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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 .

In semiliquid laboratory media, the bacterium Azospirillum brasilense migrates with the formation of swarming rings. It is demonstrated that adsorption of the sulfonated azodye Congo Red confers on A. brasilense the ability to consistently spread in a semiliquid agar with formation of microcolonies. Spontaneous variants of A. brasilense with increased swarming rate are described, as well as variants that swarm in the presence of Congo Red. It is assumed that at least two types of compounds are formed, which are necessary for swarming and/or spreading with the formation of microcolonies and are capable of interacting with Congo Red.

Three mutants of Azospirillum brasilense Sp245 incapable of both formation of the polar flagellum (Fla–-phenotype) and swarming in semisolid media (Swa–-phenotype) were characterized. These mutants were shown to have lost the 85-MDa plasmid and to carry the Tn5-Mob transposon and pSUP5011 vector in different regions of their genomes. With the use of A. brasilense Sp245 gene bank, the capacity for both polar flagellum formation and swarming was restored in the above mutants and in the previously generated transposon mutants A. brasilense Sp245 and S27. The transconjugants obtained were only slightly motile in the liquid culture. In the gene bank of Sp245, the recombinant plasmids carrying wild-type fla/swa loci were identified.

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.

An Azospirillum brasilense Sp245 spontaneous mutant, Sp245.5, lacking indigenous Plasmids of 85 (p85), 120 and 130 Mda produced about two times less indole-3-acetic acid (IAA) in the presence of tryptophan (Trp) than the wild type strain. Positive influence of p85-pSUP5011 cointegrate molecule transfer into A. brasilense Sp245.5 on IAA production in the transconjugants obtained was distinguishable only in suboptimal for IAA biosynthesis conditions. On the contrary, IAA production in Agrobacterium tumefaciens GMI9023 (p85-pSUP5011) increased several times in comparison with A. tumefaciens GMI9023 and was even greater than in the donor A. brasilense Sp245.160 mutant or in the wild type Sp245 strains. Transfer of p85-pSUP5011 into Pseudomonas putida 53a resulted in increase of IAA excretion and decrease of anthranilic acid (Ant) release. A suggestion about the dependence of an hypothetical IAA pathway probably encoded by p85 on the presence of unidentified products of Trp breakdown through Ant or of Ant itself is given.