Nematode-trapping fungi in organic and conventional cropping systems

Endosymbiotic bacteria within the nematode-trapping fungus Arthrobotrys musiformis and their potential roles in nitrogen cycling

Endosymbiotic bacteria (ESB) have important effects on their hosts, contributing to its growth, reproduction and biological functions. Although the effects of exogenous bacteria on the trap formation of nematode-trapping fungi (NTF) have been revealed, the effects of ESB on NTF remain unknown. In this study, we investigated the species diversity of ESB in the NTF Arthrobotrys musiformis using high-throughput sequencing and culture-dependent approaches, and compared bacterial profiles to assess the effects of strain source and culture media on A. musiformis. PICRUSt2 and FAPROTAX were used to predict bacterial function. Our study revealed that bacterial communities in A. musiformis displayed high diversity and heterogeneity, with Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria as the dominant phyla. The ESB between A. musiformis groups isolated from different habitats and cultured in the same medium were more similar to each other than the other groups isolated from the same habitat but cultured in different media. Function analysis predicted a broad and diverse functional repertoire of ESB in A. musiformis, and unveiled that ESB have the potential to function in five modules of the nitrogen metabolism. We isolated nitrogen-fixing and denitrifying bacteria from the ESB and demonstrated their effects on trap formation of A. musiformis. Among seven bacteria that we tested, three bacterial species Bacillus licheniformis, Achromobacter xylosoxidans and Stenotrophomonas maltophilia were found to be efficient in inducing trap formation. In conclusion, this study revealed extensive ESB diversity within NTF and demonstrated that these bacteria likely play important roles in nitrogen cycling, including nematode trap formation.

Drechslerelladaliensis and D.xiaguanensis (Orbiliales, Orbiliaceae), two new nematode-trapping fungi from Yunnan, China

Background

Nematode-trapping fungi are a highly specialised group in fungi and are essential regulators of natural nematode populations. At present, more than 130 species have been discovered in Zygomycota (Zoopagaceae), Basidiomycota (Nematoctonus), and Ascomycota (Orbiliaceae). Amongst them, nematode-trapping fungi in Orbiliaceae have become the research focus of carnivorous fungi due to their abundant species. During the investigation of carnivorous fungi in Yunnan, China, four fungal strains isolated from burned forest soil were identified as two new nematode-trapping species in Drechslerella (Orbiliaceae), based on multigene phylogenetic analysis and morphological characters.

New information

Drechslerelladaliensis sp. nov. is characterised by its ellipsoid, 1-2-septate macroconidia, clavate or bottle-shaped, 0-1-septate microconidia and unbranched, simple conidiophores. D.xiaguanensis sp. nov. is characterised by fusiform or spindle-shaped, 2-4-septate conidia and unbranched, simple conidiophores. Both of them produce constricting rings to capture nematodes. The phylogenetic analysis, based on combined ITS, TEF1-α and RPB2 sequences, determined their placement in Drechslerella. D.daliensis forms a basal lineage closely nested with D.hainanensis (YMF1.03993). D.xiaguanensis forms a sister lineage with D.bembicodes (1.01429), D.aphrobrocha (YMF1.00119) and D.coelobrocha (FWY03-25-1).

New Arthrobotrys Nematode-Trapping Species (Orbiliaceae) from Terrestrial Soils and Freshwater Sediments in China

Arthrobotrys is the most complex genus of Orbiliaceae nematode-trapping fungi. Its members are widely distributed in various habitats worldwide due to their unique nematode-trapping survival strategies. During a survey of nematophagous fungi in Yunnan Province, China, twelve taxa were isolated from terrestrial soil and freshwater sediment habitats and were identified as six new species in Arthrobotrys based on evidence from morphological and multigene (ITS, TEF, and RPB2) phylogenetic analyses. These new species i.e., Arthrobotrys eryuanensis, A. jinpingensis, A. lanpingensis, A. luquanensis, A. shuifuensis, and A. zhaoyangensis are named in recognition of their places of origin. Morphological descriptions, illustrations, taxonomic notes, and a multilocus phylogenetic analysis are provided for all new taxa. In addition, a key to known species in Arthrobotrys is provided, and the inadequacies in the taxonomic study of nematode-trapping fungi are also discussed.

Fungal communities associated with Heterodera glycines and their potential in biological control: a current update

The soybean cyst nematode (SCN) is the most important pest on soybean, a major crop worldwide. The SCN is considered both parasitic and pathogenic as it derives nutrition from the host and manipulates host physiology to do so. Currently, there are no commercially available chemicals that are specific, environmentally safe and cost effective to control SCN levels. Crop rotation, use of host resistance and other cultural practices remain the main management strategies. The need for bioprospecting other methods of controlling SCN is paramount, and fungi show promise in that respect. Several studies have evaluated fungi and fungal products as biocontrol options against plant-parasitic nematodes. This review discusses fungal genera isolated from the SCN with potential for use as biocontrol agents and the effects of their secondary metabolites on various stages of SCN development. The review also summarizes efforts to control SCN using soil amendments that could potentially impact fungal communities in the soil.

The soybean cyst nematode (SCN) is the most important pest on soybean, a major crop worldwide. The SCN is considered both parasitic and pathogenic as it derives nutrition from the host and manipulates host physiology to do so. Currently, there are no commercially available chemicals that are specific, environmentally safe and cost effective to control SCN levels. Crop rotation, use of host resistance and other cultural practices remain the main management strategies. The need for bioprospecting other methods of controlling SCN is paramount, and fungi show promise in that respect. Several studies have evaluated fungi and fungal products as biocontrol options against plant-parasitic nematodes. This review discusses fungal genera isolated from the SCN with potential for use as biocontrol agents and the effects of their secondary metabolites on various stages of SCN development. The review also summarizes efforts to control SCN using soil amendments that could potentially impact fungal communities in the soil.

Sensitivity of Nematode Community Analysis to Agricultural Management Practices and Inoculation with Local Effective Microorganisms in the Southeastern United States

In order to ensure a soil system’s sustained ability to carry out ecosystem services, indicators that assess soil health are needed. We examined the capacity of nematode maturity index (MI), structure index (SI), enrichment index (EI), and trophic groups as measures of soil health, by determining soil nematodes’ sensitivity to cropping systems: rotation, perturbation, fertilization, and inoculation with local effective microorganisms (LEM). Plots were managed for two years under different rotations, annual ryegrass/fallow (ARF) and cereal rye/edamame soybean (CRS). In the third year of the study, all of the plots were managed exactly the same as a wheat/edamame rotation. Data were collected in both winter and summer of this year. In all three years, three inoculant treatments (LEM, False-LEM and No inoculate) were applied. In CRS plots, which received the most tillage and fertilization, there were greater SI values in soils that received LEM application. Nematode community structure described by each MI, SI, and EI were sensitive enough to reflect changes due to differences in soil management practices from previous years. Principal components analysis confirmed that nitrogen mineralization is an important measure to include when using nematode community analysis in the development of a soil health index.

Nematode-Trapping Fungi

Nematode-trapping fungi are a unique and intriguing group of carnivorous microorganisms that can trap and digest nematodes by means of specialized trapping structures. They can develop diverse trapping devices, such as adhesive hyphae, adhesive knobs, adhesive networks, constricting rings, and nonconstricting rings. Nematode-trapping fungi have been found in all regions of the world, from the tropics to Antarctica, from terrestrial to aquatic ecosystems. They play an important ecological role in regulating nematode dynamics in soil. Molecular phylogenetic studies have shown that the majority of nematode-trapping fungi belong to a monophyletic group in the order Orbiliales (Ascomycota). Nematode-trapping fungi serve as an excellent model system for understanding fungal evolution and interaction between fungi and nematodes. With the development of molecular techniques and genome sequencing, their evolutionary origins and divergence, and the mechanisms underlying fungus-nematode interactions have been well studied. In recent decades, an increasing concern about the environmental hazards of using chemical nematicides has led to the application of these biological control agents as a rapidly developing component of crop protection.

Response of nematode-trapping fungi to organic substrates in a coastal grassland soil

To understand why Arthrobotrys oligospora and other nematode-trapping fungi are common and sometimes abundant in the coastal grassland soils of the Bodega Marine Reserve (BMR, Sonoma County, CA), we examined how resident trapping fungi responded to the addition of eight organic substrates (lupine leaves, grass leaves, dead isopods, dead moth larvae, isopod faeces, deer faeces, shrimp shells, and powdered chitin). We were especially interested in the effects of dead isopods because isopods are abundant at BMR and because previous studies had documented strong responses of A. oligospora to other arthropods (dead moth larvae). Soil from BMR was packed into vials (40 g dry mass equivalent per vial with water potential at -230 kPa and bulk density at 0.9 gcm(-3)), and one substrate or no substrate was added to the soil surface. After 30 d at 20 degrees C, trapping fungi were quantified by dilution plating and most probable number procedures. The response of A. oligospora was inversely related to substrate carbon:nitrogen (C:N) ratio: substrates with low C:N ratios (dead isopods, lupine leaves, dead moth larvae) usually caused large increases in A. oligospora whereas those with higher C:N ratios (isopod faeces, deer faeces, grass leaves) did not. An exception was chitin powder, which had a low C:N ratio, but which did not cause A. oligospora to proliferate. Responses of A. oligospora were directly related to the quantity of nitrogen added with each substrate, and those substrates that caused large increases in resident nematodes usually caused large increases in A. oligospora. Other trapping fungi did not respond as strongly as A. oligospora.

Effect of Fertilizers and Neem Cake Amendment in Soil on Spore Germination of Arthrobotrys dactyloides

Application of fertilizers such as urea, diammonium phosphate (DAP) and muriate of potash in soil adversely affected the spore germination of Arthrobotrys dactyloides. Amendment of soil with urea at the concentrations of 1.0%, 0.5% and 0.1% completely inhibited spore germination and direct trap formation on the conidium, whereas muriate of potash delayed and reduced the spore germination even at the lowest concentration. DAP also inhibited spore germination at 1.0% concentration, while at lower concentration the percentage of spore germination was reduced. Application of neem cake at the concentration of 0.5% also inhibited spore germination after 24 h of amendment. The inhibitory effect of neem cake was reduced after 15 days of amendment, while after 30 days after amendment the inhibitory effect was completely lost and the spore germinated by direct trap as in unamended soil. Nematodes were not attracted to ungerminated spores after 24 h of amendment. After 15 days of amendment nematodes were attracted to agar blocks containing fewer germinated spores after 24 h of incubation but after 48 h of incubation large number of nematodes were attracted and trapped by the germinated spores with direct traps. After 30 days of amendment, larger number of nematodes were attracted and trapped by direct traps.

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.

Rhizosphere Interactions and the Exploitation of Microbial Agents for the Biological Control of Plant-Parasitic Nematodes

A range of specialist and generalist microorganisms in the rhizosphere attacks plant-parasitic nematodes. Plants have a profound effect on the impact of this microflora on the regulation of nematode populations by influencing both the dynamics of the nematode host and the structure and dynamics of the community of antagonists and parasites in the rhizosphere. In general, those organisms that have a saprophytic phase in their life cycle are most affected by environmental conditions in the rhizosphere, but effects on obligate parasites have also been recorded. Although nematodes influence the colonization of roots by pathogenic and beneficial microorganisms, little is known of such interactions with the natural enemies of nematodes in the rhizosphere. As nematodes influence the quantity and quality of root exudates, they are likely to affect the physiology of those microorganisms in the rhizosphere; such changes may be used as signals for nematode antagonists and parasites. Successful biological control strategies will depend on a thorough understanding of these interactions at the population, organismal, and molecular scale.

Augmentation of Soil with the Nematophagous Fungi Hirsutella rhossiliensis and Arthrobotrys haptotyla

In previous studies, growth of Hirsutella rhossiliensis from pelletized assimilative hyphae was reduced by other soil organisms. In the current study, sensitivity to this biotic inhibition was compared when the fungus was added to soil as pelletized hyphae or as fungus-parasitized nematodes. The hypothesis was that the natural inoculum, the parasitized nematode, would be less sensitive than the artificial inoculum, pelletized hyphae. The soil was heated to 60 degrees C for 2 h to remove putative antagonists or was not heated. The soil was packed into vials (17 cm(3)) and kept at 20 degrees C in the laboratory or packed into cages (PVC pipe sealed at the ends with 480-mum pore mesh, 80 cm(3)) and buried 22 cm deep in a vineyard. After 2 or 4 weeks, assay nematodes were added to the vials or cages (recovered from the vineyard), respectively. The assay nematodes were extracted from soil after 2.0 or 2.5 days and examined for adhesive conidia of H. rhossiliensis. Consistent with the hypothesis, H. rhossiliensis was quite sensitive to biotic inhibition when formulated as pelletized hyphae but was insensitive to biotic inhibition when formulated as parasitized nematodes. These data suggest that the activity of the H. rhossiliensis pellet could be increased if the pellet better mimicked the natural inoculum. Similar experiments with the nematode-trapping fungus Arthrobotrys haptotyla, however, exhibited an opposite trend: A. haptotyla was more sensitive to biotic inhibition when added to soil as fungus-parasitized nematodes than as pelletized hyphae. Results from laboratory and field experiments were similar.