Abstract
Azorhizobium caulinodans ORS571 is a motile soil bacterium that interacts symbiotically with legume host Sesbania rostrata, forming nitrogen-fixing root and stem nodules. Bacterial chemotaxis plays an important role in establishing this symbiotic relationship. To determine the contribution of chemotaxis to symbiosis in A. caulinodans ORS571-S. rostrata, we characterized the function of TlpA1 (transducer-like protein in A. caulinodans), a chemoreceptor predicted by SMART (Simple Modular Architecture Research Tool), containing two N-terminal transmembrane regions. The tlpA1 gene is located immediately upstream of the unique che gene cluster and is transcriptionally co-oriented. We found that a ΔtlpA1 mutant is severely impaired for chemotaxis to various organic acids, glycerol and proline. Furthermore, biofilm forming ability of the strain carrying the mutation is reduced under certain growth conditions. Interestingly, competitive colonization ability on S. rostrata root surfaces is impaired in the ΔtlpA1 mutant, suggesting that chemotaxis of the A. caulinodans ORS571 contributes to root colonization. We also found that TlpA1 promotes competitive nodulation not only on roots but also on stems of S. rostrata. Taken together, our data strongly suggest that TlpA1 is a transmembrane chemoreceptor involved in A. caulinodans-S. rostrata symbiosis.
Highlights
Bacterial motility and chemotaxis are thought to play an important role in adaptation to soil and rhizosphere
N-terminal domain of TlpA1 consists of two transmembrane helices (TM) and a periplasmic ligand-binding domain (LBD), while the C-terminal region consists of a HAMP and a highly conserved MA domain (Figure 1B)
Bacterial chemotaxis is initiated by transmembrane receptor proteins that undergo conformational changes upon ligand binding (Falke and Hazelbauer, 2001)
Summary
Bacterial motility and chemotaxis are thought to play an important role in adaptation to soil and rhizosphere. Chemotaxis is an important factor in establishing the initial chemical and physical contact between the bacteria and host roots (Caetano-Anollés et al, 1988b; Miller et al, 2007; Scharf et al, 2016). Chemotaxis towards root exudates has been suggested to be the first step in bacterial colonization of plant roots (Vande Broek and Vanderleyden, 1995; Gupta, 2003; GreerPhillips et al, 2004). In the rhizobia-legume association, chemotaxis towards chemoattractants released from roots can promote infection and nodulation by guiding rhizobia to proper site for nodulation (Ames and Bergman, 1981; Bauer and Caetano-Anollés, 1990; GreerPhillips et al, 2004).
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