DOI 10.1007/s00374-005-0854-7 ORIGINAL PAPER

Abstract Plant growth-promoting rhizobacteria and ar-buscular mycorrhizal (AM) fungi represent two main groups of beneficial microorganisms of the rhizosphere. The role of different strains of Azospirillum on AM fungi development was evaluated by measuring the percentage of AM colo-nisation of the root system in durum wheat and maize plants, grown under both greenhouse and field conditions. The effect of wild-typeAzospirillum brasilense strain Sp245 and genetically modified (GM) derivatives overproducing indole-3-acetic acid was assessed at greenhouse level in (1) three different cultivars of durumwheat, in the presence of indigenous AM fungi and (2) maize plants artificially in-oculated with Glomus mosseae and Glomus macrocarpum. In addition, the establishment of natural AM fungal sym-biosis was evaluated using Azospirillum lipoferum CRT1 in maize plants at field level. Despite the stimulatory effect of the different Azospirillum inocula on root growth, no sig-nificant differences in AM colonisation were found, in-dependently of the AM fungus involved, either in wheat or in maize plants. Similarly, GMA. brasilense, which strongly stimulates root development, did not affect AM formation. Although these results were obtained in conditions in which the mycorrhization rate was moderate (15–30%), overall considered they indicate that the use of wild-type or GM Azospirillum phytostimulators does not alter mycorrhization