Characterization of the Che4 Signal Transduction Pathway in Taxis Behaviors of Azospirillum brasilense

Azospirillum brasilense is a gram negative, soil diazotroph that colonizes the roots of agronomically important crops. Studies have shown that swimming motility is an important trait for its survival and colonization of plant roots by these bacteria. Taxis responses, such as chemotaxis and aerotaxis, depend on the ability of the organism to bias its movements in the presence of gradients. The recently sequenced genome of A. brasilense has been shown to possess four chemotaxis operons, but the dominant pathway that modulates swimming behavior by affecting swimming bias in this organism is unknown. Characterization of one of the chemotaxis operons, Che1, revealed that it played a role in regulating swimming velocity in gradients and indirectly affected rate of change in swimming direction.

In this study, we determined that the Che4 pathway of Azospirillum brasilense plays a dominant role in controlling rate of change in swimming direction. Our data also suggests that cross-talk between Che4 and the previously characterized Che1 pathway possibly at the level of receptors also contributes to taxis responses in A. brasilense. Finally, characterization of double mutants lacking components of both pathways, suggest presence of additional chemosensory mechanisms that modulate taxis behavior in A. brasilense.