The impact of fluazaindolizine on free-living nematodes and the nematode community structure in a root-knot nematode infested vegetable production system.

BACKGROUND: Fluazaindolizine is a novel sulfonamide nematicide that is currently under commercialization in various countries. Four trials (two in tomato, two in zucchini) were carried out over two growing seasons in a root-knot nematode (RKN; Meloidogyne incognita)-infested plastic house to test the effects of a single application of fluazaindolizine at 1000 and 2000 g ha-1 , on RKN, the resident nematode community and to estimate any side effects on the soil food web and soil ecological functions. The composition of the nematode community was evaluated at three sampling times, pre-treatment (before soil preparation), at planting (after nematicide application) and at harvest. Nematode abundance and nematode-based ecological indices, such as maturity indices, soil food web indices (Structure, Basal, Enrichment index) and metabolic footprints were calculated and compared between the untreated and nematicide treatments at various sampling times. RESULTS: In both crops the test rates of fluazaindolizine significantly reduced both the soil numbers of RKN as well as the observed root galling at harvest. Fluazaindolizine treatments showed a low impact on the numbers of other plant-parasitic nematodes, and at harvest the numbers of some ectoparasites as Telotylenchus even showed higher values within the nematicide-treated plots than in the untreated control. Overall, fluazaindolizine treatments neither decreased the bacterivorous, fungivorous or omnivorous nematode densities, nor reduced the maturity and soil food web indices. However, we observed some slight reductions in the structure index as well as increases in the basal index that might indicate some adverse impact on soil functions resulting from the nematicide application. Still those impacts were considered small compared to the impact of the soil preparation. CONCLUSIONS: Overall, fluazaindolizine showed a good level of selectivity towards RKN and therefore could become a useful tool within integrated nematode management approaches that also complements soil health and maintaining diversity in soil.

Suitability of Zucchini and Cucumber Genotypes to Populations of Meloidogyne arenaria, M. incognita, and M. javanica.

The host suitability of five zucchini and three cucumber genotypes to Meloidogyne incognita (MiPM26) and M. javanica (Mj05) was determined in pot experiments in a greenhouse. The number of egg masses (EM) did not differ among the genotypes of zucchini or cucumber, but the eggs/plant and reproduction factor (Rf) did slightly. M. incognita MiPM26 showed lower EM, eggs/plant, and Rf than M. javanica Mj05. Examination of the zucchini galls for nematode postinfection development revealed unsuitable conditions for M. incognita MiPM26 as only 22% of the females produced EM compared to 95% of the M. javanica females. As far as cucumber was concerned, 86% of the M. incognita and 99% of the M. javanica females produced EM, respectively. In a second type of experiments, several populations of M. arenaria, M. incognita, and M. javanica were tested on zucchini cv. Amalthee and cucumber cv. Dasher II to assess the parasitic variation among species and populations of Meloidogyne. A greater parasitic variation was observed in zucchini than cucumber. Zucchini responded as a poor host for M. incognita MiPM26, MiAL09, and MiAL48, but as a good host for MiAL10 and MiAL15. Intraspecific variation was not observed among the M. javanica or M. arenaria populations. Cucumber was a good host for all the tested populations. Overall, both cucurbits were suitable hosts for Meloidogyne but zucchini was a poorer host than the cucumber.

Experimental validation of Haldane's hypothesis on the role of infection as an evolutionary force for Metazoans.

A common drawback in evolutionary science is the fact that the evolution of organisms occurs in geological timing, completely out of the time scale of laboratory experimental work. For this reason, some relevant hypotheses on evolution of Metazoans are based on correlations more than on experimental data obtained for testing the robustness of those hypotheses. In the current work, we implement an experimental methodology to analyze the role of infections as a driving force in the evolution of Metazoans (Haldane's hypothesis). To that goal, we have used simple models of virulence with short reproduction times, large populations, and that are easily testable in the laboratory. Using the bacteriovirus nematode Caenorhabditis elegans as a model organism under evolution and their infection by the environmental opportunistic bacterial pathogen Pseudomonas aeruginosa as the selective force, we have demonstrated that bacterial infection selects an evolved nematode lineage resistant to infection, with changes in its respiration and capability of consuming novel food resources. Using an experimental approach, we show that infection is a selective force in the evolution of Metazoans as proposed earlier by Haldane.

Hydrodynamic characterization of the DEAD-box RNA helicase DbpA.

The Escherichia coli DEAD-box protein A (DbpA) belongs to the highly conserved superfamily-II of nucleic acid helicases that play key roles in RNA metabolism. A central question regarding helicase activity is whether the process of coupling ATP hydrolysis to nucleic acid unwinding requires an oligomeric form of the enzyme. We have investigated the structural and functional properties of DbpA by multi-angle laser light-scattering, size-exclusion chromatography, analytical ultracentrifugation, chemical cross-linking and hydrodynamic modeling. DbpA is monomeric in solution up to a concentration of 25 microM and over the temperature range of 4 degrees C to 22 degrees C. Binding of neither nucleotide (ATP or ADP) nor peptidyl transferase center (PTC) RNA, the presumed physiological RNA substrate, favor oligomerization. The hydrodynamic parameters were used together with hydrodynamic bead modeling and structural homology in conjunction with ab initio structure prediction methods to define plausible shapes of DbpA. Collectively, the results favor models where DbpA functions as an active monomer that possesses two distinct RNA binding sites, one in the helicase core domain and the other in the carboxyl-terminal domain that recognizes 23S rRNA and interacts specifically with hairpin 92 of the PTC.

Effects of DL-methionine on hatching and activity of Meloidogyne incognita eggs and juveniles.

Several concentrations of DL-methionine were tested in the laboratory for their effects on Meloidogyne incognita (Kofoid & White) Chitwood egg hatching and juvenile activity in aqueous suspensions and infested soil. After 7 days in methionine solutions, the proportions of hatched eggs were reduced by 23.3% at 0.25 mg litre(-1) methionine and by 76.4% at 25 g litre(-1) when compared with controls in tap water. An effect of methionine solutions on juvenile activity was also apparent after 24 h in the 25 g litre(-1) treatment, where the percentage of active M incognita juveniles was reduced by 16.3%. Further reductions in nematode mobility were observed as the time of exposure increased, and at lower methionine concentrations over longer exposure times. When methionine solutions were applied to soil infested with M incognita, reductions in egg hatching and juvenile activity were observed at 0.1 and 1 mg methionine g(-1) soil in both sand and clay-loam soils. The percentage of hatched eggs one week after methionine application at rates of 1 mg g(-1) was lower in sand (5.6%) and clay-loam soil (20.8%) than in controls where egg hatch was 52.0% and 48.0% in sand and soil, respectively. Non-active juveniles were found at 1 mg methionine g(-1) soil one week after its application to soil.

Equilibrium and kinetic analysis of nucleotide binding to the DEAD-box RNA helicase DbpA.

The Escherichia coli DEAD-box protein A (DbpA) is an RNA helicase that utilizes the energy from ATP binding and hydrolysis to facilitate structural rearrangements of rRNA. We have used the fluorescent nucleotide analogues, mantADP and mantATP, to measure the equilibrium binding affinity and kinetic mechanism of nucleotide binding to DbpA in the absence of RNA. Binding generates an enhancement in mant-nucleotide fluorescence and a corresponding reduction in intrinsic DbpA fluorescence, consistent with fluorescence resonance energy transfer (FRET) from DbpA tryptophan(s) to bound nucleotides. Fluorescent modification does not significantly interfere with the affinities and kinetics of nucleotide binding. Different energy transfer efficiencies between DbpA-mantATP and DbpA-mantADP complexes suggest that DbpA adopts nucleotide-dependent conformations. ADP binds (K(d) approximately 50 microM at 22 degrees C) 4-7 times more tightly than ATP (K(d) approximately 400 microM at 22 degrees C). Both nucleotides bind with relatively temperature-independent association rate constants (approximately 1-3 microM(-1) s(-1)) that are much lower than predicted for a diffusion-limited reaction. Differences in the binding affinities are dictated primarily by the dissociation rate constants. ADP binding occurs with a positive change in the heat capacity, presumably reflecting a nucleotide-induced conformational rearrangement of DbpA. At low temperatures (<22 degrees C), the binding free energies are dominated by favorable enthalpic and unfavorable entropic contributions. At physiological temperatures (>22 degrees C), ADP binding occurs with positive entropy changes. We favor a mechanism in which ADP binding increases the conformational flexibility and dynamics of DbpA.