Characterization of CHSA, a New Gene Controlling the Chemotactic Response in Azospirillum brasilense SP7
Nitrogen-fixing bacteria of the genus Azospirillum are able to colonize the plant rhizosphere (Elmerich and Newton, 2007). Effective colonization includes survival in the soil, motility and chemotactic response to root exudates, and mechanisms of attachment to the root system through flagella and capsular polysaccharides as well as phytohormone production. Azospirillum possesses a single polar flagellum and multiple lateral flagella. The polar flagellum is responsible for the swimming motility in liquid media, whereas the lateral flagella are responsible for spreading on solid or semi-solid surfaces. Chemotaxis towards root exudates and to a variety of oxidizable substrates (including sugars, amino acids, and organic acids) as well as to O2 and redox molecules has been reported for Azospirillum. Only metabolizable substrates can serve as chemo-attractants or repellents in Azospirillum (Alexandre et al., 2000). Knowledge of the chemotaxis machinery in Azospirillum is still limited. A cluster of 5 genes with high similarity to cheA, -W, -Y, -B and -R (encoding the central transduction pathway for chemotaxis) and a gene encoding a chemoreceptor, tlp1, have now been identified in A. brasilense (Hauwaerts et al., 2002; Greer-Philips et al., 2004; Stephens et al., 2006).