Trap and conidiophore formation in Arthrobotrys superba

Recent Advances in Life History Transition with Nematode-Trapping Fungus Arthrobotrys oligospora and Its Application in Sustainable Agriculture

Parasitic nematodes cause great annual loss in the agricultural industry globally. Arthrobotrys oligospora is the most prevalent and common nematode-trapping fungus (NTF) in the environment and the candidate for the control of plant- and animal-parasitic nematodes. A. oligospora is also the first recognized and intensively studied NTF species. This review highlights the recent research advances of A. oligospora as a model to study the biological signals of the switch from saprophytism to predation and their sophisticated mechanisms for interacting with their invertebrate hosts, which is of vital importance for improving the engineering of this species as an effective biocontrol fungus. The application of A. oligospora in industry and agriculture, especially as biological control agents for sustainable purposes, was summarized, and we discussed the increasing role of A. oligospora in studying its sexual morph and genetic transformation in complementing biological control research.

Trapping devices of nematode-trapping fungi: formation, evolution, and genomic perspectives

Nematode-trapping fungi (NTF) are potential biological control agents against plant- and animal-parasitic nematodes. These fungi produce diverse trapping devices (traps) to capture, kill, and digest nematodes as food sources. Most NTF can live as both saprophytes and parasites. Traps are not only the weapons that NTF use to capture and infect nematodes, but also an important indicator of their switch from a saprophytic to a predacious lifestyle. Formation of traps and their numbers are closely related to the nematicidal activity of NTF, so the mechanisms governing trap formation have become a focus of research on NTF. Recently, much progress has been made in our understanding of trap formation, evolution, and the genome, proteome and transcriptome of NTF. Here we provide a comprehensive overview of recent advances in research on traps of NTF. Various inducers of trap formation, trap development, structural properties and evolution of traps are summarized and discussed. We specifically discuss the latest studies of NTF based on genomic, proteomic and transcriptomic analyses.

Proteomic and transcriptional analyses of Arthrobotrys oligospora cell wall related proteins reveal complexity of fungal virulence against nematodes

The nematode-trapping fungus Arthrobotrys oligospora is the best-studied fungus for understanding the interaction between fungi and nematodes. The fungus uses three-dimensional adhesive networks to capture nematodes and then penetrates into the worms through their cuticle. Here we examine the effects of fungal cell wall related proteins on morphogenesis and virulence of the fungi. We focused on the changes in its proteomic and transcriptional profiles during its transition from saprophytic to predatory phase. Isobaric tags for relative and absolute quantitation (iTRAQ) proteomics using the liquid chromatography/mass spectrometry (LC/MS) method revealed an extended set of virulence related proteins, such as adhesins and serine proteases, on the cell wall of A. oligospora. Transcription analyses of their coding genes revealed an important set of candidate virulence factors. Our analyses also show that glycosyl hydrolases likely play important roles in trap formation of A. oligospora. The adhesins on the three-dimensional adhesive networks may have two functions: to enable the mycelia to stick to nematodes and to serve as important constituents of the extracellular matrix that harbors many secreted virulence related proteins. This study is the first to systematically identify cell wall related proteins that are important in the trap formation and infection of the fungus against nematode hosts.

Phylogeny of nematode-trapping fungi based on 18S rDNA sequences

The small subunit (SSU) ribosomal DNA (18S rDNA) from 15 species of nematode-trapping fungi and closely related non-parasitic species were sequenced. Phylogenetic analysis indicated that species within the genera of Arthrobotrys, Dactylaria, Dactylella, Monacrosporium and Duddingtonia formed a monophyletic and isolated clade among an unresolved cluster of apothecial ascomycetes. The phylogenetic patterns within this clade were not concordant with the morphology of the conidia nor the conidiophores, but rather with that of the infection structures. The results from the different methods of tree reconstruction supported three lineages; the species having constricting rings, the non-parasitic species and the species having various adhesive structures (nets, hyphae, knobs and non-constricting rings) to infect nematodes.

Significance of electron dense microbodies in trap cells of the nematophagous fungus Arthrobotrys oligospora

We have studied the fate of electron dense microbodies in nematode-trapping organs (traps) of the fungus A. oligospora during the initial hours following nematode capture. The interaction studies were performed with isolated traps which had captured a nematode under conditions where the fungal cells had no access to external energy sources. Video enhanced contrast microscopy showed that under these conditions the number of dense bodies present in the trap cell that formed the penetration tube, rapidly decreased. During subsequent penetration and development of the infection bulb this decrease continued while at this time common cell organelles such as mitochondria and vacuoles were formed. This was confirmed by electron microscopy which also revealed that the dense bodies were degraded by means of an autophagic process. The organelles were degraded individually and finally turned into compartments which, based on ultrastructural criteria, were considered vacuoles. Fusion of such vacuoles into larger organelles frequently occurred. The degradation process was initiated early in the interaction since initial stages were already evident within 15 min after capture. Generally it took 1-2 h before the infection bulb had fully developed and trophic hyphae formation started. During this time the original trap cell, characterized by numerous dense bodies, was transformed into an active vegetative hyphal cell containing typical cell organelles such as nuclei, mitochondria, a strongly proliferated endoplasmic reticulum, vacuoles and "normal" microbodies but lacked dense bodies. This disappearance of dense bodies was confined to the cell that penetrated the nematode and--less frequently--its two neighbouring cells in the hyphal loop. In the other cells, constituting the trap, the dense bodies remained unaffected. As will be discussed, the present results support our current view that traps of A. oligospora contribute to the survival of the organism in its natural environment.

An electron-microscopical analysis of capture and initial stages of penetration of nematodes by Arthrobotrys oligospora

A detailed analysis was made of the capture and subsequent penetration of nematodes by the nematophagous fungus Arthrobotrys oligospora using different electron-microscopical techniques. Capture of nematodes by this fungus occurred on complex hyphal structures (traps) and was effectuated by an adhesive coating, present on these trap cells. The adhesive layer was largely fibrillar in nature and was absent on cells of normal hyphae. Following capture, penetration hyphae were formed at those sites where the trap cell wall was anchored to the nematode cuticle by the adhesive. New walls of these hyphae were formed underneath the original trap cell walls, which were partly hydrolysed to allow growth and development of the penetration tubes through the adhesive coating towards the cuticle. Our observations indicated that the cuticle of the nematode was subsequently penetrated by the penetration tubes by mechanical means. After penetration a large infection bulb was formed from which trophic hyphae arose. Cytochemical experiments indicated that the sites of penetration of the cuticle were intensely stained for acid phosphatase activity. At later stages of infection activity of this enzyme was present throughout the nematode contents; the enzyme was most probably secreted by complex membranous structures associated with the cytoplasmic membrane of the infection bulb and the trophic hyphae.