Activation of the ChvG-ChvI pathway promotes multiple survival strategies 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 or outer membrane proteins, such as PBP1a, FtsW, and AopA1, suppress ChvG-ChvI activation suggesting that providing structural integrity is a primary function of the ChvG-ChvI regulon. Here, we investigated regulon components for this function. First, the exopolysaccharide succinoglycan confers tolerance to multiple β-lactam antibiotics targeting different enzymes by forming a protective barrier around the cells. Next, a Class D β-lactamase is expressed which may contribute to the high level of β-lactam resistance in A. tumefaciens . Finally, outer membrane remodeling compensates 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.