Effect of Azospirillum inoculants on arbuscular mycorrhiza establishment in wheat and maize plants

Plant growth-promoting rhizobacteria (PGPR) and arbuscular 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 colonisation of the root system in durum wheat and maize plants, grown under both greenhouse and field conditions. The effect of wild type A. brasilense strain Sp245 and genetically-modified derivatives overproducing indole-3-acetic acid (IAA) was assessed at greenhouse level in (a) three different cultivars of durum wheat, in presence of indigenous AM fungi, and (b) maize plants artificially inoculated with Glomus mosseae and G. macrocarpum. Additionally, the establishment of natural AM fungal symbiosis was evaluated using A. lipoferum CRT1 in maize plants, at field level. Despite the stimulatory effect of the different Azospirillum inocula on root growth, no significant differences in AM colonisation were found, independently of the AM fungus involved, either in wheat or maize plants. Similarly, genetically-modified A. 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 genetically-modified Azospirillum phytostimulators does not alter mycorrhization