Bacterial chemotactic motility is important for the initiation of wheat root colonization by Azospirillum brasilense

Bacteria of the genus Azospirillum are able to colonize plant roots. Using the beta-glucuronidase (GUS) reporter system, various Azospirillum mutants, including mutants affected in chemotactic motility or extracellular polysaccharide biosynthesis, were investigated for their capacity to initiate wheat root colonization at the root hair zones. Only non-flagellated mutants and a generally non-chemotactic mutant exhibited a strongly reduced colonization ability as compared to the wild-type. No role of the Azospirillum calcofluor-binding polysaccharide in primary wheat root colonization could be observed. This is the first report demonstrating directly, by using different motility mutants, the requirement of bacterial motility in the establishment of the Azospirillum-plant root association.