The most common system responses attributed to microfloral grazers (protozoa, nema- todes, microarthropods) in the literature are increased plant growth, increased N uptake by plants, decreased or increased bacterial populations, increased CO2 evolution, increased N and P mineral- ization, and increased substrate utilization. Based on this evidence in the literature, a conceptual model was proposed in which microfloral grazers were considered as separate state variables. To help evaluate the model, the effects of microbivorous nematodes on microbial growth, nutrient cycling, plant growth, and nutrient uptake were examined with reference to activities within and outside of the rhizosphere. Blue grama grass (Bouteloua gracilis) was grown in gnotobiotic microcosms containing sandy loam soil low in inorganic N, with or without chitin amendments as a source of organic N. The soil was inoculated with bacteria (Pseudomonas paucimobilis or P. stutzeri) or fungus (Fusarium oxysporum), with half the bacterial microcosms inoculated with bacterial-feeding nematodes (Pelodera sp. or Ac- robeloides sp.) and half the fungal microcosms inoculated with fungal-feeding nematodes (Aphelenchus avenae). Similar results were obtained from both the unamended and the chitin-amended experiments. Bacteria, fungi, and both trophic groups of nematodes were more abundant in the rhizosphere than in nonrhizosphere soil. All treatments containing nematodes and bacteria had higher bacterial densities than similar treatments without nematodes. Plants growing in soil with bacteria and bacterial-feeding nematodes grew faster and initially took up more N than plants in soil with only bacteria, because of increased N mineralization by bacteria, NH4+-N excretion by nematodes, and greater initial exploi- tation of soil by plant roots. Addition of fungal-feeding nematodes did not increase plant growth or N uptake because these nematodes excreted less NH4+-N than did bacterial-feeding nematode pop- ulations and because the N mineralized by the fungus alone was sufficient for plant growth. Total shoot P was significantly greater in treatments with fungus or Pelodera sp. than in the sterile plant control or treatments with plants plus Pseudomonas stutzeri until the end of the experiment. The additional mineralization that occurs due to the activities of microbial grazers may be sig- nificant for increasing plant growth only when mineralization by microflora alone is insufficient to meet the plants' requirements. However, while the advantage of increased N mineralization by mi- crobial grazers may be short-term, it may occur in many ecosystems in those short periods of ideal conditions when plant growth can occur. Thus, these results support other claims in the literature that microbial grazers may perform important regulatory functions at critical times in the growth of plants.

Interactions of Bacteria, Fungi, and their Nematode Grazers: Effects on Nutrient Cycling and Plant Growth

Trichoderma is a genus of common filamentous fungi that display a remarkable range of lifestyles and interactions with other fungi, animals and plants. Because of their ability to antagonize plant-pathogenic fungi and to stimulate plant growth and defence responses, some Trichoderma strains are used for biological control of plant diseases. In this Review, we discuss recent advances in molecular ecology and genomics which indicate that the interactions of Trichoderma spp. with animals and plants may have evolved as a result of saprotrophy on fungal biomass (mycotrophy) and various forms of parasitism on other fungi (mycoparasitism), combined with broad environmental opportunism.

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Trichoderma : the genomics of opportunistic success

Introduction: Verticillium spp. are soil-borne plant pathogens responsible for Verticillium wilt diseases in temperate and subtropical regions; collectively they affect over 200 hosts, including many economically important crops. There are currently no fungicides available to cure plants once they are infected. Taxonomy: Kingdom: Fungi, phylum: Ascomycota, subphylum, Pezizomycotina, class: Sordariomycetes, order: Phyllachorales, genus: Verticillium. Host range and disease symptoms: Over 200 mainly dicotyledonous species including herbaceous annuals, perennials and woody species are host to Verticillium diseases. As Verticillium symptoms can vary between hosts, there are no unique symptoms that belong to all plants infected by this fungus. Disease symptoms may comprise wilting, chlorosis, stunting, necrosis and vein clearing. Brown vascular discoloration may be observed in stem tissue cross-sections. Pathogenicity: Verticillium spp. have been reported to produce cell-wall-degrading enzymes and phytotoxins that all have been implicated in symptom development. Nevertheless, evidence for a crucial role of toxins in pathogenicity is inconsistent and therefore not generally accepted. Microsclerotia and melanized mycelium play an important role in the disease cycle as they are a major inoculum source and are the primary long-term survival structures. Resistance: Different defence responses in the prevascular and the vascular stage of Verticillium wilt diseases determine resistance. Although resistance physiology is well established, the molecular processes underlying this physiology remain largely unknown. Resistance against Verticillium largely depends on the isolation of the fungus in contained parts of the xylem tissues followed by subsequent elimination of the fungus. Although genetic resistance has been described in several plant species, only one resistance locus against Verticillium has been cloned to date.

Physiology and molecular aspects of Verticillium wilt diseases caused by V. dahliae and V. albo-atrum.

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.

Rhizosphere interactions and the exploitation of microbial agents for the biological control of plant-parasitic nematodes.

Mechanisms of nematode suppression by organic soil amendments—A review

Many nematode-trapping fungi capture nematodes using an adhesive present on specific capture organs (for review see ref. 1). Until recently, the mechanism of adhesion was completely unknown. In the case of Arthrobotrys oligospora, one of the most common nematophagous fungi, nematodes are trapped in three-dimensional structures of the adhesive network type (Fig. 1a). When a suspension of nematodes is added to an agar culture of the fungus, nematodes are immediately captured and firmly held by the traps. The nematode cuticle is lysed at the point of contact and penetrated by a hypha within one hour2. An increased secretion by the fungus of a mucilaginous substance in the presence of prey has been shown by scanning and transmission electron microscopy2,3. Our hypothesis is that the firmness of attachment to the traps despite the struggle of the nematode is due to a series of events, beginning with an interaction between complementary molecular configurations on the nematode and fungal surfaces. We show evidence here for the presence of a lectin on the traps of A. oligospora which binds to a carbohydrate on the nematode surface.

Action of a nematode-trapping fungus shows lectin-mediated host–microorganism interaction

Fungal attachment to nematodes

The present investigation shows the ability of peptides to induce capture organ formation in Arthrobotrys oligospora when applied in a synthetic low nutrient medium.



Under certain conditions casitone was shown to induce capture organ formation. The active principle in casitone was concentrated and purified by alternating procedures of ion exchange chromatography and gel chromatography in pyridine-acetic acid buffers. Crude casitone solutions were applied to columns of Dowex 50 W-X2 and eluted stepwise with 0.1–1.0 M pyridine-acetic acid pH 3.2–5.1. Active portions, free from most acid and neutral amino acids, were further purified on columns of Sephadex G-10 in 0.1 M pyridine-acetic acid pH 4.6. Aromatic amino acids and large molecules in the void volume could be separated from an active peptide mixture which was subjected to renewed ion exchange chromatography on Bio-Rad AG 50 W-X2. By stepwise and/or gradient elution in 0.1–0.5 M pyridine-acetic acid pH 3.2 fairly purified peptides were obtained.



The composition of the test medium is an important factor in spontaneous capture organ formation. The peptides isolated from casitone induced capture organ formation, when given to the fungus in a synthetic mineral salt medium supplied with thiamin and biotin. Similar effects were obtained with small synthetic peptides in the same concentration (0.1 mg/ml). A large variety of peptides seem to be active when applied in a suitable medium. This was especially true for peptides with Rf > Rfleu on thin layers of cellulose developed with butanol-acetic acid-water (4: 1: 1). Of the peptides investigated valyl-peptides exerted the most drastic effect.

Peptide-induced morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora

Living nematodes induced trap formation in Arthrobotrys oligospora more rapidly than did additions of morphogenetic peptides. In nematode-induced morphogenesis, excreted substances as peptides and amino acids were only partly responsible for the effect. Additional effects were due to volatile substances from nematodes or to direct contact between living nematodes and the hyphae. Ammonia was shown by gas chromatography to be excreted in nematode suspensions in amounts that could affect trap formation. It is proposed that living nematodes act primarily through another mechanism than peptide-induced morphogenesis.

Nematode-Induced Morphogenesis in the Predacious Fungus Arthrobotrys Oligospora

The behaviour of five plant-parasitic nematodes was compared with that of a bacteria-feeding nematode. The attraction of the plant-parasitic nematodes to thirteen nematophagous fungi was tested. The two plant-parasitic nematodes also known to be mycophagous were attracted to all fungi. In the other nematodes there was no distinct attraction pattern. The ability of different fungi to attract tended to increase with increasing dependence of the fungi on nematodes for nutrients. The rapidity with which the nematodes induced trap formation depended on their motility and on the concentration of nutrients in the culture medium. The ability of a nematode to induce trap formation was not correlated with feeding behaviour (bacteria-feeding, mycophagous or plant-parasitic). All nematodes were rapidly captured when traps were present.

Birgit Nordbring-Hertz

Papers

Action of a nematode-trapping fungus shows lectin-mediated host–microorganism interaction

Fungal attachment to nematodes

Peptide-induced morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora

Nematode-Induced Morphogenesis in the Predacious Fungus Arthrobotrys Oligospora

Interactions between nematophagous fungi and plant-parasitic nematodes: attraction, induction of trap formation and capture.