Interaction Between Soil Bacteria and Ectomycorrhiza-Forming Fungi

Roots constitute important plant organs for water and nutrient uptake.However, they also release a wide range of carbon compounds of low molecular weight which are called exudates. These compounds form the basis for an environment rich in diversified microbiological populations, the rhizosphere (Hiltner 1904; the rhizosphere has been defined as a narrow zone of soil which is influenced by living roots). Bacteria are an important part of these populations. In addition, roots of most terrestrial plants develop symbiotic structures (mycorrhiza) with soil-borne fungi. In these interactions, the fungal partner provides the plant with improved access to water and nutrients in the soil due to more or less complex hyphal structures that emanate from the root surface and extend far into the soil. The plant, in return, supplies carbohydrates for fungal growth and maintenance (Smith and Read 1997; Hampp and Schaeffer 1998). Due to leakage and the turnover of mycorrhizal structures, these solutes are also released into the rhizosphere where they can be accessed by other microorganisms. The term “rhizosphere” has, therefore been extended to “mycorrhizosphere” (Oswald and Ferchau 1968). In the latter, two different zones can be distinguished: the surface of the mycorrhizal structure, affected by both root and fungus, and that occupied by fungal hyphae only. The latter has been termed “hyphosphere”(Marschner 1995). Soil free of plant and fungal components has been referred to as “bulk soil” (Andrade et al. 1997). It is reasonable to believe that these different spheres may differ in their microbial activities, and it has been shown that microbial communities within the rhizosphere are distinct from those of nonrhizosphere soil