Two isolates of Paenibacillus validus (DSM ID617 and ID618) stimulated growth of the arbuscular mycorrhizal fungus Glomus intraradices Sy167 up to the formation of fertile spores, which recolonize carrot roots. Thus, the fungus was capable of completing its life cycle in the absence of plant roots, but relied instead on the simultaneous growth of bacteria. The supernatant of a mixed batch culture of the two P. validus isolates contained raffinose and another, unidentified trisaccharide. Among the oligosaccharides tested, raffinose was most effective in stimulating hyphal mass formation on plates but could not promote growth to produce fertile spores. A suppressive subtractive hybridization library followed by reverse Northern analyses indicated that several genes with products involved in signal transduction are differentially expressed in G. intraradices SY 167 when grown in coculture with P. validus (DSM 3037). The present investigation, while likely representing a significant step forward in understanding the arbuscular mycorrhizal fungus symbioses, also confirms that its optimal establishing and functioning might rely on many, as yet unidentified factors.