Epsilon Open Archive

Abstract

The aim of this study was to determine the role of the bacteria associated with arbuscular mycorrhizal (AM) fungi in the interactions between AM fungi, plant hosts and pathogens. Mycorrhizal traits were studied in a potato host using field rhizosphere soils of 12 different plant species as inoculum. High colonisation was found with soil of Festuca ovina and Leucanthemum vulgare, which contained two dominant AMF species (Glomus mosseae and G. intraradices). Bacteria associated with spores of AM fungi (AMB) were isolated from these two AM fungal species with either of the two plant species as hosts. Identification based on fatty acid methyl esters profile analysis revealed high diversity and specific occurrence of certain taxa with either of the two AMF. Some AMB were strongly antagonistic against Rhizoctonia solani in in vitro studies and most of them were spore type-dependent and originated from G. intraradices spores. Occurrence of AMB taxa was also plant host-dependent but antagonism was not. The specificity of AMB to AMF could be due to production of specific exudates, since the results showed that exudates collected from either G. intraradices or AMB stimulated growth of the other organism in a two-compartment plate system, and that concentrations of certain compounds changed several-fold. Certain AMB (Pseudomonas sp. and Stenotrophomonas sp.) showed strong multifunctional effects, i.e. stimulated AMF colonisation of potato roots in outdoor and greenhouse studies, stimulated potato growth in vitro and demonstrated antagonism against several fungal and bacterial plant pathogens. Three AMB from these two genera grown in the presence of exudates of G. intraradices resulted in enhanced antagonistic effects. Production of extracellular enzymes and bioactive compounds varied among the AMB species, suggesting different mechanisms for their multifunctional effects. By using F. ovina inoculated with G. intraradices as a cover crop, it should be possible to enhance the occurrence of strongly pathogen-antagonistic AMB. The association of multifunctional AMB with AMF spores provides evidence that bacteria are involved in the beneficial effects of AM fungi on plants.