Nematophagous fungi can infect, kill, and digest living nematodes. Most of these fungi can also live saprophytically and some even have mycoparasitic abilities. Since most plant-parasitic nematodes attack plant roots, the rhizosphere biology of nematophagous fungi is important from a biological control point of view. We previously described that nematophagous fungi were more abundant in the rhizosphere than in bulk soil (Persmark and Jansson 1997). In recent investigations we studied the colonization of internal cells of plant roots (Bordallo et al. 2002; Lopez-Llorca et al. 2002a). In these experiments we used axenic barley and tomato plants grown in vermiculite and inoculated with the nematode-trapping fungus A. oligospora or the egg-parasite P. chlamydosporia. Roots were sequentially sampled, cryo-sectioned, and observed under light- or cryo-scanning electron microscopes. Both fungi grew inter- and intra-cellularly, formed appressoria when penetrating plant cell walls of epidermis and cortex cells, but never entered vascular tissues (Figure 2C). In contrast to Pochonia spp., appressoria had never been observed previously in A. oligospora. Using histochemical stains we could show plant defense reactions, e.g., papillae, lignitubers, and other cell wall appositions induced by nematophagous fungi, but these never prevented root colonization. Callose depositions in papillae are shown in Figure 5. Nematophagous fungi grew extensively especially in monocotyledon plants producing abundant mycelia, conidia, and chlamydospores (P. chlamydosporia). Necrotic areas of the roots were observed at initial stages of colonization by A. oligospora, but were never seen at later stages even when the fungi proliferated in epidermal and cortical cells. P. chlamydosporia colonized roots displayed higher proteolytic activity than uninoculated control roots
Figure 5 Arthrobotrys oligospora colonizing barley root. Callóse deposition in cell wall papillae (arrow) stained with Sirofluor. Bar = 20 mm. (From Bordallo et al. 2002, courtesy of Blackwell Science).
(Montfort et al. unpublished). The significance of this fact for biological control is under investigation in our laboratory.
The growth of the two nematophagous fungi in plant roots appears to be like that of an endophyte. Whether this endophytic growth induces systemic resistance to nematodes and/or plant pathogens in plants is yet unknown, but worth further investigations. Endophytic rhizobacteria reducing plant-parasitic nematodes have been described (Hallmann et al. 2001), as well as the reduction of root-knot nematodes by arbuscular mycorrhizal fungi (Waecke et al. 2001). If this is the case also in nematophagous fungi this will open up a new area of biocontrol using these fungi. The internal root colonization by egg-parasitic fungi, e.g., Pochonia spp., may give the fungi an opportunity to infect nematode eggs in egg sacks of root-knot nematodes inside the roots and reduce subsequent spread and infection of roots by the second generation of juveniles. Structures resembling trapping organs were observed in epidermal cells colonized by A. oligospora, and these may serve the purpose of trapping newly hatched juveniles escaping the roots. The ability to colonize plant roots may also be a survival strategy of these fungi and could explain soil suppressiveness to plant-parasitic nematodes in nature. The colonization of plant roots is a new area of research that deserves in-depth investigations, not the least for biocontrol purposes. This is presently underway in our laboratory.
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