Role of 2,4-Diacetylphloroglucinol in the Interactions of the Biocontrol Pseudomonad Strain F113 with the Potato Cyst Nematode Globodera rostochiensis

Pseudomonas biocontrol agents of soilborne pathogens: looking back over 30 years

ABSTRACT Pseudomonas spp. are ubiquitous bacteria in agricultural soils and have many traits that make them well suited as biocontrol agents of soilborne pathogens. Tremendous progress has been made in characterizing the process of root colonization by pseudomonads, the biotic and abiotic factors affecting colonization, bacterial traits and genes contributing to rhizosphere competence, and the mechanisms of pathogen suppression. This review looks back over the last 30 years of Pseudomonas biocontrol research and highlights key studies, strains, and findings that have had significant impact on shaping our current understanding of biological control by bacteria and the direction of future research.

Role of 2,4-diacetylphloroglucinol-producing fluorescent Pseudomonas spp. in the defense of plant roots

Plants have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to a lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, USA and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of 10 (5) CFU/g of root, the threshold required to suppress the take-all pathogen, Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the defense of plant roots in many different agroecosystems. At this time, 22 distinct genotypes of 2,4-DAPG producers (designated A - T, PfY and PfZ) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of PHLD, and phylogenetic analysis of PHLD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil and the crop species grown modulates the outcome of the competition among these genotypes in the rhizosphere. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.

Host Crop Affects Rhizosphere Colonization and Competitiveness of 2,4-Diacetylphloroglucinol-Producing Pseudomonas fluorescens

ABSTRACT Strains of Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are biocontrol agents which play a key role in the suppressiveness of some soils against soilborne pathogens. We evaluated the effect of the host plant genotype on rhizosphere colonization by both indigenous and introduced 2,4-DAPG-producing P. fluorescens. First, population densities of indigenous 2,4-DAPG producers in the rhizospheres of alfalfa, barley, bean, flax, lentil, lupine, oat, pea, and wheat grown in a Fusarium wilt-suppressive Puget silt loam were determined. Population densities differed among the various crops and among pea cultivars, with lentil and oat supporting the highest and lowest densities of 2,4-DAPG producers, respectively. Second, to determine the interactions among 2,4-DAPG producers in the rhizosphere, a Shano sandy loam was inoculated individually and with all possible combinations of P. fluorescens Q8r1-96 (genotype D), F113 (genotype K), and MVP1-4 (genotype P) and sown to wheat or pea, and the rhizosphere population dynamics of each strain was monitored. All three strains were similar in ability to colonize the rhizosphere of wheat and pea when introduced alone into the soil; however, when introduced together in equal densities, the outcome of the interactions differed according to the host crop. In the wheat rhizosphere, the population density of strain F113 was significantly greater than that of Q8r1-96 in the mixed inoculation studies, but no significant differences were observed on pea. The population density of strain Q8r1-96 was greater than that of MVP1-4 in the mixed inoculation on wheat, but the opposite occurred on pea. In the wheat rhizosphere, the population of MVP1-4 dropped below the detection limit (log 3.26 CFU g(-1) of root) in the presence of F113; however, on pea, the population density of MVP1-4 was higher than that of F113. When all three strains were present together, F113 had the greatest density in the wheat rhizosphere, but MVP1-4 was dominant in the pea rhizosphere. Finally, eight pea cultivars were grown in soil inoculated with either MVP1-4 or Q8r1-96. The effect of the pea cultivar on rhizosphere colonization was dependent on the bacterial strain inoculated. Rhizosphere population densities of MVP1-4 did not differ significantly among pea cultivars, whereas population densities of Q8r1-96 did. We conclude from these studies that the host crop plays a key role in modulating both rhizosphere colonization by 2,4-DAPG-producing P. fluorescens and the interactions among different genotypes present in the same rhizosphere.

phlD-based genetic diversity and detection of genotypes of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens

Diversity within a worldwide collection of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens strains was assessed by sequencing the phlD gene. Phylogenetic analyses based on the phlD sequences of 70 isolates supported the previous classification into 18 BOX-PCR genotypes (A-Q and T). Exploiting polymorphisms within the sequence of phlD, we designed and used allele-specific PCR primers with a PCR-based dilution endpoint assay to quantify the population sizes of A-, B-, D-, K-, L- and P-genotype strains grown individually or in pairs in vitro, in the rhizosphere of wheat and in bulk soil. Except for P. fluorescens Q8r1-96, which strongly inhibited the growth of P. fluorescens Q2-87, inhibition between pairs of strains grown in vitro did not affect the accuracy of the method. The allele-specific primer-based technique is a rapid method for studies of the interactions between genotypes of 2,4-diacetylphloroglucinol producers in natural environments.

Isolation and characterization of a novel banana rhizosphere bacterium as fungal antagonist and microbial adjuvant in micropropagation of banana

AIM

Isolation and characterization of a bacterial isolate (strain FP10) from banana rhizosphere with innate potential as fungal antagonist and microbial adjuvant in micropropagation of banana.

METHODS AND RESULTS

Bacterium FP10 was isolated from the banana rhizosphere and identified as Pseudomonas aeruginosa based on phenotypic, biochemical traits and sequence homology of partial 622-bp fragment of 16S ribosomal DNA (rDNA) amplicon, with the ribosomal database sequences. Strain FP10 displayed antibiosis towards fungi causing wilt and root necrosis diseases of banana. Production of plant growth hormone, indole-3-acetic acid (IAA), siderophores and phosphate-solubilizing enzyme in FP10 was determined. Strain FP10 tested negative for hydrogen cyanide, cellulase and pectinase, the deleterious traits for plant growth. Screening of antibiotic genes was carried out by polymerase chain reaction using gene-specific primers. Amplification of a 745-bp DNA fragment confirmed the presence of phlD, which is a key gene involved in the biosynthesis of 2,4-diacetylphloroglucinol (DAPG) in FP10. The antibiotic produced by FP10 was confirmed as DAPG using thin layer chromatography, high performance liquid chromatography and Fourier transform infrared and tested for fungal antibiosis towards banana pathogens. Procedures for encapsulation of banana shoot tips with FP10 are described.

CONCLUSIONS

Strain FP10 exhibited broad-spectrum antibiosis towards banana fungi causing wilt and root necrosis. DAPG by FP10 induced bulb formation and lysis of fungal mycelia. Encapsulation of banana shoot tips with FP10 induced higher frequency of germination (plantlet development) than nontreated controls on Murashige and Skoog basal medium. Treatment of banana plants with FP10 enhanced plant height and reduced the vascular discolouration as a result of Fusarium oxysporum f. sp. cubense FOC.

SIGNIFICANCE AND IMPACT OF THE STUDY

Because of the innate potential of fungal antibiosis by DAPG antibiotic and production of siderophore, plant-growth-promoting IAA and phosphatase, the strain FP10 can be used as biofertilizer as well as a biocontrol agent.

Genetic diversity and biocontrol potential of fluorescent pseudomonads producing phloroglucinols and hydrogen cyanide from Swiss soils naturally suppressive or conducive to Thielaviopsis basicola-mediated black root rot of tobacco

Pseudomonas populations producing the biocontrol compounds 2,4-diacetylphloroglucinol (Phl) and hydrogen cyanide (HCN) were found in the rhizosphere of tobacco both in Swiss soils suppressive to Thielaviopsis basicola and in their conducive counterparts. In this study, a collection of Phl+ HCN+Pseudomonas isolates from two suppressive and two conducive soils were used to assess whether suppressiveness could be linked to soil-specific properties of individual pseudomonads. The isolates were compared based on restriction analysis of the biocontrol genes phlD and hcnBC, enterobacterial repetitive intergenic consensus (ERIC)-PCR profiling and their biocontrol ability. Restriction analyses of phlD and hcnBC yielded very concordant relationships between the strains, and suggested significant population differentiation occurring at the soil level, regardless of soil suppressiveness status. This was corroborated by high strain diversity (ERIC-PCR) within each of the four soils and among isolates harboring the same phlD or hcnBC alleles. No correlation was found between the origin of the isolates and their biocontrol activity in vitro and in planta. Significant differences in T. basicola inhibition were however evidenced between the isolates when they were grouped according to their biocontrol alleles. Moreover, two main Pseudomonas lineages differing by the capacity to produce pyoluteorin were evidenced in the collection. Thus, Phl+ HCN+ pseudomonads from suppressive soils were not markedly different from those from nearby conducive soils. Therefore, as far as biocontrol pseudomonads are concerned, this work yields the hypothesis that the suppressiveness of Swiss soils may rely on the differential effects of environmental factors on the expression of key biocontrol genes in pseudomonads rather than differences in population structure of biocontrol Pseudomonas subcommunities or the biocontrol potential of individual Phl+ HCN+ pseudomonad strains.

Influence of plant species on population dynamics, genotypic diversity and antibiotic production in the rhizosphere by indigenous Pseudomonas spp

The population dynamics, genotypic diversity and activity of naturally-occurring 2,4-diacetylphloroglucinol (DAPG)-producing Pseudomonas spp. was investigated for four plant species (wheat, sugar beet, potato, lily) grown in two different soils. All four plant species tested, except lily and in some cases wheat, supported relatively high rhizosphere populations (5 x 10(4) to 1 x 10(6) CFU/g root) of indigenous DAPG-producing Pseudomonas spp. during successive cultivation in both a take-all suppressive and a take-all conducive soil. Although lily supported on average the highest population densities of fluorescent Pseudomonas spp., it was the least supportive of DAPG-producing Pseudomonas spp. of all four plant species. The genotypic diversity of 492 DAPG-producing Pseudomonas isolates, assessed by Denaturing Gradient Gel Electrophoresis (DGGE) analysis of the phlD gene, revealed a total of 7 genotypes. Some of the genotypes were found only in the rhizosphere of a specific plant, whereas the predominant genotypes were found at significantly higher frequencies in the rhizosphere of three plant species (wheat, sugar beet and potato). Statistical analysis of the phlD(+) genotype frequencies showed that the diversity of the phlD(+) isolates from lily was significantly lower than the diversity of phlD(+) isolates found on wheat, sugar beet or potato. Additionally, soil type had a significant effect on both the phlD(+) population density and the phlD(+) genotype frequencies, with the take-all suppressive soil being the most supportive. HPLC analysis further showed that the plant species had a significant effect on DAPG-production by the indigenous phlD(+) population: the wheat and potato rhizospheres supported significantly higher amounts of DAPG produced per cell basis than the rhizospheres of sugar beet and lily. Collectively, the results of this study showed that the host plant species has a significant influence on the dynamics, composition and activity of specific indigenous antagonistic Pseudomonas spp.

Differential impact of some Aspergillus species on Meloidogyne javanica biocontrol by Pseudomonas fluorescens strain CHA0

AIMS

The aim was to determine the influence of some Aspergillus species on the production of nematicidal agent(s) in vitro and biocontrol of Meloidogyne javanica in tomato by Pseudomonas fluorescens strains CHA0 and CHA0/pME3424.

METHODS AND RESULTS

Six species of Aspergillus, isolated from the rhizosphere of certain crops, produced a variety of secondary metabolites in vitro. Culture filtrate (CF) obtained from Ps. fluorescens strain CHA0 and its2,4-diacetylphloroglucinol overproducing mutant CHA0/pME3424 grown in King's B liquid medium caused significant mortality of M. javanica juveniles in vitro. Bacterial growth medium amended with CF of A. niger enhanced nematicidal and beta-galactosidase activities of fluorescent pseudomonads while A. quadrilineatus repressed such activities. Methanol or ethyl acetate extracts of the CF of A. niger markedly optimized bacterial efficacy to cause nematode deaths while hexane extract of the fungus had no influence on the nematicidal activity of the bacterial strains. A. niger applied alone or in conjunction with the bacterial inoculants inhibited root-knot nematode galling in tomato. On the other hand, A. quadrilineatus used alone or together with CHA0 did not inhibit nematode galling but when used in combination with strain CHA0/pME3424 did reduce galling intensity.

CONCLUSIONS

Aspergillus niger enhances the production of nematicidal compounds by Ps. fluorescensin vitro and improves biocontrol potential of the bacterial inoculants in tomato while A. quadrilineatus reduces bacterial performance to suppress root-knot nematodes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Rhizosphere harbours a variety of micro-organisms including bacteria, fungi and viruses. Aspergillus species are ubiquitous in most agricultural soils and generally produce a variety of secondary metabolites. Such metabolites synthesized by Aspergillus species may influence the production of nematicidal agents and subsequent biocontrol performance of the bacterial inoculants against plant-parasitic nematodes. This fact needs to be taken into consideration when using biocontrol strains in an agriculture system.

Trichoderma harzianum enhances the production of nematicidal compounds in vitro and improves biocontrol of Meloidogyne javanica by Pseudomonas fluorescens in tomato

AIMS

To determine the influence of soil-borne fungus Trichoderma harzianum on the biocontrol performance of Pseudomonas fluorescens strain CHA0 and its 2,4-diacetylphloroglucinol (DAPG) overproducing derivative CHA0/pME3424 against Meloidogyne javanica.

METHODS AND RESULTS

Amendment of the culture filtrate (CF) or methanol extract of the CF of a T. harzianum strain Th6 to P. fluorescens growth medium enhanced the production of nematicidal compound(s) by bacterial inoculants in vitro. In addition, bacteria overwhelmingly expressed phl'-'lacZ reporter gene when the medium was amended with CF of T. harzianum. Pseudomonas fluorescens and T. harzianum applied together in unsterilized sandy loam soil caused greater reduction in nematode population densities in tomato roots.

CONCLUSIONS

Trichoderma harzianum improves root-knot nematode biocontrol by the antagonistic rhizobacterium P. fluorescens both in vitro and under glasshouse conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

The synergistic effect of T. harzianum on the production of nematicidal compound(s) critical in biocontrol may improve the efficacy of biocontrol bacteria against plant-parasitic nematodes. Considering the inconsistent performance of the biocontrol agents under field conditions, application of a mixture of compatible T. harzianum and P. fluorescens would more closely mimic the natural situation and might broaden the spectrum of biocontrol activity with enhanced efficacy and reliability of control.

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.

Induced systemic resistance in Arabidopsis thaliana in response to root inoculation with Pseudomonas fluorescens CHA0

Root inoculation of Arabidopsis thaliana ecotype Columbia with Pseudomonas fluorescens CHA0r partially protected leaves from the oomycete Peronospora parasitica. The molecular determinants of Pseudomonas fluorescens CHA0r for this induced systemic resistance (ISR) were investigated, using mutants derived from strain CHA0: CHA400 (pyoverdine deficient), CHA805 (exoprotease deficient), CHA77 (HCN deficient), CHA660 (pyoluteorin deficient), CHA631 (2,4-diacetylphloroglucinol [DAPG] deficient), and CHA89 (HCN, DAPG- and pyoluteorin deficient). Only mutations interfering with DAPG production led to a significant decrease in ISR to Peronospora parasitica. Thus, DAPG production in Pseudomonas fluorescens is required for the induction of ISR to Peronospora parasitica. DAPG is known for its antibiotic activity; however, our data indicate that one action of DAPG could be due to an effect on the physiology of the plant. DAPG at 10 to 100 microM applied to roots of Arabidopsis mimicked the ISR effect. CHA0r-mediated ISR was also tested in various Arabidopsis mutants and transgenic plants: NahG (transgenic line degrading salicylic acid [SA]), sid2-1 (nonproducing SA), npr1-1 (non-expressing NPR1 protein), jar1-1 (insensitive to jasmonic acid and methyl jasmonic acid), ein2-1 (insensitive to ethylene), etr1-1 (insensitive to ethylene), eir1-1 (insensitive to ethylene in roots), and pad2-1 (phytoalexin deficient). Only jar1-1, eir1-1, and npr1-1 mutants were unable to undergo ISR. Sensitivity to jasmonic acid and functional NPR1 and EIR1 proteins were required for full expression of CHA0r-mediated ISR. The requirements for ISR observed in this study in Peronospora parasitica induced by Pseudomonas fluorescens CHA0r only partially overlap with those published so far for Peronospora parasitica, indicating a great degree of flexibility in the molecular processes leading to ISR.

Non-pathogenic Fusarium solani represses the biosynthesis of nematicidal compounds in vitro and reduces the biocontrol of Meloidogyne javanica by Pseudomonas fluorescens in tomato

AIMS

The aim of the present investigation was to determine the influence of various Fusarium solani strains on the production of nematicidal agent(s) in vitro and biocontrol of Meloidogyne javanica in tomato by Pseudomonas fluorescens strains CHA0 and CHA0/pME3424.

METHODS AND RESULTS

Culture filtrates (CF) of P. fluorescens strain CHA0 and its diacetylphloroglucinol-overproducing derivative CHA0/pME3424 caused substantial mortality of M. javanica juveniles in vitro. Bacterial growth medium amended with the growth medium of F. solani repressed the nematicidal activity of the bacteria. Methanol extract of F. solani CF resulting from Czapek's Dox liquid (CDL) medium without zinc amendment repressed the nematicidal activity of the bacteria while the CF obtained from CDL medium amended with zinc did not. Conidial suspension of F. solani strain Fs5 (repressor strain for the biosynthesis of nematicidal compounds in P. fluorescens) reduced biocontrol potential of the bacterial inoculants against M. javanica in tomato while strain Fs3 (non-repressor) did not.

CONCLUSIONS

Fusarium solani strains with increased nematicidal activity repress the biosynthesis of nematicidal compounds by P. fluorescens strains in vitro and greatly alter its biocontrol efficacy against root-knot nematode under natural conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fusarium solani strains are distributed worldwide and found in almost all the agricultural fields which suggest that some mycotoxin-producing strains will also be found in almost any soil sample taken. Besides the suppressive effect of these metabolite-producing strains on the production of nematicidal compound(s) critical in biocontrol, F. solani strains may also affect the performance of mycotoxin-sensitive biocontrol bacteria effective against plant-parasitic nematodes.

Interactions Between Strains of 2,4-Diacetylphloroglucinol-Producing Pseudomonas fluorescens in the Rhizosphere of Wheat

ABSTRACT Strains of fluorescent Pseudomonas spp. that produce the antibiotic 2,4-diacetylphoroglucinol (2,4-DAPG) are among the most effective rhizobacteria controlling diseases caused by soilborne pathogens. The genotypic diversity that exists among 2,4-DAPG producers can be exploited to improve rhizosphere competence and biocontrol activity. Knowing that D-genotype 2,4-DAPG-producing strains are enriched in some take-all decline soils and that P. fluorescens Q8r1-96, a representative D-genotype strain, as defined by whole-cell repetitive sequence-based polymerase chain reaction (rep-PCR) with the BOXA1R primer, is a superior colonizer of wheat roots, we analyzed whether the exceptional rhizosphere competence of strain Q8r1-96 on wheat is characteristic of other D-genotype isolates. The rhizosphere population densities of four D-genotype strains and a K-genotype strain introduced individually into the soil were significantly greater than the densities of four strains belonging to other genotypes (A, B, and L) and remained above log 6.8 CFU/g of root over a 30-week cycling experiment in which wheat was grown for 10 successive cycles of 3 weeks each. We also explored the competitive interactions between strains of different genotypes inhabiting the same soil or rhizosphere when coinoculated into the soil. Strain Q8r1-96 became dominant in the rhizosphere and in nonrhizosphere soil during a 15-week cycling experiment when mixed in a 1:1 ratio with either strain Pf-5 (A genotype), Q2-87 (B genotype), or 1M1-96 (L genotype). Furthermore, the use of the de Wit replacement series demonstrated a competitive disadvantage for strain Q2-87 or strong antagonism by strain Q8r1-96 against Q2-87 in the wheat rhizosphere. Amplified rDNA restriction analysis and sequence analysis of 16S rDNA showed that species of Arthrobacter, Chryseobacterium, Flavobacterium, Massilia, Microbacterium, and Ralstonia also were enriched in culturable populations from the rhizosphere of wheat at the end of a 30-week cycling experiment in the presence of 2,4-DAPG producers. Identifying the interactions among 2,4-DAPG producers and with other indigenous bacteria in the wheat rhizosphere will help to elucidate the variability in biocontrol efficacy of introduced 2,4-DAPG producers and fluctuations in the robustness of take-all suppressive soils.

Effect of 2,4-diacetylphloroglucinol on pythium: cellular responses and variation in sensitivity among propagules and species

ABSTRACT The antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) plays an important role in the suppression of plant pathogens by several strains of Pseudomonas spp. Based on the results of this study, there is variation within and among Pythium spp. to 2,4-DAPG. Also, various propagules of Pythium ultimum var. sporangiiferum, that are part of the asexual stage of the life cycle, differ considerably in their sensitivity to 2,4-DAPG. Mycelium was the most resistant structure, followed by zoosporangia, zoospore cysts, and zoospores. Additionally, we report for the first time that pH has a significant effect on the activity of 2,4-DAPG, with a higher activity at low pH. Furthermore, the level of acetylation of phloroglucinols is also a major determinant of their activity. Transmission electron microscopy studies revealed that 2,4-DAPG causes different stages of disorganization in hyphal tips of Pythium ultimum var. sporangiiferum, including alteration (proliferation, retraction, and disruption) of the plasma membrane, vacuolization, and cell content disintegration. The implications of these results for the efficacy and consistency of biological control of plant-pathogenic Pythium spp. by 2,4-DAPG-producing Pseudomonas spp. are discussed.

Improvement of Pseudomonas fluorescens CHA0 biocontrol activity against root-knot nematode by the addition of ammonium molybdate

AIMS

To improve the efficacy of Pseudomonas fluorescens CHA0 and its genetically modified (GM) derivatives by adding ammonium molybdate to control Meloidogyne javanica, the root-knot nematode in mungbean.

METHODS AND RESULTS

Culture filtrate of P. fluorescens CHA0 and its GM derivative (antibiotic overproducing strain CHA0/pME3424 and antibiotic-deficient CHA89) obtained from nutrient broth yeast extract medium amended with 1, 2 or 4 mm of ammonium molybdate (NH4-Mo) caused substantial mortality of M. javanica juveniles in vitro. Pseudomonas fluorescens CHA0 or CHA0/pME3424 applied in conjunction with NH4-Mo caused greater reduction of nematode penetration in mungbean roots compared with the bacterial application alone. Ammonium molybdate at 4 mg kg-1 of soil along with CHA0 also enhanced plant height while shoot weight remained unaffected. Either used alone or in conjunction with NH4-Mo, strain CHA89 did not reduce nematode invasion compared with the controls. Bacterial strains did not differ significantly in their colonization potential in the mungbean rhizosphere. Efficacy of the biocontrol bacteria to control root-knot nematode was accentuated when soil was treated with NH4-Mo and zinc (both at 1 mg kg-1 of soil).

CONCLUSION

The addition of ammonium molybdate enhances the production of nematicidal compounds by P. fluorescensin vitro and improves bacterial efficacy against root-knot nematode under glasshouse conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Application of minerals such as ammonium molybdate is appealing because they are cheap and can easily be applied under field conditions to improve biocontrol potential of the bacterial inoculants. They also significantly reduce the amount of biocontrol inoculant biomass required to achieve root-knot disease control, with a consequent reduction in cost.

Regulation of antibiotic production in root-colonizing Peudomonas spp. and relevance for biological control of plant disease

Certain strains of fluorescent pseudomonads are important biological components of agricultural soils that are suppressive to diseases caused by pathogenic fungi on crop plants. The biocontrol abilities of such strains depend essentially on aggressive root colonization, induction of systemic resistance in the plant, and the production of diffusible or volatile antifungal antibiotics. Evidence that these compounds are produced in situ is based on their chemical extraction from the rhizosphere and on the expression of antibiotic biosynthetic genes in the producer strains colonizing plant roots. Well-characterized antibiotics with biocontrol properties include phenazines, 2,4-diacetylphloroglucinol, pyoluteorin, pyrrolnitrin, lipopeptides, and hydrogen cyanide. In vitro, optimal production of these compounds occurs at high cell densities and during conditions of restricted growth, involving (i) a number of transcriptional regulators, which are mostly pathway-specific, and (ii) the GacS/GacA two-component system, which globally exerts a positive effect on the production of extracellular metabolites at a posttranscriptional level. Small untranslated RNAs have important roles in the GacS/GacA signal transduction pathway. One challenge in future biocontrol research involves development of new strategies to overcome the broad toxicity and lack of antifungal specificity displayed by most biocontrol antibiotics studied so far.

Zinc and glycerol enhance the production of nematicidal compounds in vitro and improve the biocontrol of Meloidogyne javanica in tomato by fluorescent pseudomonads

AIMS

To assess the effects of various carbon and mineral sources on the nematicidal potential of biocontrol inoculants of Pseudomonas aeruginosa IE-6S+ and Ps. fluorescens CHA0 under laboratory and glasshouse conditions.

METHODS AND RESULTS

Culture filtrates of strains IE-6S+ and CHA0, cultured in nutrient yeast extract broth, caused substantial mortality of the juveniles of Meloidogyne javanica. The nematicidal activities of the culture filtrates were altered after amendment with various carbon and mineral sources. Soil amendment with zinc alone or in combination with glycerol improved the biocontrol efficacy against root-knot nematode, promoted tomato plant growth and enhanced bacterial rhizosphere and endophytic colonization.

CONCLUSIONS

Appropriate quantities of glycerol and zinc alone or in combination enhance the nematicidal activity of Ps. aeruginosa and Ps. fluorescens. Glucose reduces the activity of these bacteria against nematodes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Minerals and carbon sources are appealing because they are easy and economical to provide during liquid fermentation of inoculants or as fertilizer amendments to improve the biocontrol activity of indigenous and introduced bacteria.

Differential ability of genotypes of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens strains to colonize the roots of pea plants

Indigenous populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent Pseudomonas spp. that occur naturally in suppressive soils are an enormous resource for improving biological control of plant diseases. Over 300 isolates of 2,4-DAPG-producing fluorescent Pseudomonas spp. were isolated from the rhizosphere of pea plants grown in soils that had undergone pea or wheat monoculture and were suppressive to Fusarium wilt or take-all, respectively. Representatives of seven genotypes, A, D, E, L, O, P, and Q, were isolated from both soils and identified by whole-cell repetitive sequence-based PCR (rep-PCR) with the BOXA1R primer, increasing by three (O, P, and Q) the number of genotypes identified previously among a worldwide collection of 2,4-DAPG producers. Fourteen isolates representing eight different genotypes were tested for their ability to colonize the rhizosphere of pea plants. Population densities of strains belonging to genotypes D and P were significantly greater than the densities of other genotypes and remained above log 6.0 CFU (g of root)(-1) over the entire 15-week experiment. Genetic profiles generated by rep-PCR or restriction fragment length polymorphism analysis of the 2,4-DAPG biosynthetic gene phlD were predictive of the rhizosphere competence of the introduced 2,4-DAPG-producing strains.

Comparison of Three Methods for Monitoring Populations of Different Genotypes of 2,4-Diacetylphloroglucinol-Producing Pseudomonas fluorescens in the Rhizosphere

ABSTRACT Pseudomonas fluorescens strains producing the antibiotic 2,4-diacetylphloroglucinol (DAPG) have biocontrol activity against a broad spectrum of root and seedling diseases. In this study, we determined the effect of genotype on the ability to isolate and quantify introduced 2,4-DAPG producers from the rhizosphere of wheat using three different methods: traditional dilution plating on selective media, colony hybridization followed by polymerase chain reaction (PCR), and phlD-specific PCR-based dilution endpoint assay. Regression analysis of the population densities of 10 2,4-DAPG-producing P. fluorescens, representing five genotypes, determined by the three different methods demonstrated that the relationship was linear (P < 0.001) and the techniques were very similar (i.e., slopes equal to 1.0). The phlD-specific PCR-based assay had a slightly lower limit of detection than the other two methods (log 3.3 versus log 4.0 CFU/g of fresh root weight). With the colony hybridization procedure, we observed that the phlD probe, derived from strain P. fluorescens Q8r1-96, hybridized more strongly to colonies of BOX-PCR genotypes D (strains W2-6, L5.1-96, Q8r1-96, and Q8r2-96) and K (strain F113) compared with strains of genotypes A (Pf-5 and CHA0), B (Q2-87), and L (1M1-96 and W4-4). Colony hybridization alone overestimated the actual densities of some strains, thus requiring an additional PCR step to obtain accurate estimates. In contrast, population densities estimated for three of the bacterial treatments (strains CHA0, W2-6, and Q8r2-96) with the PCR-based assay were significantly (P < 0.041) smaller by 7.6 to 9.2% and 6.4 to 9.4% than population densities detected by the dilution plating and colony hybridization techniques, respectively. In this paper, we discuss the relative advantages of the different methods for detecting 2,4-DAPG producers.

Microbial populations responsible for specific soil suppressiveness to plant pathogens

Agricultural soils suppressive to soilborne plant pathogens occur worldwide, and for several of these soils the biological basis of suppressiveness has been described. Two classical types of suppressiveness are known. General suppression owes its activity to the total microbial biomass in soil and is not transferable between soils. Specific suppression owes its activity to the effects of individual or select groups of microorganisms and is transferable. The microbial basis of specific suppression to four diseases, Fusarium wilts, potato scab, apple replant disease, and take-all, is discussed. One of the best-described examples occurs in take-all decline soils. In Washington State, take-all decline results from the buildup of fluorescent Pseudomonas spp. that produce the antifungal metabolite 2,4-diacetylphloroglucinol. Producers of this metabolite may have a broader role in disease-suppressive soils worldwide. By coupling molecular technologies with traditional approaches used in plant pathology and microbiology, it is possible to dissect the microbial composition and complex interactions in suppressive soils.

Autoinduction of 2,4-diacetylphloroglucinol biosynthesis in the biocontrol agent Pseudomonas fluorescens CHA0 and repression by the bacterial metabolites salicylate and pyoluteorin

The antimicrobial metabolite 2,4-diacetylphloroglucinol (2,4-DAPG) contributes to the capacity of Pseudomonas fluorescens strain CHA0 to control plant diseases caused by soilborne pathogens. A 2, 4-DAPG-negative Tn5 insertion mutant of strain CHA0 was isolated, and the nucleotide sequence of the 4-kb genomic DNA region adjacent to the Tn5 insertion site was determined. Four open reading frames were identified, two of which were homologous to phlA, the first gene of the 2,4-DAPG biosynthetic operon, and to the phlF gene encoding a pathway-specific transcriptional repressor. The Tn5 insertion was located in an open reading frame, tentatively named phlH, which is not related to known phl genes. In wild-type CHA0, 2, 4-DAPG production paralleled expression of a phlA'-'lacZ translational fusion, reaching a maximum in the late exponential growth phase. Thereafter, the compound appeared to be degraded to monoacetylphloroglucinol by the bacterium. 2,4-DAPG was identified as the active compound in extracts from culture supernatants of strain CHA0 specifically inducing phlA'-'lacZ expression about sixfold during exponential growth. Induction by exogenous 2,4-DAPG was most conspicuous in a phlA mutant, which was unable to produce 2, 4-DAPG. In a phlF mutant, 2,4-DAPG production was enhanced severalfold and phlA'-'lacZ was expressed at a level corresponding to that in the wild type with 2,4-DAPG added. The phlF mutant was insensitive to 2,4-DAPG addition. A transcriptional phlA-lacZ fusion was used to demonstrate that the repressor PhlF acts at the level of transcription. Expression of phlA'-'lacZ and 2,4-DAPG synthesis in strain CHA0 was strongly repressed by the bacterial extracellular metabolites salicylate and pyoluteorin as well as by fusaric acid, a toxin produced by the pythopathogenic fungus Fusarium. In the phlF mutant, these compounds did not affect phlA'-'lacZ expression and 2, 4-DAPG production. PhlF-mediated induction by 2,4-DAPG and repression by salicylate of phlA'-'lacZ expression was confirmed by using Escherichia coli as a heterologous host. In conclusion, our results show that autoinduction of 2,4-DAPG biosynthesis can be countered by certain bacterial (and fungal) metabolites. This mechanism, which depends on phlF function, may help P. fluorescens to produce homeostatically balanced amounts of extracellular metabolites.

Small, stable shuttle vectors based on the minimal pVS1 replicon for use in gram-negative, plant-associated bacteria

The minimal replicon of the Pseudomonas plasmid pVS1 was genetically defined and combined with the Escherichia coli p15A replicon, to provide a series of new, oligocopy cloning vectors (5.3 to 8.3 kb). Recombinant plasmids derived from these vectors were stable in growing and nongrowing cells of root-colonizing P. fluorescens strains incubated under different environmental conditions for more than 1 month.

Biological control of Pythium ultimum by Stenotrophomonas maltophilia W81 is mediated by an extracellular proteolytic activity

Stenotrophomonas maltophilia strain W81, isolated from the rhizosphere of field-grown sugar beet, produced the extracellular enzymes chitinase and protease and inhibited the growth of the phytopathogenic fungus Pythium ultimum in vitro. The role of these lytic enzymes in the interaction between W81 and P. ultimum was investigated using Tn5 insertion mutants of W81 incapable of producing extracellular protease (W81M1), extracellular chitinase (W81M2) or the two enzymes (W81A1). Lytic enzyme activity was restored in W81A1 following introduction of a 15 kb cosmid-borne fragment of W81 genomic DNA. Incubation of P. ultimum in the presence of commercial purified protease or cell-free supernatants from cultures of wild-type W81, the chitinase-negative mutant W81M2 or the complemented derivative W81A1 (pCU800) resulted in hyphal lysis and loss of subsequent fungal growth ability once re-inoculated onto fresh plates. In contrast, commercial purified chitinase or cell-free supernatants from cultures of the protease-negative mutant WS1M1 or the chitinase- and protease-negative mutant W81A1 had no effect on integrity of the essentially chitin-free Pythium mycelium, and did not prevent subsequent growth of the fungus. In soil microcosms containing soil naturally infested by Pythium spp., strains W81, W81M2 and W81A1(pCU800) reduced the ability of Pythium spp. to colonize the seeds of sugar beet and improved plant emergence compared with the untreated control, whereas W81A1 and W21M1 failed to protect sugar beet from damping-off. Wild-type W81 and its mutant derivatives colonized the rhizosphere of sugar beet to similar extents, it was concluded that the ability of S. maltophilia W81 to protect sugar beet from Pythium -mediated damping-off was due to the production of an extracellular protease.