The bacterial community of tomato rhizosphere is modified by inoculation with arbuscular mycorrhizal fungi but unaffected by soil enrichment with mycorrhizal root exudates or inoculation with Phytophthora nicotianae Laëtitia Lioussanne a , François Perreault a , Mario Jolicoeur b , Marc St-Arnaud a, * a Institut de recherche en biologie végétale, Université de Montréal and Jardin botanique de Montréal, 4101 East Sherbrooke street, Montreal, QC H1X 2B2, Canada b Bio-P2 Research Unit, Department of Chemical Engineering, École Polytechnique de Montréal, P.O. Box 6079, Centre-ville Station, Montréal, QC H3C 3A7, Canada article info Article history: Received 19 March 2009 Received in revised form 23 November 2009 Accepted 29 November 2009 Available online 8 December 2009 Keywords: Mycorrhizosphere Arbuscular mycorrhizal fungi Phytophthora nicotianae Bacterial community Exudates DGGE abstract Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizo- sphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmen- talized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR- Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction AM fungi have been shown to reduce disease development in a wide range of plant-pathogen associations (St-Arnaud and Vuja- novic, 2007). These ubiquitous fungi are grouped into the phylum Glomeromycota (Schüßler et al., 2001) and form a mutualistic symbi- osis with most land plants, receiving carbon from their host, and delivering minerals and water back (Smith and Read, 2008). They benefit plant growth in a large part through their ability to colonize a wider soil volume and to exploit resources more efficiently than roots. Recently, Maherali and Klironomos (2007) provided evidences supporting that functional ability to protect from soil pathogens may be conserved within AM fungi evolutionary lineages. AM fungi impact the other soil microorganisms development leading to the formation of the specific zone of soil called the mycorrhizosphere (Linderman, 1988). Root colonization with AM fungi has most of the time been shown to decrease (Christensen and Jakobsen, 1993; Bansal and Mukerji, 1994; Cavagnaro et al., 2006), but also to increase (Posta et al., 1994; Albertsen et al., 2006) or have no effect (Olsson et al., 1996) on the microbial biomass within not only the rhizosphere but also within the mycosphere, the zone of soil under the influence of the mycorrhizal mycelium only. They were also shown to have species-specific impacts by stimulating or inhibiting the growth of specific microbial taxa (Marschner and Timonen, 2006). As some rhizobacteria are known to inhibit pathogen proliferation through various mechanisms (Bowen and Rovira, 1999; Whipps, 2001), one way AM fungi may reduce disease development is therefore by inducing the formation of a bacterial community unfavourable to pathogens development * Corresponding author. Tel.: þ1 514 872 1439; fax: þ1 514 872 9406. E-mail address: marc.st-arnaud@umontreal.ca (M. St-Arnaud). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2009.11.034 Soil Biology & Biochemistry 42 (2010) 473e483