*Corresponding author. E-mail : babana_ama@yahoo.fr Intercontinental Journal of Microbiology Vol. 1(1), pp. 01-07, 29 November, 2012 Available online at http://www.intercontinentaljournals.org/IJBS.htm Full Length Research Paper Effect of Pseudomonas sp. on wheat roots colonization by mycorhizal fungi and phosphate-solubilizing microorganisms, wheat growth and P-uptake Amadou Hamadoun Babana 1 *, Hani Antoun 2 , Amadou Hamadoun Dicko 1 , Kadia Maïga 1 and Diakaridia traoré 1 1 Laboratory of Research in microbiology and microbial biotechnology, Faculty of Sciences and Techniques, University of Sciences Techniques and Technologies of Bamako, Bamako Mali BP E3206. 2 Department of Soils and Agrofood engineering, Faculty´of Agriculture and food sciences, Pavillon Charles-Eugene Marchand, Laval Universityl, Quebec Qc, Canada G1K 7P4. Accepted 29 November, 2012 The effect of Pseudomonas sp. BR2 on the distribution of phosphate-solubilizing microorganisms (PSM) in wheat rhizosphere and wheat roots colonization by indigenous and introduced mycorhizal fungi was checked in a soil-plant test. Wheat plants were cultivated during 4 weeks in a soil with low phosphorus content, fertilized with Tilemsi rock phosphate (TRP). Inoculation treatments include Pseudomonas sp. BR2 and Glomus intraradices. According to soil types (sterilized or not), the rate of seed inoculation affect differently wheat roots colonization by BR2, but no significant difference was observed between soil types. Inoculation with BR2, in absence of TRP fertilization, had no effect on the total microbial population, but strongly increased PSM number in wheat rhizosphere. Co-inoculation with G. intraradices and BR2 significantly reduced phosphate-solubilizing bacteria (PSB) population size and, at the same time, increased the population size of phosphate-solubilizing fungi (PSF). After TRP application, BR2 increased significantly the population of phosphate-solubilizing microorganisms, with a maximum effectiveness in presence of G. intraradices. The percentage of the mycorhized wheat roots was significantly stimulated by Pseudomonas sp. BR2 after in TRP application. In addition to phosphate solubilization, BR2 stimulated wheat growth by improving the establishment of mycorhizes and phosphate solubilizing microorganisms in wheat rhizosphère. Key words: Pseudomonas sp., PGPR, root colonization, wheat, rock phosphate, solubilization, Mali. INTRODUCTION Phosphorus (P) deficiency is a major constraint to crop, particularly wheat, production in Mali. However, the the use of the Tilemsi phosphate rock (TPR) of Mali, a good and cheaper alternative to imported phosphate fertilizers as phosphorus source is limited by its least inefficacency in many agricultural soils. While many soil microorganisms can also mobilize sparingly soluble inorganic phosphates, no biological fertilizer is available in Mali. This work investigates the possibility to use Pseudomonas sp. BR2, to formulate a biofertilizer. The essential aim of biofertilizer technology is the development of inoculant composed of selected microorganisms to minimize the application of chemical fertilizers and maximize plants growth and nutrition (Probanza et al., 2002). So, microorganisms, to be used to formulate a biological inoculant, must be selected in a way to improve the positive effect produced by each group of microorganisms thereby improving plant growth in infertile soils (Saleh et al., 1998; Sundara et al., 2002). Mycorrhizal fungi are recognized for their ability to increase the productivity of host plants by facilitating the absorption of phosphorus and other elements (Barea et al., 2002; Villegas and Fortin, 2001). Mycorrhized plants increase phosphorus absorption using several mechanisms including the: physical exploration of soil, which reduce the diffusion distance between phosphate ions and increase the surface area for absorption of these ions; solubilization of rock phosphate by ectomycorrhizal fungi; storage of the absorbed phosphorus; transfert of phosphorus to plant roots and the its uses in the plant (Bolan et al., 1991). Mycorrhizal fungi differ in their ability to improve the absorption of nutrients, and roots colonized by mycorrhizal fungi may alter growth conditions of certain groups of microorganisms (Schreiner et al., 1997). Some rhizospheric bacteria are known for their ability to