this study was to develop an efficient protocol for the isolation of bacteria, which controls TFRR based on selection of enhanced competitive root-colonizing bacteria from total rhizosphere soil samples.

METHODS AND RESULTS

A total of 216 potentially enhanced bacterial strains were isolated from 17 rhizosphere soil samples after applying a procedure to enrich for enhanced root tip colonizers. Amplified ribosomal DNA restriction analysis, in combination with determination of phenotypic traits, was introduced to evaluate the presence of siblings. One hundred sixteen strains were discarded as siblings. Thirty-eight strains were discarded as potential pathogens based on the sequence of their 16S rDNA. Of the remaining strains, 24 performed equally well or better than the good root colonizer Pseudomonas fluorescens WCS365 in a competitive tomato root tip colonization assay. Finally, these enhanced colonizers were tested for their ability to control TFRR in stonewool, which resulted in seven new biocontrol strains.

CONCLUSIONS

The new biocontrol strains, six Gram-negative and one Gram-positive bacteria, were identified as three Pseudomonas putida strains and one strain each of Delftia tsuruhatensis, Pseudomonas chlororaphis, Pseudomonas rhodesiae and Paenibacillus amylolyticus.

SIGNIFICANCE AND IMPACT OF THE STUDY

We describe a fast method for the isolation of bacteria able to suppress TFRR in stonewool, an industrial plant growth substrate. The procedure minimizes the laborious screens that are a common feature in the isolation of biocontrol strains.

The mycorrhiza helper Pseudomonas fluorescens BBc6R8 has a specific priming effect on the growth, morphology and gene expression of the ectomycorrhizal fungus Laccaria bicolor S238N

The mycorrhiza helper Pseudomonas fluorescens BBc6R8 promotes the presymbiotic survival and growth of the ectomycorrhizal fungus Laccaria bicolor S238N in the soil. An in vitro fungal-bacterial confrontation bioassay mimicking the promoting effects of the bacteria on fungal growth was set up to analyse the fungal morphological and transcriptional changes induced by the helper bacteria at three successive stages of the interaction. The specificity of the P. fluorescens BBc6R8 effect was assessed in comparison with six other rhizobacterial strains possessing mycorrhiza helper or pathogen antagonistic abilities. The helper BBc6R8 strain was the only strain to induce increases in the radial growth of the colony, hyphal apex density and branching angle. These morphological modifications were coupled with pleiotropic alterations of the fungal transcriptome, which varied throughout the interaction. Early stage-responsive genes were presumably involved in recognition processes and transcription regulation, while late stage-responsive genes encoded proteins of primary metabolism. Some of the responsive genes were partly specific to the interaction with P. fluorescens BBc6R8, whereas others were mutually regulated by different rhizobacteria. The results highlight the fact that the helper BBc6R8 strain has a specific priming effect on growth, morphology and gene expression of its fungal associate L. bicolor S238N.

Effects of the tomato pathogen Fusarium oxysporum f. sp. radicis-lycopersici and of the biocontrol bacterium Pseudomonas fluorescens WCS365 on the composition of organic acids and sugars in tomato root exudate

The effects of the pathogenic fungus Fusarium oxysporum f. sp. radicis-lycopersici and of the bacterial biocontrol strain Pseudomonas fluorescens WCS365, and of both microbes, on the amounts and composition of root exudate components of tomato plants grown in a gnotobiotic stonewool substrate system were studied. Conditions were selected under which introduction of F. oxysporum f. sp. radicis-lycopersici caused severe foot and root rot, whereas inoculation of the seed with P. fluorescens WCS365 decreased the percentage of diseased plants from 96 to 7%. This is a much better disease control level than was observed in potting soil. Analysis of root exudate revealed that the presence of F. oxysporum f. sp. radicis-lycopersici did not alter the total amount of organic acids, but that the amount of citric acid decreased and that of succinic acid increased compared with the nontreated control. In contrast, in the presence of the P. fluorescens biocontrol strain WCS365, the total amount of organic acid increased, mainly due to a strong increase of the amount of citric acid, whereas the amount of succinic acid decreased dramatically. Under biocontrol conditions, when both microbes are present, the content of succinic acid decreased and the level of citric acid was similar to that in the nontreated control. The amount of sugar was approximately half that of the control sample when either one of the microbes was present alone or when both were present. Analysis of the interactions between the two microbes grown together in sterile tomato root exudate showed that WCS365 inhibited multiplication of F. oxysporum f. sp. radicis-lycopersici, whereas the fungus did not affect the number of CFU of the bacterium.

Survival of gfp-tagged antagonistic bacteria in the rhizosphere of tomato plants and their effects on the indigenous bacterial community

The survival and colonization patterns of Pseudomonas putida PRD16 and Enterobacter cowanii PRF116 in the rhizosphere of greenhouse-grown tomato plants and the effects of their inoculation on the indigenous bacterial community were followed by selective plating, molecular fingerprinting, and confocal laser scanning microscopy (CLSM) over 3 weeks. Both strains, which showed in vitro antagonistic activity against Ralstonia solanacearum, were previously tagged with gfp. Seed and root inoculation were compared. Although plate counts decreased for both gfp-tagged antagonists, PRD16 showed a better survival in the rhizosphere of tomato roots independent of the inoculation method. Analysis of 16S rRNA gene fragments amplified from total community DNA by denaturing gradient gel electrophoresis and CLSM confirmed the decrease in the relative abundance of the inoculant strains. Pronounced differences in the Pseudomonas community patterns for plants inoculated with PRD16 compared to the control were detected 3 weeks after root inoculation, indicating a longer-lasting effect. Analysis by CLSM showed rather heterogeneous colonization patterns for both inoculant strains. In comparison with seed inoculation, root inoculation led to a much better colonization as evidenced by all three methods. The colonization patterns observed by CLSM provide important information on the sampling strategy required for monitoring inoculant strains in the rhizosphere.

Enrichment for enhanced competitive plant root tip colonizers selects for a new class of biocontrol bacteria

Our group studies tomato foot and root rot, a plant disease caused by the fungus Forl (Fusarium oxysporum f.sp. radicis-lycopersici ). Several bacteria have been described to be able to control the disease, using different mechanisms. Here we describe a method that enables us to select, after application of a crude rhizobacterial mixture on a sterile seedling, those strains that reach the root tip faster than our best tomato root colonizer tested so far, the Pseudomonas fluorescens biocontrol strain WCS365. Of the five tested new isolates, four appeared to be able to reduce the number of diseased plants. Analysis of one of these strains, P. fluorescens PCL1751, suggests that it controls the disease through the mechanism 'competition for nutrients and niches', a mechanism novel for biocontrol bacteria. Moreover, this is the first report describing a method to enrich for biocontrol strains from a crude mixture of rhizobacteria. Another advantage of the method is that four out of five strains do not produce antifungal metabolites, which is preferential for registration as a commercial product.

Colonization of tomato root by pathogenic and nonpathogenic Fusarium oxysporum strains inoculated together and separately into the soil

In soil, fungal colonization of plant roots has been traditionally studied by indirect methods such as microbial isolation that do not enable direct observation of infection sites or of interactions between fungal pathogens and their antagonists. Confocal laser scanning microscopy was used to visualize the colonization of tomato roots in heat-treated soil and to observe the interactions between a nonpathogenic strain, Fo47, and a pathogenic strain, Fol8, inoculated onto tomato roots in soil. When inoculated separately, both fungi colonized the entire root surface, with the exception of the apical zone. When both strains were introduced together, they both colonized the root surface and were observed at the same locations. When Fo47 was introduced at a higher concentration than Fol8, it colonized much of the root surface, but hyphae of Fol8 could still be observed at the same location on the root. There was no exclusion of the pathogenic strain by the presence of the nonpathogenic strain. These results are not consistent with the hypothesis that specific infection sites exist on the root for Fusarium oxysporum and instead support the hypothesis that competition occurs for nutrients rather than for infection sites.

Applications of autofluorescent proteins for in situ studies in microbial ecology

When autofluorescent proteins (AFPs), such as green fluorescent protein (GFP) and Discosoma striata red fluorescent protein (DsRed), are excited with light of a specific wavelength, they emit light of a longer wavelength, without the further addition of substrates. A range of AFPs have been identified and cloned from marine organisms, and mutagenesis techniques have been employed to develop improved variant AFPs for applications in biological research. In recent years, AFP technology has become an important tool for microbiologists and microbial ecologists studying processes such as microbe-plant interactions, biosensors, biofilm formation, and horizontal gene transfer. The ability to use AFPs with differing fluorescent spectra within a single cell has allowed simultaneous monitoring of several aspects of microbial physiology and gene expression in situ in real time. This provides a tremendous insight into microbial function and behavior in natural environments. Furthermore, the integration of AFP reporters with other markers and technologies is facilitating a systems approach to research in microbial ecology.

Paenibacillus polymyxa invades plant roots and forms biofilms

Paenibacillus polymyxa is a plant growth-promoting rhizobacterium with a broad host range, but so far the use of this organism as a biocontrol agent has not been very efficient. In previous work we showed that this bacterium protects Arabidopsis thaliana against pathogens and abiotic stress (S. Timmusk and E. G. H. Wagner, Mol. Plant-Microbe Interact. 12:951-959, 1999; S. Timmusk, P. van West, N. A. R. Gow, and E. G. H. Wagner, p. 1-28, in Mechanism of action of the plant growth promoting bacterium Paenibacillus polymyxa, 2003). Here, we studied colonization of plant roots by a natural isolate of P. polymyxa which had been tagged with a plasmid-borne gfp gene. Fluorescence microscopy and electron scanning microscopy indicated that the bacteria colonized predominantly the root tip, where they formed biofilms. Accumulation of bacteria was observed in the intercellular spaces outside the vascular cylinder. Systemic spreading did not occur, as indicated by the absence of bacteria in aerial tissues. Studies were performed in both a gnotobiotic system and a soil system. The fact that similar observations were made in both systems suggests that colonization by this bacterium can be studied in a more defined system. Problems associated with green fluorescent protein tagging of natural isolates and deleterious effects of the plant growth-promoting bacteria are discussed.

Influence of fusaric acid on phenazine-1-carboxamide synthesis and gene expression of Pseudomonas chlororaphis strain PCL1391

Production of the antifungal metabolite phenazine-1-carboxamide (PCN) byPseudomonas chlororaphisstrain PCL1391 is essential for the suppression of tomato foot and root rot caused by the soil-borne fungusF. oxysporumf. sp.radicis-lycopersici. The authors have shown previously that fusaric acid (FA), a phytotoxin produced byFusarium oxysporum, represses the production of PCN and of the quorum-sensing signalN-hexanoyl-l-homoserine lactone (C6-HSL). Here they report that PCN repression by FA is maintained even during PCN-stimulating environmental conditions such as additional phenylalanine, additional ferric iron and a low Mg2+concentration. Constitutive expression ofphzIorphzRincreases the production of C6-HSL and abolishes the repression of PCN production by FA. Transcriptome analysis usingP. chlororaphisPCL1391 microarrays showed that FA represses expression of the phenazine biosynthetic operon (phzABCDEFGH) and of the quorum-sensing regulatory genesphzIandphzR. FA does not alter expression of the PCN regulatorsgacS,rpoSandpsrA. In conclusion, reduction of PCN levels by FA is due to direct or indirect repression ofphzRandphzI. Microarray analyses identified genes of which the expression is strongly influenced by FA. Genes highly upregulated by FA are also upregulated by iron starvation inPseudomonas aeruginosa. This remarkable overlap in the expression profile suggests an overlapping stress response to FA and iron starvation.

Visualization of interactions between a pathogenic and a beneficial Fusarium strain during biocontrol of tomato foot and root rot

The soilborne fungus Fusarium oxysporum f. sp. radicis-lycopersici causes tomato foot and root rot (TFRR), which can be controlled by the addition of the nonpathogenic fungus F. oxysporum Fo47 to the soil. To improve our understanding of the interactions between the two Fusarium strains on tomato roots during biocontrol, the fungi were labeled using different autofluorescent proteins as markers and subsequently visualized using confocal laser scanning microscopy. The results were as follows. i) An at least 50-fold excess of Fo47over F. oxysporum f. sp. radicis-lycopersici was required to obtain control of TFRR. ii) When seedlings were planted in sand infested with spores of a single fungus, Fo47 hyphae attached to the root earlier than those of F. oxysporum f. sp. radicis-lycopersici. iii) Subsequent root colonization by F. oxysporum f. sp. radicis-lycopersici was faster and to a larger extent than that by Fo47. iv) Under disease-controlling conditions, colonization of tomato roots by the pathogenic fungus was significantly reduced. v) When the inoculum concentration of Fo47 was increased, root colonization by the pathogen was arrested at the stage of initial attachment to the root. vi) The percentage of spores of Fo47 that germinates in tomato root exudate in vitro is higher than that of the pathogen F. oxysporum f. sp. radicis-lycopersici. Based on these results, the mechanisms by which Fo47 controls TFRR are discussed in terms of i) rate of spore germination and competition for nutrients before the two fungi reach the rhizoplane; ii) competition for initial sites of attachment, intercellular junctions, and nutrients on the tomato root surface; and iii) inducing systemic resistance.

Role of chemotaxis toward fusaric acid in colonization of hyphae of Fusarium oxysporum f. sp. radicis-lycopersici by Pseudomonas fluorescens WCS365

Pseudomonas fluorescens WCS365 is an excellent competitive colonizer of tomato root tips after bacterization of seed or seedlings. The strain controls tomato foot and root rot (TFRR) caused by the phytopathogenic fungus Fusarium oxysporum f. sp. radicis-lycopersici. Under biocontrol conditions, fungal hyphae were shown to be colonized by WCS365 bacteria. Because chemotaxis is required for root colonization by WCS365 cells, we studied whether chemotaxis also is required for hyphae colonization. To that end, an in vitro assay was developed to study hyphae colonization by bacteria. The results indicated that cells of the cheA mutant FAJ2060 colonize hyphae less efficiently than cells of wild-type strain WCS365, when single strains were analyzed as well as when both strains were applied together. Cells of WCS365 show a chemotactic response toward the spent growth medium of F. oxysporum f. sp. radicis-lycopersici, but those of its cheA mutant, FAJ2060, did not. Fusaric acid, a secondary metabolite secreted by Fusarium strains, appeared to be an excellent chemo-attractant. Supernatant fluids of a number of Fusarium strains secreting different levels of fusaric acid were tested as chemo-attractants. A positive correlation was found between chemo-attractant activity and fusaric acid level. No chemotactic response was observed toward the low fusaric acid-producer FO242. Nevertheless, the hyphae of FO242 still were colonized by WCS365, suggesting that other metabolites also play a role in this process. The possible function of hyphae colonization for the bacterium is discussed.