Mutualistic Interactions

Litter- and wood-degrading basidiomycetes have developed strategies for suboptimal concentrations of nitrogen in their substrates. These strategies include recycling of nitrogen from senescent mycelium, reallocation from intracellular stored proteins and uptake and translocation of nitrogen from soil to wood/litter (Cowling and Merrill, 1966; Watkinson et al., 2001; Lindahl and Finlay, 2005). As mentioned in the previous section, it has been proposed that lysis of bacteria may be another strategy in which saprotrophic basidiomycetes obtain nitrogen (Greaves, 1971; Tsuneda and Thorn, 1994b). This hypothesis can be extended to a mutualistic/predatory relationship with nitrogen-fixing bacteria. Nitrogen-fixing bacteria that are adapted to grow in the vicinity of wood-degrading basidiomycetes may provide a continuous source of nitrogen to the fungi. A mutualistic/predatory relationship would imply that only some of the bacteria are being lysed so that the bacterial density can be maintained due to growth of the bacteria on oligomers released by the fungal enzymes.

The importance of nitrogen input by nitrogen-fixing bacteria for decay activities of wood-rot fungi has already been proposed (Cowling and Merrill, 1966). Several reports have indicated the occurrence of nitrogen-fixation in decaying wood (e.g. Jurgensen et al., 1989; Hendrickson, 1991; Brunner and Kimmins, 2003). Brunner and Kimmins (2003) showed that the highest nitrogen-fixation rates were found in the more advanced stages of decay.

Knowledge of the composition of the nitrogen-fixing bacterial communities in decaying wood is very limited. Older studies were largely based on physiological properties to identify culturable bacteria, and conclusions from such studies, for example the absence of the free-living nitrogen-fixing bacterium Azospirillum in decaying wood, should be considered with great care (Jurgensen et al., 1989). Obviously, DNA- and/or RNA-based techniques are required to obtain a proper identification of both culturable and non-culturable bacteria that inhabit wood during basidiomycetal decay.

As indicated in the previous section, we observed that colonization of beech wood blocks by the white-rot fungus H. fasciculare coincided with a strong reduction in numbers of wood-inhabiting bacteria. A relatively high proportion (25%) of the culturable bacteria that survived the bactericidal effects belonged to the order Rhizobiales which are potentially nitrogen-fixing bacteria. These bacteria were not detected in beech wood blocks without H. fasciculare, but they may have been masked by the presence of high numbers of Burkholderia- and Xanthomonas strains. The occurrence of strains related to the methanotrophic bacterium Methylocapsa acidiphilia, family Beijerinckiaceae of the order Rhizobiales, in wood blocks colonized by H. fasciculare is of particular interest. M. acidiphilia is a nitrogen-fixing methanotrophic bacterium that was isolated from an acidic Sphagnum peat bog (Dedysh et al., 2002). It grows with methane and methanol as substrates. Methanol may be the actual substrate for these bacteria, that is they may be methylotrophs rather than methanotrophs, in decaying wood as this is a side-product of fungal ligninolytic activities (Ander and Eriksson, 1984). So, the presence of these putative nitrogen-fixing, methylotrophic strains in wood decayed by white-rot fungi may indicate a mutualistic/predatory interaction namely growth of the bacteria on methanol released by fungal lignolytic activities and supply of nitrogen to the fungus via nitrogen-fixation and cell lysis (Figure 2).

Besides interactions with nitrogen-fixing bacteria, other mutualistic interactions between wood-decomposing basidiomycetes and bacteria have been proposed. Bacteria may provide essential growth factors, for example thiamine (Henningsson, 1967). However, this possibility has not been examined further. Greaves (1971) suggested that bacteria may also stimulate growth and activity of wood-degrading fungi by degrading toxic compounds. In fresh wood several compounds, collectively called wood extractives, occur that may inhibit growth and activity of basidiomycetes. Degradation of such compounds by bacteria has been reported (Burnes et al., 2000; Kallionen et al., 2003). However, this should perhaps be considered as facilitation rather than as mutualism. Only in those cases in which the toxic compounds are produced by the fungi themselves does

Figure 2 Hypothetical mutualistic interaction between wood-degrading basidiomycetes and nitrogen-fixing, methanol-degrading bacteria (methylotrophs). Methanol, a side-product of fungal lignin degradation, is the growth substrate for nitrogen-fixing bacteria. Part of the biomass of the methylotrophic bacteria is lysed by the fungus and used as nitrogen source. LMW, law molucular weight.

Figure 2 Hypothetical mutualistic interaction between wood-degrading basidiomycetes and nitrogen-fixing, methanol-degrading bacteria (methylotrophs). Methanol, a side-product of fungal lignin degradation, is the growth substrate for nitrogen-fixing bacteria. Part of the biomass of the methylotrophic bacteria is lysed by the fungus and used as nitrogen source. LMW, law molucular weight.

the term mutualism seem to be appropriate, as, for example in the aforementioned removal of methanol, a toxic side-product of fungal lignolysis, by wood-inhabiting methanotrophs.

Bacteria that detoxify fungal cell membrane disrupting compounds produced by bacterial pathogens of fruit bodies (see above) may also be considered as mutualists of basidiomycetes. Tsukamoto et al. (2002) isolated several tolaasin-detoxifying strains from wild Agaricales. Perhaps the presence of tolaasin-detoxifying strains on wild mushrooms explains why P. tolaasii is much more frequently isolated from cultivated mushrooms than from wild ones (Bessette, 1984). More detailed investigations are needed to understand the nutritional requirements of the antagonists of P. tolaasii. If they are preferentially selected by the fungus, for example via a resistance to antibacterial compounds and are growing on fungal exudates, this would be true mutualism.

Fruit body formation of several edible mushrooms is dependent on the presence of certain bacteria (Rainey et al., 1990; Cho et al., 2003). Evidence has been presented that this is due to the removal of fungal autoinhibitors by bacteria (Noble et al., 2003). However, the bacteria may also exert a stress on the fungus which triggers fruit body formation.

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