Inter-root movement of Azospirillum brasilense and subsequent root colonization of crop and weed seedlings growing in soil

Inter-root movement and dispersion of the beneficial bacterium Azospirillum brasilense were monitored in root systems of wheat seedlings growing in the field and in growth chamber soil trays. Two strains were used, a motile wild-type strain (Cd, mot⁺) and a motility deficient strain (mot^–), which was derived from the Cd strain. Root colonization by two wild-type strains (Cd and Sp-245) was studied in 64 plant species growing in pots in the greenhouse. The two wild-type strains of A. brasilense were capable of colonizing all tested plant species. In soil trays and in the field, mot⁺ cells moved from inoculated roots to non-inoculated roots of either wheat plants or weeds growing in the same field plot, but the mot^– strain did not move toward non-inoculated roots of either plant species. In the field, both mot⁺ and mot^– strains of A. brasilense survived well in the rhizosphere of wheat for 30 days, but only mot⁺ moved between different weeds, regardless of the species, botanical family, or whether they were annuals or perennials. In plant-free, water-saturated soils, either in columns or in the field, both strains remained at the inoculation site and did not move.It is proposed (a) that A. brasilense is not a plant-specific bacterium and that (b) colonization of the entire root system in soil is an active process determined by bacterial motility; it is not plant specific, but depends on the presence of plants.Correspondence to: Y. Bashan