Mycorrhiza Helper Bacteria
Mycorrhizal symbiosis should not be considered merely as a bipartite plant-fungus interaction, but should instead incorporate the associated organisms. These mycorrhiza-associated organisms are known to influence each other mutually, the outcome of which
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Introduction
Mycorrhizal symbiosis should not be considered merely as a bipartite plant–fungus interaction, but should instead incorporate the associated organisms. These mycorrhizaassociated organisms are known to influence each other mutually, the outcome of which is described as the “mycorrhizosphere” (Foster and Marks 1966; Meyer and Linderman 1986; Frey-Klett and Garbaye 2005). The mycorrhizosphere comprises mycorrhizas, extramatrical mycelium and the associated microorganisms. In the same way the rhizospheres exert a pressure on microbial populations (Barea et al. 2005), the mycorrhizal roots and hyphae of mycorrhizal fungi (MF) shape the bacterial species composition due to root and hyphal exudation and turnover (Bowen 1993; Morgan et al. 2005). This “mycorrhizosphere effect” may lead to improved plant nutrition, growth and disease resistance (Linderman 1988; FreyKlett et al. 2005). Determining the functional significance of the mycorrhizosphere organisms for plant productivity presents a major challenge for the future (Artursson et al. 2006). The presence of bacteria that are directly involved in mycorrhiza formation was first indicated by the studies of Bowen and Theodorou (1979) which showed that some bacterial isolates promoted and others inhibited the colonization of Pinus radiata roots by Rhizopogon luteolus. In subsequent work the presence of bacteria able to promote mycorrhiza formation was confirmed in ectomycorrhiza (ECM) (Garbaye and Bowen 1987; de Oliveira and Garbaye 1989), in arbuscular mycorrhiza (AM) (Meyer and Linderman 1986; Ames 1989) and suggested in orchid mycorrhizal associations (Wilkinson et al. 1989). The bacteria able to promote mycorrhizal development were then collectively named as MHB (mycorrhiza helper bacteria; Duponnois and Garbaye 1991; Garbaye 1994). Their presence in other types of mycorrhizal associations may be expected, but has not been investigated thus far.
Mika T. Tarkka UFZ, Helmholtz-Centre for Environmental Research, Department of Soil Ecology, Theodor-Lieser-Strasse 4, 06120 Halle, Germany e-mail: [email protected]
A. Varma (ed.) Mycorrhiza, © Springer-Verlag Berlin Heidelberg 2008
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M. T. Tarkka, P. Frey-Klett
In this review, we will briefly cover the origin and taxonomy of the MHB. An emphasis will be given for the thus far characterized mechanisms that lead to enhanced development of mycorrhizal symbiosis, and on the specificity of this tripartite interaction, which is reflected by the promotion of some and inhibition of other fungal species by individual MHB isolates. Finally, a short overview will be given on the possible applications for MHB in forestry and agriculture. For a deeper insight into bacterium–fungus interactions in mycorrhizas, mycorrhiza–PGPR interactions and other beneficial interactions occurring in the mycorrhizosphere, the reader is advised to take notice of recent reviews (Barea et al. 2005; Morgan et al. 2005; Artursson et al. 2006; Marschner and Timonen 2006; Reddy and Satyanarayana 2006; Frey-Klett et al. 2007; Gamalero
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