Activation of the ChvG-ChvI pathway promotes survival during cell wall stress in Agrobacterium tumefaciens

Agrobacterium tumefaciens shifts from a free-living soil bacterium to a plant-invading state upon encountering the plant root microenvironment. The acid-induced two-component sensor system ChvG-ChvI drives this shift and triggers a complex transcriptional program that promotes host invasion and survival against host immune defenses. Remarkably, ChvG-ChvI is also activated under cell wall stress conditions, suggesting that the transcriptional response may have a broader function. Here, we find that blocking cell wall synthesis either genetically or chemically leads to ChvG-ChvI activation. Mutations in key cell wall synthesis enzymes, such as penicillin-binding protein 1a and FtsW, suppress ChvG-ChvI activation in cell wall stress inducing conditions, suggesting that providing structural integrity is a primary function of the ChvG-ChvI regulon. Here, we investigated regulon components for this function. First, deletion of exoA, a gene required for production of the exopolysaccharide succinoglycan, confers resistance to multiple β-lactam antibiotics targeting different enzymes. Next, a class D β-lactamase is expressed that may contribute to the high level of β-lactam resistance in A. tumefaciens. Finally, outer membrane proteins are upregulated, suggesting that outer membrane remodeling may compensate for the accumulation of cell wall damage by providing structural integrity. Overall, we expand our understanding of mechanisms driving ChvG-ChvI activation and β-lactam resistance in a bacterial plant pathogen.