Evidence for the involvement of calcium in the hatching of Globodera rostochiensis

Low concentrations of ruthenium red and lanthanum chloride inhibited hatching of juveniles from eggs in cysts of Globodera rostochiensis in potato root diffusate. Pro-bit analysis for 31 cysts at seven concentrations of ruthenium red showed that 50% inhibition with 95% fiducial limits occured at 47 ± 23 μm; a similar value of 59 ± 14 μm was obtained using eggs removed from cysts. Results for 10 to 20 cysts at six concentrations of lanthanum chloride suggested a somewhat higher value for 50% inhibition of 110 ± 83 μM. In contrast hatching of eggs in cysts of Heterodera schachtii in water was unaffected by 5 ITIM lanthanum chloride and 625 μM ruthenium red, concentrations which cause over 90% inhibition of hatch in G. rostochiensis.
Two calcium ionophores synergised hatching of a 1971 population of G. rostochiensis in dilute diffusate. Optimal concentrations of 2 μM for A23187 and 10 μM for BrX537A increased the hatch from 17 ± 3–6 juveniles/cyst to 114 ± 44 juveniles/cyst and 138 ± 40 juveniles/cyst respectively. Ionophores in the absence of diffusate hatched very few eggs of this population but caused a greater hatch in a second (1975) population which gave a high hatch in water of 43 ± 10 juveniles/cyst. This was increased by A23187 to 181 ± 41 juveniles/cyst. The results with both the inhibitors and the ionophores suggest that hatching in G. rostochiensis might be a calcium-mediated process.     

Toxicity of 2,4-diacetylphloroglucinol (DAPG) to Plant-parasitic and Bacterial-feeding Nematodes

The antibiotic 2,4-diacetylphloroglucinol (DAPG) is produced by some isolates of the beneficial bacterium Pseudomonas fluorescens. DAPG is toxic to many organisms, and crop yield increases have been reported after application of DAPG-producing P. fluorescens. This study was conducted to determine whether DAPG is toxic to selected nematodes. The plant-parasitic nematodes Heterodera glycines, Meloidogyne incognita, Pratylenchus scribneri and Xiphinema americanum, and the bacterial-feeding nematodes Caenorhabditis elegans, Pristionchus pacificus, and Rhabditis rainai, were immersed in concentrations ranging from 0 to 100 μg/ml DAPG. Egg hatch and viability of juveniles and adults were determined. DAPG was toxic to X. americanum adults, with an LD(50) of 8.3 μg/ml DAPG. DAPG decreased M. incognita egg hatch, but stimulated C. elegans hatch during the first hours of incubation. Viability of M. incognita J2 and of C. elegans J1 and adults was not affected. There were no observed effects on the other nematodes. The study indicated that DAPG is not toxic to all nematodes, and did not affect the tested species of beneficial bacterial-feeding nematodes. Augmentation of DAPG-producing P. fluorescens populations for nematode biocontrol could be targeted to specific nematode species known to be affected by this compound and by other antibiotics produced by the bacteria, or these bacteria could be used for other possible effects, such as induced plant resistance.     

Mutational Disruption of the Biosynthesis Genes Coding for the Antifungal Metabolite 2,4-Diacetylphloroglucinol Does Not Influence the Ecological Fitness of Pseudomonas fluorescens F113 in the Rhizosphere of Sugarbeets

The ability of Pseudomonas fluorescens F113 to produce the antibiotic 2,4-diacetylphloroglucinol (DAPG) is a key factor in the biocontrol of the phytopathogenic fungus Pythium ultimum by this strain. In this study, a DAPG-producing strain (rifampin-resistant mutant F113Rif) was compared with a nearly isogenic DAPG-negative biosynthesis mutant (Tn5::lacZY derivative F113G22) in terms of the ability to colonize and persist in the rhizosphere of sugarbeets in soil microcosms during 10 plant growth-harvest cycles totaling 270 days. Both strains persisted similarly in the rhizosphere for 27 days, regardless of whether they had been inoculated singly onto seeds or coinoculated in a 1:1 ratio. In order to simulate harvest and resowing, the roots were removed from the soil and the pots were resown with uninoculated sugarbeet seeds for nine successive 27-day growth-harvest cycles. Strains F113Rif and F113G22 performed similarly with respect to colonizing the rhizosphere of sugarbeet, even after nine cycles without reinoculation. The introduced strains had a transient effect on the size of the total culturable aerobic bacterial population. The results indicate that under these experimental conditions, the inability to produce DAPG did not reduce the ecological fitness of strain F113 in the rhizosphere of sugarbeets.     

A method for estimating the contribution of seed potatoes, machinery and soil tare in field infestations with potato cyst nematodes on a national scale

In order to make a cost benefit analysis for the management of the potato cyst nematodes Globodera rostochiensis and G. pallida, we developed a method to estimate the relative importance of three basic distribution channels of potato cyst nematodes: seed potatoes, machinery and soil tare. The baseline is determined by the area planted with potatoes, the area infested with potato cysts, the proportion of resistant potato cultivars and the distribution of cysts trough different channels. This quantification forms a basis for the evaluation of the effects of different control measures for potato cyst nematode on a national scale. The method can be useful as an example for application in other countries.     

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.     

EVect of the Entomopathogenic Nematodes, Steinernema feltiae and S. carpocapsae , on the Potato Cyst Nematode, Globodera rostochiensis , in Pot Trials

Two pot experiments, one in a glasshouse and the other in an outdoor sand plunge, were conducted to examine the influence of the entomopathogenic nematodes, Steinernema feltiae and S. carpocapsae , on the invasion and development of the potato cyst nematode, Globodera rostochiensis . Of a total of eight diVerent treatments with entomopathogenic nematodes in the glasshouse trial, three reduced the invasion of G. rostochiensis and one reduced the numbers of new cysts that were produced compared with controls. In the outdoor experiment, seven of the 12 treatments gave a reduction in invasion but none resulted in changes in the numbers of cysts found at plant senescence. In general, invasion of G. rostochiensis juveniles was reduced more eVectively by S. carpocapsae than by S. feltiae , and was greatest in the outdoor trial where larger inocula of entomopathogenic nematodes were used. Overall, the results indicated that use of S. feltiae and S. carpocapsae is unlikely to provide a viable control strategy for G. rostochiensis .     

Host Status of Different Potato (Solanum tuberosum) Varieties and Hatching in Root Diffusates of Globodera ellingtonae

Globodera ellingtonae was detected in Oregon in 2008. In order to make decisions regarding the regulation of this nematode, knowledge of its biology is required. We determined the host status of a diversity of potato (Solanum tuberosum) varieties in soil-based experiments and identified hatching stimulants in in vitro hatching assays. ‘Russet Burbank,’ ‘Desiree,’ ‘Modac,’ ‘Norland,’ ‘Umatilla,’ and ‘Yukon Gold’ were good hosts (RF > 14) for G. ellingtonae. Potato varieties ‘Maris Piper,’ ‘Atlantic,’ and ‘Satina,’ all which contain the Ro1 gene that confers resistance to G. rostochiensis, were not hosts for G. ellingtonae. In in vitro hatching assays, G. ellingtonae hatched readily in the presence of diffusates from potato (PRD) and tomato (Solanum lycopersicum; TRD). Egg hatch occurred in an average of between 87% and 90% of exposed cysts, with an average of between 144 and 164 juveniles emerging per cyst, from PRD- and TRD-treated cysts, respectively. This nematode hatched rapidly in the presence of PRD and TRD, with at least 66% of total hatch occurring by day 3 of exposure. There was no dose-response of egg hatch to concentrations of PRD or TRD ranging from 1:5 to 1:100 diffusate to water. When G. ellingtonae was exposed to root diffusates from 21 different plants, hatch occurred in 0% to 70% of exposed cysts, with an average of between 0 to 27 juveniles emerging per cyst. When root diffusate-exposed cysts were subsequently transferred to PRD to test viability, root diffusates from arugula (Eruca sativa), sudangrass (Sorghum bicolor subsp. drummondii), and common vetch (Vicia sativa) continued to inhibit egg hatch compared with the other root diffusates or water in which hatch occurred readily (60 to 182 juveniles emerging per cyst). Previously known hatching stimulants of G. rostochiensis and G. pallida, sodium metavanadate, sodium orthovanadate, and sodium thiocyanate, stimulated some egg hatch. Although, Globodera ellingtonae hatched readily in PRD and TRD and reproduced on potato, the pathogenicity of this nematode on potato remains to be determined.     

In-vitro Assays of Meloidogyne incognita and Heterodera glycines for Detection of Nematode-antagonistic Fungal Compounds

In-vitro methods were developed to test fungi for production of metabolites affecting nematode egg hatch and mobility of second-stage juveniles. Separate assays were developed for two nematodes: root-knot nematode (Meloidogyne incognita) and soybean cyst nematode (Heterodera glycines). For egg hatch to be successfully assayed, eggs must first be surface-disinfested to avoid the confounding effects of incidental microbial growth facilitated by the fungal culture medium. Sodium hypochlorite was more effective than chlorhexidine diacetate or formaldehyde solutions at surface-disinfesting soybean cyst nematode eggs from greenhouse cultures. Subsequent rinsing with sodium thiosulfate to remove residual chlorine from disinfested eggs did not improve either soybean cyst nematode hatch or juvenile mobility. Soybean cyst nematode hatch in all culture media was lower than in water. Sodium hypochlorite was also used to surface-disinfest root-knot nematode eggs. In contrast to soybean cyst nematode hatch, root-knot nematode hatch was higher in potato dextrose broth medium than in water. Broth of the fungus Fusarium equiseti inhibited root-knot nematode egg hatch and was investigated in more detail. Broth extract and its chemical fractions not only inhibited egg hatch but also immobilized second-stage juveniles that did hatch, confirming that the fungus secretes nematode-antagonistic metabolites.     

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.     

Carbon fractions in the rhizosphere of pea inoculated with 2,4 diacetylphloroglucinol producing and non-producing Pseudomonas fluorescens F113

The aim of this work was to determine the effect of wild type and functionally modified Pseudomonas fluorescens strains on C fractions in the rhizosphere of pea. The lac ZY marked F113 strain produces the antibiotic 2,4 diacetylphloroglucinol (DAPG) useful in plant disease control. The modified strain of F113 was represented in production of DAPG, creating the DAPG negative strain F113 G22. The F113 treatment resulted in a significantly lower shoot/root ratio. The F113 G22 treatment had a significantly greater indigenous and total fluorescent Pseudomonas population than the control and F113 (DAPG₊) treatment. Both strains significantly increased the water soluble carbohydrates and the total water soluble carbon in the pea rhizosphere soil. Strain F113 significantly increased the soil protein content relative to the control, but not in relation to the F113 G22 treatment. The F113 treatment had a significantly greater organic acid content than the control and F113 G22 treatments, whilst the F113 G22 treatment was also significantly greater than the control. Both inocula resulted in significantly lower phosphate contents than the control. The F113 inocula significantly increased alkaline phosphatase, sulphatase and urease activities, and reduced β glucosidase activities indicating increased carbon availability. Both inocula increased C availability ; however, antibiotic production by strain F113 reduced the utilisation of organic acids released from the plant resulting in differing effects of the two strains on nutrient availability, plant growth, soil enzyme activities and Pseudomonas populations.     

The effect of oxamyl and the growing of susceptible and resistant potato cultivars on the population dynamics of Globodera rostochiensis throughout the soil profile

Comparisons are made between the population dynamics of potato cyst nematode and root growth of Pentland Crown and Maris Piper potato cultivars. Large changes in the number of eggs occurred to a depth of 48 cm, particularly in a peaty loam and in plots treated with oxamyl. Oxamyl delayed the hatch of eggs in the peaty loam but not in the sandy loam, giving the protected plants several weeks without invasion damage during which they became bigger with their roots better established deeper in the soil. Oxamyl killed or impaired the movement of second stage juveniles but appeared to have little systemic activity to hinder juvenile development within roots.     

Effects of Initial Nematode Density on Population Dynamics of Globodera rostochiensis on Resistant and Susceptible Potatoes

The influence of resistant and susceptible potato cultivars on Globodera rostochiensis population density changes was studied at different nematode inoculum levels (Pi) in the greenhouse and field. Soil in which one susceptible and two resistant cultivars were grown and fallow soil in pots was infested with cysts to result in densities of 0.04-75 eggs/cm³ soil. A resistant cultivar was grown in an infested field with Pi of 0.7-16.7 eggs/cm³ soil. Pi was positively correlated with decline of soil population densities due to hatch where resistant potatoes were grown in the greenhouse and in the field but not in fallow soil. However, Pi was not correlated with in vitro hatch of G. rostochiensis cysts in water or potato root diffusate. Under continuous culture o f a resistant cultivar, viable eggs per cyst declined 60-90% per plant growth cycle (4 weeks) and the number of cysts containing viable eggs had decreased by 77% after five cycles. The rate of G. rostochiensis reproduction on both resistant and susceptible cultivars was negatively correlated with Pi. These data were used to predict the effect of resistant and susceptible potato cultivars on G. rostochiensis soil population dynamics.     

Impact of wild-type and genetically modified Pseudomonas fluorescens on soil enzyme activities and microbial population structure in the rhizosphere of pea

The aim of this study was to determine the impact of wild-type along with functionally and nonfunctionally modified Pseudomonas fluorescens strains in the rhizosphere. The wild-type F113 strain carried a gene encoding the production of the antibiotic 2,4-diacetylphloroglucinol (DAPG) useful in plant disease control, and was marked with a lacZY gene cassette. The first modified strain was a functional modification of strain F113 with repressed production of DAPG, creating the DAPG-negative strain F113 G22. The second paired comparison was a nonfunctional modification of wild-type (unmarked) strain SBW25, constructed to carry marker genes only, creating strain SBW25 EeZY-6KX. Significant perturbations were found in the indigenous bacterial population structure, with the F113 (DAPG+) strain causing a shift towards slower growing colonies (K strategists) compared with the nonantibiotic-producing derivative (F113 G22) and the SBW25 strains. The DAPG+ strain also significantly reduced, in comparison with the other inocula, the total Pseudomonas populations but did not affect the total microbial populations. The survival of F113 and F113 G22 were an order of magnitude lower than the SBW 25 strains. The DAPG+ strain caused a significant decrease in the shoot-to-root ratio in comparison to the control and other inoculants, indicating plant stress. F113 increased soil alkaline phosphatase, phosphodiesterase and aryl sulphatase activities compared to the other inocula, which themselves reduced the same enzyme activities compared to the control. In contrast to this, the β-glucosidase, β-galactosidase and N -acetyl glucosaminidase activities decreased with the inoculation of the DAPG+ strain. These results indicate that soil enzymes are sensitive to the impact of inoculation with genetically modified microorganisms (GMMs).     

Effect of introduced Pseudomonas fluorescens strains on the uptake of nitrogen by wheat from 15N-enriched organic residues

The effects of an antibiotic-producing Pseudomonas fluorescens strain (F113) carrying the marker gene cassette lacZY and a marked, non-producing strain (F113G22) on the uptake of nitrogen from ¹⁵N-enriched organic residues incorporated into a sandy soil were investigated in microcosm studies. Strain F113 produces the antibiotic 2,4-diacetylphloroglucinol (DAPG), whilst its modified derivative strain F113G22 has DAPG production deleted by Tn5 mutagenesis. Uptake of nitrogen by wheat (Triticum aestivum) from ¹⁵N-enriched organic residues was estimated using stable isotope-ratio mass spectrometry of shoot and root material of 17-day-old plants. In addition, plant growth and active microbial biomass in soil were monitored. In contrast to results obtained in our previous study on pea (Pisum sativum), it was found that in wheat, inoculation with either strain F113 or F113G22 decreased the proportion of nitrogen derived from ¹⁵N-labelled organic residues incorporated into soil as compared to non-inoculated controls. It is therefore suggested that these strains decreased mineralization of organic residues in the rhizosphere of wheat, making less inorganic N (¹⁵N) available for plant uptake. The results of this study indicate that the effects of introduced Pseudomonas fluorescens strains on nitrogen mineralization in the rhizosphere are plant-species dependent, and highlight the importance of testing microbial inocula on a range of plant species.     

Effect of genetically modified Pseudomonas fluorescens strains on the uptake of nitrogen by pea from 15N enriched organic residues

The effects of an antibiotic-producing Pseudomonas fluorescens strain (F113) carrying the marker gene cassette lac ZY and a marked, non-producing strain (F113G22) on the uptake of nitrogen from ¹⁵N enriched organic residues incorporated into a sandy soil were investigated in microcosm studies. Strain F113 produces the antibiotic 2,4-diacetylphloroglucinol (DAPG), while its modified derivative strain F113G22 has DAPG production deleted by Tn 5 mutagenesis. Uptake of nitrogen by pea ( Pisum sativum ) was estimated using isotope-ratio mass spectrometry. In addition, plant growth and microbial activity in soil were monitored. Both strains F113 and F113G22 enhanced the uptake of nitrogen from mineralized organic residues, even though the antibiotic producing strain F113 significantly reduced microbial activity in soil. It is suggested that the effect on nitrogen uptake was due to increased mineralization of organic residues by the introduced organisms, making greater quantities of inorganic nitrogen available for plant uptake. Unlike studies assessing impact in terms of perturbation to indigenous microbial communities, this study provides direct evidence of a change in ecosystem function as a result of the introduction of strains of a genetically marked bacterium, irrespective of whether its natural antibiotic-producing capacity has been genetically deleted.     

大豆胞囊线虫的休眠

以前研究发现,辽宁地区大豆生长期间及收获期土壤中胞囊孵出的二龄幼虫量很少,推测线虫卵的休眠与大豆生长时期或季节相关。为明确该地区大豆胞囊线虫的休眠特点,2002-2003年采用田间随机多点取样、室内分离及模拟自然条件孵化等方法对大豆胞囊线虫的休眠进行深入研究。结果表明:在生长季节,感病品种辽豆10根围土壤中的白色雌虫、卵囊及褐色的胞囊均可孵出二龄幼虫,且孵化持续时间较长,第21d仍有幼虫孵出,白色雌虫及卵囊内的卵孵化率高于褐色胞囊;不同作物对其根围土壤中胞囊内卵的孵化影响不大,寄主作物大豆、非寄主作物玉米根围及休闲地土壤中的胞囊在条件适宜均可孵出二龄幼虫;季节对胞囊内卵的孵化有较大的影响,出苗期孵化率最高,收获期最低,2周时平均1个胞囊孵出幼虫分别为83.8和9.7条;胞囊皮对线虫卵的孵化有显著的影响。表明沈阳地区大豆胞囊线虫在正常和逆境条件下均有部分卵表现休眠。     

Naturally induced secretions of the potato cyst nematode co-stimulate the proliferation of both tobacco leaf protoplasts and human peripheral blood mononuclear cells.

Naturally induced secretions from infective juveniles of the potato cyst nematode Globodera rostochiensis co-stimulate the proliferation of tobacco leaf protoplasts in the presence of the synthetic phytohormones alpha-naphthaleneacetic acid (NAA) and 6-benzylaminopurine (BAP). With the use of a protoplast-based bioassay, a low-molecular-weight peptide(s) (< 3 kDa) was shown to be responsible for the observed effect. This mitogenic oligopeptide(s) is functionally dissimilar to auxin and cytokinin and, in addition, it does not change the sensitivity of the protoplasts toward these phytohormones. In combination with the mitogen phytohemagglutinin (PHA), cyst nematode secretions also co-stimulated mitogenesis in human peripheral blood mononuclear cells (PBMC). The stimulation of plant cells isolated from nontarget tissue--these nematodes normally invade the roots of potato plants--suggests the activation of a general signal transduction mechanism(s) by an oligopeptide(s) secreted by the nematode. Whether a similar oligopeptide-induced mechanism underlies human PBMC activation remains to be investigated. Reactivation of the cell cycle is a crucial event in feeding cell formation by cyst nematodes. The secretion of a mitogenic low-molecular-weight peptide(s) by infective juveniles of the potato cyst nematode could contribute to the redifferentiation of plant cells into such a feeding cell.     

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.     

Analysis of the sensory responses of parasitic nematodes using electrophysiology

Use of the electrophysiological technique to examine the sensory perception of live, intact nematodes has provided detailed analysis of responses to known concentrations of test chemicals. The use of larger nematodes, such as the animal-parasite Syngamus trachea, enabled direct extracellular recordings from individual sensilla; with smaller nematodes, the recording electrode was inserted close to the cephalic region. Extracellular recordings from the cephalic region of second-stage juveniles and males of the potato cyst nematodes, Globodera rostochiensis and Globodera pallida, were obtained after exposure to a variety of semiochemicals, including sex pheromones and certain putative phagostimulatory compounds. The responses of adult females of the animal-parasitic nematode, Brugia pahangi, to some possible host cues, and the inhibition by ivermectin of the response to a known allelochemical were investigated. Exposure to acetylcholine was used to compare the concentration-dependent responses of second-stage juveniles of G. rostochiensis and adult females of B. pahangi and the insect-parasitic nematode Leidynema appendiculata. Use of a perfusion system enabled sequential exposure of individual nematodes to different test chemicals or to different concentrations of the same chemical. Incubating second-stage juveniles of G. rostochiensis for 24 h in a mAb showing specificity to amphidial secretions resulted in blocking of the normal response to host root diffusates. The potential of the electrophysiology technique for analysing perturbation of sensory perception is discussed.     

Impact on Arbuscular Mycorrhiza Formation of Pseudomonas Strains Used as Inoculants for Biocontrol of Soil-Borne Fungal Plant Pathogens

The arbuscular mycorrhizal symbiosis, a key component of agroecosystems, was assayed as a rhizosphere biosensor for evaluation of the impact of certain antifungal Pseudomonas inoculants used to control soil-borne plant pathogens. The following three Pseudomonas strains were tested: wild-type strain F113, which produces the antifungal compound 2,4-diacetylphloroglucinol (DAPG); strain F113G22, a DAPG-negative mutant of F113; and strain F113(pCU203), a DAPG overproducer. Wild-type strain F113 and mutant strain F113G22 stimulated both mycelial development from Glomus mosseae spores germinating in soil and tomato root colonization. Strain F113(pCU203) did not adversely affect G. mosseae performance. Mycelial development, but not spore germination, is sensitive to 10 μM DAPG, a concentration that might be present in the rhizosphere. The results of scanning electron and confocal microscopy demonstrated that strain F113 and its derivatives adhered to G. mosseae spores independent of the ability to produce DAPG.