Bacterial response to ecological opportunities offered by soil fungi

Microbial life plays a key role in soil functioning, as it is involved in the biogeochemical cycles of carbon, nitrogen, sulfur and phosphorus, and also influences the growth and development of plants in soil (Elsas van et al., 2007). This way, the soil microbiota exerts an influence on not only the soil system, but, via effects on the atmosphere, on all other ecosystems of the world. The mycorrhizal fungi can be seen as important players in this functioning of the soil, as they form an essential symbiotic relation with plants, including a range of tree species (Finlay, 2007; Garbaye, 1994). These fungi provide the plant with different nutrients (primarily phosphorus and nitrogen sources) and, in return, they obtain the fixed carbon from the plants. In the last years, it is becoming increasingly known that the role of bacteria in the plant-fungal interaction is important for the optimal functioning of the system (Frey-Klett et al., 2007; Johansson et aI., 2004). Soil is regarded as a generally carbon-limited environment, in which the possibilities for bacterial growth are very restricted (Elsas van et al., 2007). The emergence of fungal hyphae in the soil may provide novel ecological opportunities for particular resident bacteria in soil (de Boer et al., 2005). For these bacteria, it is essential to quickly respond to the conditions offered by the emerging fungi such that they can benefit from these new ecological opportunities. It is likely that, over evolutionary time, this has incited the emergence in certain soil bacteria of mechanisms dedicated to their evolutionarily-beneficial interaction with fungal hosts. Bacteria that developed the "better" mechanisms may have emerged as the more successful ones, thus eventually becoming dominant in bacterial communities that are able to thrive in the mycosphere.