CheD is a Key Protein Involved in Regulating the Chemotaxis and Pathogenic Life Cycle of Agrobacterium fabrum

Agrobacterium fabrum is the causal agent of crown gall disease. It possesses a chemotaxis system that enables it to detect signals from wounded plants and to initiate the infection process. However, the mechanisms underlying chemotaxis signal transduction and regulation in A. fabrum, as well as their impact on tumorigenesis, remain largely unexplored. In this study, we investigated the chemotaxis protein CheD in A. fabrum. CheD exhibited a sequence similarity of 25 to 27.27% with the previously well-characterized CheD in Bacillus subtilis and Borrelia burgdorferi and featured a C-terminally extended polypeptide. Protein interaction analysis revealed that CheD interacts with two transmembrane chemoreceptors and two non-transmembrane chemoreceptors of A. fabrum. Chemotaxis assays demonstrated that the in-frame deletion of the cheD gene led to impaired chemotactic responses of A. fabrum towards nutritional substrates and plant-derived chemical signals. The CheD mutant lacking the C-terminal 22 amino acids failed to restore chemotactic function in the cheD-deficient mutant. These findings suggest that CheD is essential for the detection of external chemical cues and the mediation of chemotaxis signal transduction in A. fabrum, with its C-terminal extension being critical for functional activity. Furthermore, the deletion of cheD also resulted in the impairment of biofilm formation, exopolysaccharide (EPS) production, and tumorigenesis in A. fabrum. This indicates that CheD influences the Agrobacterium-plant interaction by regulating the early stages of infection. Thus, CheD plays an essential role in the chemotaxis and pathogenic life cycle of A. fabrum.