Short communication Mycorrhizal fungi increase biocontrol potential of Pseudomonas fluorescens Eleni Siasou, Dominic Standing, Ken Killham, David Johnson * Institute of Biological and Environmental Sciences, Cruickshank Building, St Machar Drive, University of Aberdeen, Aberdeen AB24 3UU, UK article info Article history: Received 6 November 2008 Received in revised form 9 February 2009 Accepted 24 February 2009 Available online 16 March 2009 Keywords: Take-all Arbuscular mycorrhiza Gaeumannomyces graminis var. tritici Carbon DAPG Pseudomonas abstract Wheat roots are susceptible to colonisation by soil-borne pathogens, such as Gaeumannomyces graminis var. tritici (Ggt), which causes the globally important disease take-all, and mutualistic arbuscular mycorrhizal fungi (AMF). Certain rhizosphere fluorescent Pseudomonas strains have received much attention as potential biocontrol agents given their ability to produce antibiotics, such as 2,4-diacetyl- phloroglucinol (DAPG), that confer a measure of plant protection. Here we show that Pseudomonas flu- orescens only produced DAPG in the presence of soluble carbon from soil containing either Ggt or AMF, and production increased by two orders of magnitude in response to both AMF and Ggt. Encouragement of mycorrhizal colonisation may therefore offer a sustainable strategy for protection against take-all. Ó 2009 Elsevier Ltd. All rights reserved. Take-all is considered the most important root disease in cereals, particularly wheat, and has an enormous impact on global grain production (Freeman and Ward, 2004). The disease is caused by the ascomycete fungus Gaeumannomyces graminis var. tritici (Ggt) that persists saprotrophically in soil and crop debris. Concerns about the costs, effectiveness and deleterious environmental effects of chemical treatment of crop pathogens has resulted in the exploration of biocontrol as a potentially sustainable, alternative strategy for control of take-all, and other diseases. Common rhi- zobacteria of the genus Pseudomonas have been shown to have significant effects on take-all largely because of their capacity to produce the antimicrobial secondary metabolite 2,4-diacetyl- phloroglucinol (DAPG) (Cook, 2003). DAPG is a broad-range anti- biotic with antibacterial, antifungal, antihelminthic, and phytotoxic properties (Thomashow and Weller, 1996). The activity of rhizobacteria is primarily driven by carbon from plant roots, and so the nature of rhizosphere carbon flow is likely to be a major determinant of DAPG production. In host plants such as wheat, rhizosphere carbon flow is greatly affected by the presence of mutualistic arbuscular mycorrhizal (AM) fungi, which substan- tially modify root release of plant photosynthate (Johnson et al., 2002a,b) to create a distinct mycorrhizosphere (Bending et al., 2006), and are major components of soil microbial biomass. In farming systems receiving few or no inorganic fertilisers (e.g. organic cropping systems), crop plants are likely to be more dependent on mycorrhizal fungi for their obtaining an adequate supply of nutrients. Several studies have demonstrated increased abundance and diversity (Helgason et al., 1998) of AM fungi in more undisturbed plant communities (Mathimaran et al., 2005; Douds and Millner, 1999; Gosling et al., 2006). Carbon from AM fungal mycelium is rapidly incorporated into microbial biomass (Paterson et al., 2008) and so these fungi have the potential to be important conduits of energy into rhizosphere bacteria like Pseudomonas fluorescens that have biocontrol potential. The demonstration that barley plants had reduced root infection by Ggt when they were grown in the mycorrhizal condition with the AM fungus Glomus mosseae (Khaosaad et al., 2007) provides circumstantial evidence for the existence of this process. Here we test the hypothesis that carbon from plants colonised by AM fungi is a more efficient energy source for DAPG production by the rhizosphere bacteria P. fluorescens. Wheat seedlings Triticum aestivum L., Robigus were subjected to four treatments: control (no Ggt infection-no AM colonisation), AMF (with Glomus intraradices 5% v/v), Ggt (with Ggt strain 571.8), and AMF þ Ggt (with G. intraradices and Ggt). Each treatment had 4 replicates. Surface sterilised (H 2 O 2 for 20 min, and twice rinsed with sterile deionised water) wheat (cv. Robigus) seedlings were germinated for 5 days at 25  C before planting and grown for a further 3 months in pots containing autoclaved dune grassland sand. The AMF inoculum was added by layering within the sand, while the Ggt inoculum (seeds infected with Ggt strain 571.8 for 5 days) was added to the rooting zone along with fragments of * Corresponding author. Tel.: þ44 1224 273857; fax: þ44 1224 272703. E-mail address: d.johnson@abdn.ac.uk (D. Johnson). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2009.02.028 Soil Biology & Biochemistry 41 (2009) 1341–1343