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Summary The plant-beneficial bacterium Azospirillum brasilense can swim in liquids and swarm or migrate with the formation of microcolonies in soft media. To get closer to understanding the influence of natural environments on A. brasilense motility, we studied the individual and social movement of the bacterium in the presence of various plant lectins. The lectins with specificity for N -acetyl- β - d -glucosamine oligomers (wheat germ, Solanum tuberosum and Ulex europeus agglutinins) decreased A. brasilense swimming speed and induced the formation of branched–granular colonies instead of the swarming rings. These effects seemed to be a consequence of specific interactions between the agglutinins and the lectin-binding polymers present in the A. brasilense cell envelope. Concanavalin A (with an affinity for terminal α - d -mannosyl and α - d -glucosyl residues) and Phaseolus vulgaris phytohemagglutinin P (with unknown specificity) almost did not affect the motility of A. brasilense .

Indole acetic acid (IAA) production in Azospirillum brasilense strain Sp245 is controlled by a 85 MDa plasmid naturally in this bacterium. In the presence of l-tryptophan, anthranilic acid production and almost no IAA production occurs in a derivative strain harbouring a Tn5-Mob insertion in the 85 MDa plasmid. Agrobacterium tumefaciens strain GM19023, upon transfer of Tn5-Mob labelled 85 MDa plamid of A. brasilense Sp245, gains the ability to produce antranilic acid.