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In agricultural ecosystems, most crop cultivars are bred for pest resistance with the aim of resisting aboveground herbivores, but it is unclear if this resistance also affects abundance, composition, and activity of underground pests and content of soil labile pools. A fully factorial greenhouse experiment was carried out, using four rice cultivars with distinct resistance to shoot herbivore brown planthopper, Nilaparvata lugens, with and without rice root nematodes, Hirschmanniella spp., to test two hypotheses: (1) rice cultivars resistant to brown planthopper are more susceptible to rice root nematodes and (2) rice root nematodes of resistant cultivars cause greater modification of labile pools of rhizosphere soil than those of susceptible cultivars by inducing more leakage of plant metabolites. Nematode abundance, soluble sugar and soluble amino acid contents of plant, and microbial biomass C and N, dissolved organic C and N, and soluble sugar and amino acid contents of rhizosphere soil were analyzed. The abundance of rice root nematodes substantially increased under all rice cultivars following root nematode inoculation. Rice cultivar resistance to brown planthopper did not affect the numbers of rice root nematodes but decreased negative effects of nematodes on root growth, shoot soluble sugar and amino acid contents, and labile pools of rhizosphere soil, probably implying that integrated complex mechanisms other than only resource allocation were involved in rice resistance to aboveground and belowground herbivores. Our study will contribute to a better understanding of aboveground-belowground interactions and to the development of integrated pest managements by considering belowground ecosystem responses in crop systems. [ABSTRACT FROM AUTHOR]

Abstract: The combined toxicity of four heavy metals (copper, zinc, cadmium and chromium) to the nematode Caenorhabditis elegans was determined by using feeding as an endpoint. Six equivalent-effect concentration ratio (EECR) mixtures and six uniform design concentration ratio (UDCR) mixtures were designed to fully explore the combined toxicities of these heavy metals. Observed toxicities were compared with predictions calculated by two basic models, concentration addition (CA) and independent action (IA). All the concentration–response relationships of the mixtures can be well characterized and described by the Weibull function. CA provided a relatively better prediction for the mix-toxicity of the four heavy metals, which share a similar mode of action on the feeding of C. elegans, although the prediction calculated by IA was also reliable, from the viewpoint of model prediction. [Copyright &y& Elsevier]

Earthworms naturally secrete substantial amounts of mucus (relative to their body size) from the cuticle when moving through the soil. However, how the metabolites in the epidermally derived mucus affect soil microfauna is largely unknown. The objective of this study was to determine how earthworm mucus affected the physiology of the model nematode Caenorhabditis elegans and that of two soil-dwelling bacterial-feeding nematodes: Mesorhabditis sp. and Protorhabditis sp. The traits examined included phobotaxis, reproduction, mortality, body size, feeding inhibition rate, and feeding preference. Two nematode species escaped from the earthworm mucus, whereas earthworm mucus reduced the reproduction of C. elegans and increased the mortality of C. elegans and Mesorhabditis sp. Moreover, earthworm mucus reduced the feeding rate of C. elegans and Protorhabditis sp. and changed the feeding preferences of all three species of nematodes. At 48 h, earthworm mucus increased feeding on Bacillus amyloliquefaciens JX1 for C. elegans and Protorhabditis sp. and reduced feeding on Escherichia coli OP50 for Mesorhabditis sp. These results suggest that earthworm mucus can be detrimental to bacterial-feeding nematodes and provide insights into the non-trophic interactions between earthworms and other soil biota. • Two of the three bacterial-feeding nematodes escaped from earthworm mucus. • Earthworm mucus reduced the reproduction of C. elegans. • Earthworm mucus increased the mortality of the C. elegans and Mesorhabditis sp. • Earthworm mucus inhibited the feeding rate of C. elegans and Protorhabditis sp and changed the feeding preferences of all the bacterial-feeding nematodes. [ABSTRACT FROM AUTHOR]

Bacterivorous nematodes are one of the dominant microbial organisms and play an important role in shaping the bacterial community and hormone dynamics in rhizosphere soils. It has been found that the presence of bacterivorous nematodes potentially regulate indole-3-acetic acid (IAA) content in soils by interacting with the status of IAA-producing bacteria, and eventually act on the growth of plant roots. However, the mechanism behind this interaction in broad soil ecosystems remains unknown. This study reveals the interaction between bacterivorous nematodes and the soil bacterial community, especially IAA-producing bacteria, based on denaturing gradient gel electrophoresis (DGGE) analysis. The results of DGGE showed that the bacterial diversity in soil was increased by inoculation of nematodes during certain periods of plant culture, and was also affected by the genotype of the Arabidopsis thaliana (Johannes Thal, 1577) plants. The variation in soil bacterial communities among the treatments with different genotypes of A. thaliana and nematodes indicated that the soil bacterial community structure was affected by both genotype and nematode grazing. Furthermore, we measured the root architectures of four genotypes of A. thaliana , including wild type (Col-0), two IAA-insensitive mutants (Axr5 and Axr3–1), and DR5::GUS transgenic type (IAA sensitive) under different conditions of bacterivorous nematode inoculation. We found that the root architecture of the IAA-sensitive A. thaliana developed more tips and slender roots under inoculation with activated bacterivorous nematodes, while the root architectures of auxin-insensitive A. thaliana mutants showed no significant differences between alive and inactivated bacterivorous nematodes, even though the IAA content increased in the soil under inoculation with alive nematodes. This study confirms that the presence of bacterivorous nematodes in soils increases the soil IAA content significantly by regulating the bacterial community structure, which leads to an accumulation of auxin in the root tips of A. thaliana , and eventually enhances the plant root growth. [ABSTRACT FROM AUTHOR]

Abstract: Microcosm experiments were carried out to study the effects of bacterial-feeding nematodes and prometryne on soil bacterial communities in contaminated soil. Prometryne (5 or 10mgkg−1 dry soil, that is, P5 or P10) and bacterial-feeding nematodes (5 or 10 individuals g−1 dry soil, that is, N5 or N10), singly and in combination (P5N5, P5N10, P10N5, P10N10), were added to a nematode-free soil. An uncontaminated nematode-free soil was studied for comparison (Control). Bacterial-feeding nematode grazing boosted soil enzyme activities in contaminated soils, thus speeding up prometryne degradation. In the initial stage of the experiment, prometryne enhanced the soil enzyme activities too, but served the opposite purpose later. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that prometryne contamination and nematode grazing over the incubation period exerted an obvious impact on Species richness (S), Shannon–Wiener index (H′) and Evenness (E H ) of soil bacteria, which increased initially, then decreased and increased again later. The cluster analysis of DGGE profiles showed that the similarity of soil bacterial communities in all treatments with indigenous microbes, P5, P5N5, P5N10, P10, P10N5, and P10N10 and the Control was 75%, 44%, 78% and 49% at Day 0, Day 8, Day 18 and Day 30, respectively. Compared to the Control, DGGE profiles displayed a varying characteristic bands pattern in all treatments over the incubation period with certain bands present in the treatments while not in the Control and vice versa, suggesting that bacterial-feeding nematode grazing and prometryne contamination affected soil bacterial communities evidently. Consequently, when added to contaminated soil, bacterial-feeding nematodes can contribute to restoration of contaminated sites by degrading toxic compounds like prometryne through enhanced microbial activity. [Copyright &y& Elsevier]

Abstract: The effects of bacterial-feeding nematodes on nitrification and the ammonia-oxidizing bacteria (AOB) community composition were studied in soil microcosms. Sterilized soils were inoculated with mixed soil bacteria (obtained by filtering) or with bacteria and bacterial-feeding nematodes, after which the dynamic inorganic nitrogen concentration was measured weekly. After 28 days of incubation, denaturing gradient gel electrophoresis (DGGE) based on PCR amplification of the amoA gene was used to analyze the AOB community composition. In addition, a clone library from the amoA gene fragments was established using clones randomly selected and sequenced from the two treatments. The results showed that the presence of bacterial-feeding nematodes led to significantly greater NH4 + and NO3 − contents over the entire incubation period, indicating that bacterial-feeding nematodes promoted both N mineralization and nitrification. The results of DGGE showed that the AOB community composition was significantly changed in the presence of bacterial-feeding nematodes. Furthermore, the sequencing results suggested that Nitrosospira sp. was the dominant species in the treatment without nematodes, while Nitrosomonas sp. and Nitrosospira sp. were the dominant species in the treatment with nematodes. Such changes in the AOB community may be one of explanation of the important role that nematodes play in promoting nitrification. [Copyright &y& Elsevier]