Predacity by nematophagous fungi and its relation to the attraction of nematodes

Isolation, Identification, and Characterization of the Nematophagous Fungus Arthrobotrys oligospora from Kyrgyzstan

PURPOSE

Predatory fungi have been the subject of fundamental studies and their potential as biological control agents against parasitic plant nematodes has been assessed. The aim of the present study was to isolate and identify predatory fungi, performing in vitro and in vivo screening to select highly active strains to control parasitic nematodes.

METHODS

Different nutrient media were used to isolate predatory fungi and determine their morphological and cultural properties. Identification was performed by classical and molecular biology methods. In vitro and in vivo screening was conducted to select highly active strains.

RESULTS

Twelve isolates of Arthrobotrys oligospora (Orbiliomycetes) found in nature were investigated for their predaceous efficacy against garlic stem nematodes (Ditylenchus dipsaci). The effect of temperature and pH on the growth rate and trap formation of representative isolates was determined and isolates were characterized by light microscopy and molecular markers. BLAST was used to sequence the rDNA internal transcribed spacer of A. oligospora isolate KTMU-7. The optimum growth of A. oligospora strains was achieved at 20-25 °C on 1-2% corn meal agar (CMA) within the pH range of 5.6-8.6. The factors responsible for the trap formation of these fungal strains were identified. In vitro and in vivo experiments were performed to evaluate the nematicidal activity of local predatory fungal isolates against soil nematodes.

CONCLUSIONS

Preliminary studies proved A. oligospora to be a potentially effective biological control agent, immobilizing 85.7 ± 2.19% of garlic stem nematodes in soil from the rhizosphere of potato plants.

Fungi-Nematode Interactions: Diversity, Ecology, and Biocontrol Prospects in Agriculture

Fungi and nematodes are among the most abundant organisms in soil habitats. They provide essential ecosystem services and play crucial roles for maintaining the stability of food-webs and for facilitating nutrient cycling. As two of the very abundant groups of organisms, fungi and nematodes interact with each other in multiple ways. Here in this review, we provide a broad framework of interactions between fungi and nematodes with an emphasis on those that impact crops and agriculture ecosystems. We describe the diversity and evolution of fungi that closely interact with nematodes, including food fungi for nematodes as well as fungi that feed on nematodes. Among the nematophagous fungi, those that produce specialized nematode-trapping devices are especially interesting, and a great deal is known about their diversity, evolution, and molecular mechanisms of interactions with nematodes. Some of the fungi and nematodes are significant pathogens and pests to crops. We summarize the ecological and molecular mechanisms identified so far that impact, either directly or indirectly, the interactions among phytopathogenic fungi, phytopathogenic nematodes, and crop plants. The potential applications of our understanding to controlling phytophagous nematodes and soilborne fungal pathogens in agricultural fields are discussed.

Predator-prey interactions of nematode-trapping fungi and nematodes: both sides of the coin

Nematode-trapping fungi develop complex trapping devices to capture and consume nematodes. The dynamics of these organisms is especially important given the pathogenicity of nematodes and, consequently, the potential application of nematode-trapping fungi as biocontrol agents. Furthermore, both the nematodes and nematode-trapping fungi can be easily grown in laboratories, making them a unique manipulatable predator-prey system to study their coevolution. Several different aspects of these fungi have been studied, such as their genetics and the different factors triggering trap formation. In this review, we use the nematode-trapping fungus Arthrobotrys oligospora (which forms adhesive nets) as a model to describe the trapping process. We divide this process into several stages; namely attraction, recognition, trap formation, adhesion, penetration, and digestion. We summarize the latest findings in the field and current knowledge on the interactions between nematodes and nematode-trapping fungi, representing both sides of the predator-prey interaction.

Multitrophic Effects of Belowground Parasitoid Learning

The ability to learn allows organisms to take advantage of dynamic and ephemeral opportunities in their environment. Here we show that learning in belowground entomopathogenic nematodes has cascading multitrophic effects on their hosts, other nematodes, and nematophagous fungal predators. In addition to quantifying these effects, we show that social behavioral plasticity in these belowground parasitoids can amplify signaling by plant defense pathways and results in an almost doubling of insect herbivore infection by entomopathogenic nematodes. Cumulatively, these effects point to the critical role of plant signaling in regulating community structure while suggesting an equally important role for behavioral plasticity in shaping community dynamics.

Substrate modulation, group effects and the behavioral responses of entomopathogenic nematodes to nematophagous fungi

Laboratory experiments were conducted on the behavioral responses of five species of entomopathogenic nematodes (EPNs; Steinernema diaprepesi, Steinernema sp. glaseri-group, Steinernema riobrave, Heterorhabditis zealandica, Heterorhabditis indica) to three species of nematophagous fungi (NF; trapping fungus Arthrobotrys gephyropaga; endoparasites Myzocytium sp., Catenaria sp.). We hypothesized that EPN responses to NF and their putative semiochemicals might reflect the relative susceptibility of EPNs to particular NF species. EPN responses to "activated" NF (i.e., induced to form traps or sporangia by previous interactions with nematodes) versus controls of non-activated NF or heat-killed EPNs were compared in choice experiments on water agar in Petri dishes (dia=9 cm) and in horizontal sand columns (8 cm L×2.7 cm dia). On agar, all EPN species were attracted to all activated NF species except for S. riobrave, which was neutral. In sand, all EPN species were repelled by activated Arthrobotrys but attracted to activated Myzocytium and Catenaria, except H. indica (neutral to Myzocytium) and Steinernema sp. (neutral to Catenaria). EPN behavioral responses appeared unrelated to relative susceptibility to NF except that H. indica exhibited low susceptibility and a neutral response to Myzocytium in sand whereas the remaining EPNs were highly susceptible and attracted. These results indicate potential complexity (i.e., mixed responses, aggregation or group movement) and species specificity in the responses of EPNs to NF, demonstrate that results on agar can differ markedly from those in sand, and underline the potential importance of utilizing natural substrates to properly assess the role of semiochemicals in nematode-fungus interactions.

Effects of transgenic hybrid aspen overexpressing polyphenol oxidase on rhizosphere diversity

This study assessed the potential effects of transgenic aspen overexpressing a polyphenol oxidase gene on diversity in rhizosphere communities. Cultivation-independent methods were used to better delineate bacterial and fungal populations associated with transgenic and nontransgenic trees. Gene libraries for the bacterial component of the rhizosphere were established using 16S rRNA and chaperonin-60 (CPN-60) gene sequences, while the fungal community was characterized using 18S rRNA gene sequences. The 16S rRNA gene libraries were dominated by alphaproteobacterial sequences, while the CPN-60 gene libraries were dominated by members of the Bacteroidetes/Chlorobi group. In both the CPN-60 and 16S rRNA libraries, there were differences in only minor components of the bacterial community between transgenic and unmodified trees, and no significant differences in species diversity were observed. Compared to the bacterial gene libraries, greater coverage of the underlying population was achieved with the fungal 18S rRNA libraries. Members of the Zygomycota, Chytridiomycota, Ascomycota, and Basidiomycota were recovered from both libraries. The dominant groups of fungi associated with each tree type were very similar, although there were some qualitative differences in the recovery of less-abundant fungi, likely as a result of the underlying heterogeneity of the fungal population. The methods employed revealed only minor differences between the bacterial and fungal communities associated with transgenic and unmodified trees.

Interactions Among a Soil Organic Amendment, Nematodes, and the Nematode-Trapping Fungus Dactylellina candidum

ABSTRACT When alfalfa leaves (Medicago sativa) are added to soil, both the nematode-trapping fungus Dactylellina candidum and microbivorous nematodes increase. To determine whether the response of the fungus to alfalfa depends on consumption of bacterivorous and fungivorous nematodes, soil microcosm experiments were performed. D. candidum did not increase if alfalfa leaves were added to soil lacking nematodes, but did increase if nematodes were added to soil lacking alfalfa leaves. Although these results indicate that the response of D. candidum to alfalfa depends on nematodes, D. candidum and microbivorous nematodes did not exhibit classical predator-prey dynamics (i.e., D. candidum seldom reduced numbers of resident nematodes and, after initially increasing in alfalfa amended soil, numbers of D. candidum then decreased, whereas numbers of nematodes continued to increase). Fungivorous nematodes were abun dant in alfalfa-amended soil, and their potential to suppress trapping fungi requires more research.

Correlations between most probable number and activity of nematode-trapping fungi

ABSTRACT Soil cages were used to determine whether nematode-trapping fungi population density, as measured by most probable number (MPN) procedures, was correlated with the trapping of nematodes. Fungi studied (and trap type) were Arthrobotrys oligospora (adhesive networks), A. eudermata (adhesive networks), A. dactyloides (constricting rings), Dactylellina ellipsospora (adhesive knobs), and D. haptotyla (adhesive knobs). The fungi were formulated as assimilative hyphae in dried alginate pellets. Pellets were added to field soil, the soil was packed into 80-cm(3) cages (PVC pipe, 3.0 cm long and 3.9 cm in diameter), and the cages were buried in vineyards. After 14 to 61 days, the cages were recovered, and MPN data and trapping activity were determined. For all five fungi, MPN data were correlated with the number of pellets added. Regardless of fungus population density, A. oligospora and A. eudermata trapped few if any nematodes in soil, and consequently, trapping and fungus population density were not correlated. The correlation between population density and trapping was weak for A. dactyloides but relatively strong for D. ellipsospora and D. haptotyla. High levels of trapping by the latter two fungi required more than 10(2) fungus propagules per gram of soil.

Predacious activity of the nematode-destroying fungus Arthrobotrys oligospora in dependence of the medium composition

The nematophagous fungus Arthrobotrys oligospora Fres. can live saprophytically as well as predatorily. As a predator it forms sticky reticulate traps in the presence of living nematodes which enable it to kill and consume the animals. In laboratory experiments the nutrient acquisition of the fungus was tested on agar media with various N- and C-sources in different concentrations. The intensity of predacious response was determined by counting the traps formed on the agar surface after induction by nematodes. Predacious activity (PA) served as a unit of the strength of nematophagous response. PA was defined as the number of traps/100 mm2. In contrast to carnivorous higher plants, which consume insects in order to compensate their N-deficit, the fungus seems to kill nematodes to get both, N as well as C. If the agar substrate contains a certain concentration of N and C, the fungus does not form traps and lives as a saprophyte. The level which determines the switch towards pure saprophytism under laboratory conditions ranges around 0.12 M C at a N-concentration of 0.05 M. If no nitrogen is added to the medium the fungus forms nematode-induced traps even in the most concentrated C-media. On N-media without any carbon source the fungus shows a much stronger PA compared with N-free media. Furthermore, more chlamydospores were formed on N-media.

The potential of nematophagous fungi to control the free-living stages of nematode parasites of sheep: screening studies

Ninety-four species of fungi with known nematophagous activity were tested for their ability to reduce the number of infective larvae of sheep nematodes in faecal cultures, and also for their ability to produce nematode-attractant and nematocidal substances against these free-living stages under in vitro conditions. Reductions of infective larval numbers exceeding 80% were consistently recorded when 100-250 conidia g-1 faeces of various species from the genera Arthrobotrys, Geniculifera and Monacrosporium were used. Even concentrations as low as 10 conidia g-1 faeces resulted in a significant reduction in infective larval numbers compared to control cultures. This study demonstrates that whilst many fungal species exhibit nematophagous activity against a variety of free-living nematodes, few show efficient activity against the free-living stages of parasitic nematodes in the sheep faecal environment. For the most active, there were six species of Arthrobotrys, two species of Geniculifera and two species of Monacrosporium which showed comparable activity to the extensively studied species A. oligospora.

Quantification of predatory and endoparasitic nematophagous fungi in soil

Methods were developed to quantify predatory and endoparasitic fungi in soil. The methods were based on previously developed detection techniques and combined with a most probable number estimation. The methods were applied to an agricultural soil fertilized with farmyard manure. Large amounts of farmyard manure resulted in increased amounts of organic matter, numbers of propagules of predatory and endoparasitic fungi, and numbers of bacteria and nematodes.