THE POLAR FLAGELLUM MEDIATES AZOSPIRILLUM-BRASILENSE ADSORPTION TO WHEAT ROOTS

Azospirillum brasilense in a motile Gram-negative bacterium that can adapt its flagellation to different environments. Cells growing in a liquid culture possess only a single polar flagellum; growth on a solid surface additionally induces multiple lateral flagella. The polar flagellum is primarily used for swimming, i.e. locomotion of the bacterium in a liquid environment, whereas the lateral flagella allow the bacteria to swarm over a solid surface. We have previously described a completely non-motile A. brasilense mutant (Sp7 p90D084), and shown that this mutant has a drastically reduced adsorption capacity to wheat roots. In the present work, we present several lines of evidence demonstrating that adsorption to wheat roots is mediated by the polar flagellum of A. brasilense. First, the non-adsorbing mutant Sp7 p90D084 forms no polar and no lateral flagella, but is otherwise undistinguishable from wild-type A. brasilense. Second, disintegration of the flagella by heat or acid eliminates adsorption. Third, using a polyclonal antiserum against the polar flagellum filament protein (Fla1), we have isolated out of a collection of 3000 Tn5-B30-induced mutants, three additional and genetically different non-flagellate mutants. Like Sp7 p90D084, these mutants show a severely reduced adsorption capacity to wheat roots. Finally, purified polar flagella bind to wheat roots in vitro.