Root-to-Root Travel of the Beneficial Bacterium Azospirillum brasilense

Abstract

The root-to-root travel of the beneficial bacterium Azospirillum brasilense on wheat and soybean roots in agar, sand, and light-textured soil was monitored. We used a motile wild-type (Mot) strain and a motility-deficient (Mot) strain which was derived from the wild-type strain. The colonization levels of inoculated roots were similar for the two strains. Mot cells moved from inoculated roots (either natural or artificial roots in agar, sand, or light-textured soil) to noninoculated roots, where they formed a band-type colonization composed of bacterial aggregates encircling a limited part of the root, regardless of the plant species. The Mot strain did not move toward noninoculated roots of either plant species and usually stayed at the inoculation site and root tips. The effect of attractants and repellents was the primary factor governing the motility of Mot cells in the presence of adequate water. We propose that interroot travel of A. brasilense is an essential preliminary step in the root-bacterium recognition mechanism. Bacterial motility might have a general role in getting Azospirillum cells to the site where firmer attachment favors colonization of the root system. Azospirillum travel toward plants is a nonspecific active process which is not directly dependent on nutrient deficiency but is a consequence of a nonspecific bacterial chemotaxis, influenced by the balance between attractants and possibly repellents leaked by the root.