Identification of Chemotaxis Compounds in Root Exudates and Their Sensing Chemoreceptors in Plant-Growth-Promoting Rhizobacteria Bacillus amyloliquefaciens SQR9

Abstract

Chemotaxis-mediated response to root exudates, initiated by sensing-specific ligands through methyl-accepting chemotaxis proteins (MCP), is very important for root colonization and beneficial functions of plant-growth-promoting rhizobacteria (PGPR). Systematic identification of chemoattractants in complex root exudates and their sensing chemoreceptors in PGPR is helpful for enhancing their recruitment and colonization. In this study, 39 chemoattractants and 5 chemorepellents, including amino acids, organic acids, and sugars, were identified from 98 tested components of root exudates for the well-studied PGPR strain Bacillus amyloliquefaciens SQR9. Interestingly, mutant stain SQR9Δ8mcp, with all eight putative chemoreceptors completely deleted, lost the chemotactic responses to those 44 compounds. Gene complementation, chemotaxis assay, and isothermal titration calorimetry analysis revealed that McpA was mainly responsible for sensing organic acids and amino acids, while McpC was mostly for amino acids. These two chemoreceptors may play important roles in the rhizosphere chemotaxis of SQR9. In contrast, the B. amyloliquefaciens-unique chemoreceptor McpR was specifically responsible for arginine, and residues Tyr-78, Thr-131, and Asp-162 were critical for arginine binding. This study not only deepened our insights into PGPR-root interaction but also provided useful information to enhance the rhizosphere chemotaxis mobility and colonization of PGPR, which will promote their application in agricultural production.

Recognition of dominant attractants by key chemoreceptors mediates recruitment of plant growth‐promoting rhizobacteria

“… Note: Apart from the 116 compounds listed in this Table; there is evidence for the presence of at least 72 other compounds that, however, could not be identified. Compounds identified as attractants or repellents (r) in a previous study (Feng et al, ) are shown in bold. Chemicals detected in this study but absent in the previous study are underlined. a .…”

“…From the chemotaxis and colonization data it is clear that McpA and McpC are central receptors. A previous study has identified the ligands for both receptors (Feng et al, ). From the 188 compounds detected in root exudates at the cotyledon stage, McpA and McpC were found to respond to 29 compounds of which 28 served as chemoattractants while capric acid was a repellent (Feng et al, ).…”

“…To get a comprehensive understanding of the dose–response relationships of these compounds, we summarize in Fig. the tactic responses of wild‐type SQR9 to the 20 attractants as determined here and in our previous study (Feng et al, ). It can be observed that most chemotactic response was dependent on ligand dose concentration, but the EC 50 value (the concentration at which the compound can induce half of the maximal response) for the different compounds was quite variable.…”

“…This strain is chemotactically attracted to cucumber root exudates (Weng et al, ; Zhang et al, ). We have recently determined the chemical composition of cucumber root exudates and demonstrated that 39 compounds (mainly include amino acids, organic acids and sugars) caused chemoattraction whereas five compounds were found to repel the strain (Feng et al, ). Of the eight chemoreceptors in SQR9, we have shown that six of them, McpA, McpB, McpC, TlpA, TlpB and McpR, sensed at least one chemoeffector (Feng et al, ).…”

Role and exploitation of underground chemical signaling in plants

“…demonstrated that the benzoxazinoid 2,4‐dihydroxy‐7‐methoxy‐2 H ‐1,4‐benzoxazin‐3(4 H )‐one (DIMBOA) producing wild‐type maize attracted significantly higher numbers of P. putida than the DIMBOA‐deficient bx1 mutant . Sometimes bacteria are also attracted by a blend of chemo‐attractants secreted by plants, for example the PGPBs Bacillus amyloliquefaciens and Pseudomonas fluorescens are attracted by amino acids and organic acids produced by cucumber and tomato roots …”