brasilense (Thirunavukkarasu et al., 2008; Mishra Omipalisib cell line et al., 2011). Chemotaxis is the ability bacteria have to sense gradients of compounds and to drive motility toward the most appropriate niche and is an important trait for survival in the rhizosphere and in plant–microbe interactions (Alexandre, 2010). Signal transduction systems enable cells to detect and

adapt to these changes by executing appropriate cellular responses, such as regulation of gene expression or modulation of the swimming pattern. The best characterized signal transduction system is the one regulating the run or tumble swimming bias via chemotaxis in Escherichia coli (Wadhams & Armitage, 2004). This signal transduction system consists of a set of conserved proteins, which includes CheA, CheW, CheY, CheB, and CheR and a set of chemoreceptors known as methyl-accepting proteins that perceive environmental cues. In A. brasilense, energy taxis is dominant (Fig. 1), Ibrutinib with responses to most stimuli in this bacterium being triggered

by changes in the electron transport system (Alexandre et al., 2000). Greer-Phillips et al. (2004) identified a novel chemoreceptor-like protein, named Tlp1, which serves as an energy taxis transducer. A tlp1 mutant was shown to be deficient in chemotaxis toward several rapidly oxidizable substrates, to taxis to the terminal electron acceptors oxygen and nitrate, and to redox taxis, suggesting that Tlp1 controls energy taxis in A. brasilense. The tlp1 mutant is also impaired in colonization of plant roots (Greer-Phillips et al., 2004). Stephens et al. (2006) characterized the CheB and CheR components of the chemotaxis-like signal transduction pathway Che1 in A. brasilense. Characterization of cheB, cheR, and cheBR null mutants showed that these genes significantly influence chemotaxis and aerotaxis but are not essential for these behaviors, suggesting that multiple chemotaxis systems

are present and contribute to chemotaxis and aerotaxis in A. brasilense. A further study characterized mutants for genes cheA1 and cheY1, also components of the Che1 system. As for the cheB/cheR mutants, these mutants were defective but not null for chemotaxis and aerotaxis, and showed a minor defect in swimming pattern. Detailed characterizations of these second mutants lead the authors to propose that the Che1 chemotaxis-like pathway modulates cell length as well as flocculation (Bible et al., 2008). Recently, Carreño-López et al. (2009) identified gene chsA as an important component of the chemotaxis signaling pathway in A. brasilense. The encoded protein, ChsA, displays characteristic signaling protein architecture, containing a PAS sensory domain and an EAL domain. The authors showed that a chsA null mutant was impaired in surface motility and chemotactic response, although it was not affected in synthesis of polar and lateral flagella, thus strengthening a key role of this gene in chemotaxis.