plant surface microbiology.pdf

12 Interaction Between Soil Bacteria and
Ectomycorrhiza-Forming Fungi
Rüdiger Hampp and Andreas Maier
1 Introduction
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 (Curl and Truelove
1986; Whipps and Lynch 1986).
Interactions between soil bacteria and symbiotic fungi can be both negative
and positive. Mycorrhiza-forming fungi have been shown to reduce bacterial
viability (Meyer and Linderman 1986). Due to the transfer and exudation
of plant-derived organic compounds to soil microsites not accessible to
roots, fungi can promote bacterial growth and survival (Hobbie 1992; Söder-
Plant Surface Microbiology
A.Varma, L. Abbott, D. Werner, R. Hampp (Eds.)
© Springer-Verlag Berlin Heidelberg 2004