Plant growth is influenced by the presence of bacteria and fungi, and their interactions are particularly common in the rhizospheres of plants with high relative densities of microbes
]. Pro- and eukaryotic microorganisms compete for simple plant-derived substrates and have thus developed antagonistic strategies. Bacteria have found niches with respect to the utilization of fungal-derived substrates as well, with their nutritional strategies ranging from hyphal exudate consumption to endosymbiosis and mycophagy
]. Current applications related to bacterial-fungal interactions include biocontrol of fungal plant diseases
] and controlled stimulation of mycorrhizal infection
]. Better insight into the co-existence mechanisms of soil bacteria and fungi is crucial in order to improve existing applications and to invent new ones.
Abundant in the rhizospheres of plants, the streptomycetes are best known for their capacity to control plant diseases (reviewed by
]). The fact that many streptomycetes are able to produce antifungal compounds indicates that they may be competitors of fungi. Direct inhibition of fungal parasites may lead to plant protection and is often based on antifungal secondary metabolites
]. In parallel to antibiotics, the streptomycetes produce a repertoire of other small molecules, including for instance root growth-inducing auxins
] and iron acquisition-facilitating siderophores
Ectomycorrhiza formation between filamentous fungi and forest tree roots is crucial to satisfying the nutritional needs of forest trees
]. The ectomycorrhizas (EM) and the symbiotic fungal mycelia, the mycorrhizosphere, are associated with diverse bacterial communities. Until now, studies on the functional significance of EM associated bacteria have been rare
]. Nevertheless, diverse roles have been implicated for these bacteria, including stimulation of EM formation, improved nutrient acquisition and participation in plant protection (reviewed in
An important question to be addressed with EM associated bacteria is whether there is a specific selection for particular bacterial strains by mycorrhizas, since this would indicate an established association between the bacteria, the EM fungus, and/or the plant root. Frey-Klett et al.
] observed such interdependency: the community of fluorescent pseudomonads from EM with the fungus Laccaria bicolor
was more antagonistic against plant pathogenic fungi than the bulk soil community. This suggested that mycorrhiza formation does select for antifungal compound-producing pseudomonads from the soil. Moreover, these bacteria were not particularly inhibitory to ectomycorrhiza formation with L. bicolor
, indicating some form of adaptation of this ectomycorrhizal fungus to the Pseudomonas
Fungus specificity, i.e. selective inhibition or inhibition of one but stimulation of another fungus, is commonly observed in bacterium-fungus co-culture bioassays. Garbaye and Duponnois
], for instance, observed that bacteria which stimulate growth and mycorrhiza formation by L. bicolor
may be inhibitory to Hebeloma cylindrosporum
.To date, the study on metabolites related to fungus specificity of mycorrhiza associated bacteria has focused on one Streptomyces
isolate. Riedlinger et al.
] observed that Streptomyces
505 stimulated the growth of the mutualist Amanita muscaria
, while inhibiting the plant parasite Heterobasidion annosum
]. EM formation with A. muscaria
was stimulated by Streptomyces
505, and at the same time Norway spruce roots were protected from H. annosum
root rot by the same strain
]. The sole inhibition of H. annosum
was related to its low level of tolerance to an exudate produced by AcH 505, an antifungal substance WS-5995
B. This indicates that production of antibiotics by mycorrhiza associated bacteria is of central importance in relation to fungus specificity, controlled stimulation of mycorrhizal infection, and plant protection.
There is evidence that inoculation of roots with non-pathogenic bacteria may render plants disease resistant. This phenomenon was studied in detail in the interaction between Arabidopsis thaliana
and fluorescent pseudomonads and has been termed “priming”
]. Streptomycetes have also been implicated in the induction of a priming-like state in plants. The inoculation of Arabidopsis
seedlings with Streptomyces
sp. EN27 led to suppression of Fusarium oxysporum
wilt disease in roots and Erwinia carotovora
soft rot in leaves
]. Upon pathogen challenge, the endophyte-treated plants demonstrated higher levels of defence gene expression compared with the non-Streptomyces
-treated controls, indicating a priming-like state in the plant. Streptomyces
sp. GB 4-2 acted in a similar manner against Heterobasidion
root and butt rot in Norway spruce seedlings
]. While the sole inoculation with the plant pathogen led to the lysis of the roots, an anatomical barrier against the root pathogen was formed in the presence of Streptomyces
4-2. The needles of Norway spruce were also protected from Botrytis cinerea
gray mold infection, indicating a systemic response.
Here, we report an assessment study of fungal, bacterial, and plant responses to mycorrhiza-associated streptomycetes. Based on our earlier work with mycorrhizosphere streptomycetes
], we formulated the following hypotheses: (i) streptomycetes impact fungi and bacteria in a streptomycete strain specific manner, (ii) few strains promote the growth of mycorrhizal fungi, and (iii) induction of plant defence responses is not widespread among streptomycetes.
We restricted our investigations to the genus Streptomyces
, since it includes well known antagonists of fungi
], as well as isolates which affect plant resistance against microbial pathogens
] and stimulate mycorrhiza formation
]. Since production of multiple secondary metabolites is commonplace in Streptomyces
] we expected that the mechanisms underlying fungal specificity are related to the specific patterns of secondary metabolite production.