The arbuscular mycorrhiza development has been studied in transgenic tobacco plants expressing an agrobacterium iaa M gene of auxin biosynthesis (iaaM-plants) and plants with a heterologous gene of isoprenoid biosynthesis pathway hmg1 from Arabidopsis thaliana in the sense and antisense orientations relative to the promoter (hmg1- and as-hmg1-plants, respectively). The mycorrhiza occurrence frequency has been shown to be higher in iaaM- and hmg1-plants as compared to control and as-hmg1-plants.