Azospirillum brasilense Chemotaxis Depends on Two Signaling Pathways Regulating Distinct Motility Parameters

Abstract

The genomes of most motile bacteria encode two or more chemotaxis (Che) systems, but their functions have been characterized in only a few model systems. Azospirillum brasilense is a motile soil alphaproteobacterium able to colonize the rhizosphere of cereals. In response to an attractant, motile A. brasilense cells transiently increase swimming speed and suppress reversals. The Che1 chemotaxis pathway was previously shown to regulate changes in the swimming speed, but it has a minor role in chemotaxis and root surface colonization. Here, we show that a second chemotaxis system, named Che4, regulates the probability of swimming reversals and is the major signaling pathway for chemotaxis and wheat root surface colonization. Experimental evidence indicates that Che1 and Che4 are functionally linked to coordinate changes in the swimming motility pattern in response to attractants. The effect of Che1 on swimming speed is shown to enhance the aerotactic response of A. brasilense in gradients, likely providing the cells with a competitive advantage in the rhizosphere. Together, the results illustrate a novel mechanism by which motile bacteria utilize two chemotaxis pathways regulating distinct motility parameters to alter movement in gradients and enhance the chemotactic advantage. IMPORTANCEChemotaxis provides motile bacteria with a competitive advantage in the colonization of diverse niches and is a function enriched in rhizosphere bacterial communities, with most species possessing at least two chemotaxis systems. Here, we identify the mechanism by which cells may derive a significant chemotactic advantage using two chemotaxis pathways that ultimately regulate distinct motility parameters. Bacterial chemotaxis provides a competitive advantage by guiding motile cells in gradients of chemoeffectors toward environments that support growth and metabolism. Chemotaxis contributes to the establishment of various associations of bacteria with eukaryotic hosts (mammals, insects, and plants) and promotes virulence, symbiosis, and the establishment of microbial communities (1). Bacterial chemotaxis and motility are widespread traits encoded in the genomes of bacteria inhabiting diverse environments, and these functions are specifically enriched in microorganisms found in soils (2), suggesting that they provide a significant competitive advantage in this environment. Consistent with these findings, comparative genome analyses of chemotaxis in diverse motile bacteria suggested that most bacteria possess two chemotaxis systems and soil-dwelling bacteria often have more than two chemotaxis systems (3).The molecular mechanism of chemotaxis signal transduction has been deciphered in most detail in the model organism Escherichia coli, which possesses a single chemotaxis system. In E. coli, the chemotaxis signal transduction pathway consists of membrane-bound receptors clustered in dense arrays at the cell poles, where their C-terminal domains associate with cytoplasmic CheA kinase and the CheW scaffolding protein. When stimu...

Optogenetic Manipulation of Cyclic Di-GMP (c-di-GMP) Levels Reveals the Role of c-di-GMP in Regulating Aerotaxis Receptor Activity in Azospirillum brasilense

“…Specifically, we identify the binding of c-di-GMP to particular receptors as a mechanism to modulate the contribution of these receptors to the aerotaxis signaling output. Therefore, A. brasilense aerotaxis depends not only on signaling by dedicated chemotaxis receptors (5,26,30), as expected, but also on intracellular c-di-GMP levels.…”

“…A. brasilense cells respond tactically to oxygen gradients via signaling by two Che systems that coordinately control transient changes in the swimming speed and in the probability of swimming reversals (25,26). Therefore, we next examined the effect of transient changes in intracellular c-di-GMP levels on these motility parameters under steady-state conditions.…”

Uncharacterized Bacterial Structures Revealed by Electron Cryotomography

“…Cells attach to plant roots through a two-step process (32), a rapid reversible adsorption, thought to be mediated by the polar flagellum, and a slow irreversible anchoring, thought to be mediated by an as-yet unidentified surface polysaccharide (33). A recent study reported that mutants in components of the Che4 chemotaxis system are defective in this root colonization (34). A. brasilense cells also attach to conspecifics in the presence of elevated oxygen levels (35).…”

Distinct Domains of CheA Confer Unique Functions in Chemotaxis and Cell Length in Azospirillum brasilense Sp7

“…However, deletion of che1 or of the gene for the histidine kinase of the Che1 pathway, cheA1, has a minor effect on chemotaxis (17). An additional Che pathway, which has a more dominant role in controlling chemotaxis, has recently been identified (19). In addition to regulating transient increases in swimming speed, Che1 is implicated in regulating cell length: mutations that render cheA1, che1, or cheY1 nonfunctional also result in cells of relatively shorter lengths than the wild type, while mutations that abolish the function of enzymes required for chemotaxis receptor adaptation, CheB1 and CheR1, result in cells significantly longer than the wild type, with these effects most evident in dividing cells (17).…”