Nematode succession and microfauna-microorganism interactions during root residue decomposition

The quality of plant material affects the vigor of the decomposition process and composition of the decomposer biota. Root residues from hairy vetch (Vicia villosa Roth), rye (Secale cereale L.) and vetch+rye, packed in litterbags were placed in pots of soil at 15 C and the content of the bags was analyzed after 2, 4, 8 and 12 weeks. Bacterial biomass did not differ between residues with contrasting composition. Among bacterivores groups of nematodes that require high bacterial production dominated in fast decomposing resources whereas flagellates with smaller requirements prevail in slower decomposing resources. Biomass of bacterial feeding nematodes correlated positively with early phase (0-2 wk) decomposition that increased in the order: rye< vetch+rye<vetch. Bacterial biomass therefore seems to be under top-down (predation) control during early decomposition. In contrast, the fungal biomass differed between resources with highest values for rye. Moreover, this increase in fungal biomass occurred later during succession and was correlated with decomposition activity for rye in that period. Fungal biomass therefore seems to be under bottom-up (resource) control. The composition of the nematode assemblages (composed of 25 taxa) showed a clear relationship to initial plant resource quality as well as decomposition phase. Early successional microbivorous nematodes vary according to resource quality with demanding bacterivores+predators (Neodiplogasteridae) dominating in vetch and less demanding bacterivores (Rhabditidae) and fungivores (Aphelenchus) being equally common in vetch and rye. Later in the succession (2-4 wk) bacterivorous Cephalobidae and fungivorous Aphelenchoides prevailed similarly on the different root materials whereas bacterivorous protozoa and the amoebal fraction thereof dominated in rye. At week 12 no species dominated the nematode assemblages that were similar between the resources. The differences between nematode assemblages among plant resources at 2 week were similar to the results of a field study sampled after 6 weeks with the same soil and plant resources. This lends support to the relevance of the successional patterns observed in this incubation study.     

Non-target effects of a carbamate and the proteins avidin and aprotinin on in vitro development of a bacterial feeding nematode

A 24-well plate trial was conducted to determine the effects of conventional (oxamyl) and transgenic (avidin and aprotinin) insect control chemicals on the non-target bacterial-feeding nematode Bursilla sp. (Rhabditidae: Nematoda). Treatments were added to agar as pure oxamyl, avidin from egg whites or aprotinin from bovine lung, which was then inoculated with the bacteria Xanthamonas campestris as a food source for the nematodes. Oxamyl was toxic to the nematodes at a concentration of 400 ppm, significantly decreasing adult survival, egg laying and consequent progeny development and survival. The 400 ppm concentration of oxamyl also had a significant negative effect on bacterial growth in two of three experiments. Avidin is a biotin-binding protein and concentrations of 100 and 400 ppm significantly decreased the number of progeny produced by first generation nematodes through an effect on egg development. This effect was not significant (P>0.05) at either 10 or 25 ppm. This is the first report of avidin affecting nematode egg development and suggests a role for biotin in this process. Avidin had no effect on visual bacterial growth. Aprotinin is a protease inhibitor and concentrations up to 400 ppm had no significant effect on nematode development or bacterial growth. These lab-based findings need to be further investigated using plants producing avidin and aprotinin, growing in field soil in order to quantify their impact on environmental processes such as decomposition.     

Evidence that nematodes may vector the soft rot‐causing enterobacterial phytopathogens

Bacterial soft rot is a globally significant plant disease that causes major losses in the production of many popular crops, such as potato. Little is known about the dispersal and ecology of soft‐rot enterobacteria, and few animals have been identified as vectors for these pathogens. This study investigates whether soil‐living and bacterial‐feeding nematodes could act as vectors for the dispersal of soft‐rot enterobacteria to plants. Soft‐rot enterobacteria associated with nematodes were quantified and visualized through bacterial enumeration, GFP‐tagging, and confocal and electron scanning microscopy. Soft‐rot enterobacteria were able to withstand nematode grazing, colonize the gut of Caenorhabditis elegans and subsequently disperse to plant material while remaining virulent. Two nematode species were also isolated from a rotten potato sample obtained from a potato storage facility in Finland. Furthermore, one of these isolates (Pristionchus sp. FIN‐1) was shown to be able to disperse soft‐rot enterobacteria to plant material. The interaction of nematodes and soft‐rot enterobacteria seems to be more mutualistic rather than pathogenic, but more research is needed to explain how soft‐rot enterobacteria remain viable inside nematodes.