Phosphorus limitation increases attachment in Agrobacterium tumefaciens and reveals a conditional functional redundancy in adhesin biosynthesis

Bacterial responses to phosphorus limitation, commonly inorganic phosphate (Pi), are important survival mechanisms in a variety of environments. The two-component sensor kinase PhoR and its cognate response regulator PhoB are central to the Pi limitation response of many bacteria and control the large Pho regulon. Limitation for Pi significantly increased attachment and biofilm formation by the plant pathogen Agrobacterium tumefaciens, and this was driven by PhoB. Surprisingly, it was also found that both phoR and phoB were essential in A. tumefaciens. Expression of a plasmid-borne copy of the low affinity Pi transporter (pit) from Sinorhizobium meliloti in A. tumefaciens abolished the phoB and phoR essentiality in A. tumefaciens and allowed direct demonstration of the requirement for this regulatory system in the biofilm response. Increased attachment under Pi limitation required a unipolar polysaccharide (UPP) adhesin. Mutation of a polyisoprenylphosphate hexose-1-phosphate transferase (PHPT) called uppE abolished UPP production and prevented surface attachment under Pi-replete conditions, but this was rescued under Pi limitation, and this rescue required phoB. In low Pi conditions, either uppE or a paralogous gene Atu0102 is functionally redundant, but only uppE functions in UPP synthesis and attachment when Pi is replete. This conditional functional redundancy illustrates the influence of phosphorus availability on A. tumefaciens surface colonization.