Abstract
Chemotactic bacteria sense environmental changes via dedicated receptors that bind to extra- or intracellular cues and relay this signal to ultimately alter direction of movement toward beneficial cues and away from harmful environments. In complex environments, such as the rhizosphere, bacteria must be able to sense and integrate diverse cues. Azospirillum brasilense is a microaerophilic motile bacterium that promotes growth of cereals and grains. Root surface colonization is a prerequisite for the beneficial effects on plant growth but how motile A. brasilense navigates the rhizosphere is poorly studied. Previously only 2 out of 51 A. brasilense chemotaxis receptors have been characterized, AerC and Tlp1, and only Tlp1 was found to be essential for wheat root colonization. Here we describe another chemotaxis receptor, named Aer, that is homologous to the Escherichia coli Aer receptor, likely possesses an FAD cofactor and is involved in aerotaxis (taxis in an air gradient). We also found that the A. brasilense Aer contributes to sensing chemical gradients originating from wheat roots. In addition to A. brasilense Aer having a putative N-terminal FAD-binding PAS domain, it possesses a C-terminal PilZ domain that contains all the conserved residues for binding c-di-GMP. Mutants lacking the PilZ domain of Aer are altered in aerotaxis and are completely null in wheat root colonization and they also fail to sense gradients originating from wheat roots. The PilZ domain of Aer is also vital in integrating Aer signaling with signaling from other chemotaxis receptors to sense gradients from wheat root surfaces and colonizing wheat root surfaces.
Highlights
The rhizosphere is a chemical milieu, and rhizobacteria must navigate this environment to find habitable niches
This chemotaxis receptor is predicted to be membrane anchored and to possess a cytoplasmic N-terminal sensory domain comprised of a PAS/PAC domain followed by, two transmembrane domains, a HAMP domain, a MA, and a C-terminal PilZ domain (Figure 1A)
When we overexpressed AZOBR_p280026 in A. brasilense, the total FAD content of cells increased, suggesting this chemotaxis receptor likely behaves like E. coli Aer and A. brasilense AerC (Figure 1C)
Summary
The rhizosphere is a chemical milieu, and rhizobacteria must navigate this environment to find habitable niches Chemotactic bacteria adjust their motility in response to environmental chemicals, swimming toward favorable signals (attractants) and away from harmful chemicals. Soil bacteria are characterized by both a large number of chemotaxis receptors and multiple chemotaxis signaling systems (Buchan et al, 2010; Scharf et al, 2016), and plant-associated soil bacteria are more likely to encode chemotaxis genes than non-plant associated bacteria (Levy et al, 2018) These features are thought to provide a competitive advantage within the chemically rich and complex environment of the rhizosphere (Scharf et al, 2016)
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