Lectins and polysaccharide EPS I have flow-responsive roles in the attachment and biofilm mechanics of plant pathogenicRalstonia
Authors: Carter Mariama D. , Tran Tuan M. , Cope-Arguello Matthew L. , Weinstein Sofia , Li Hanlei , Hendrich Connor , Prom Jessica L. , Li Jiayu , Chu Lan Thanh , Bui Loan , Manikantan Harishankar , Lowe-Power Tiffany , Allen Caitilyn 
Published: June 2024

Bacterial biofilm formation and attachment to hosts are mediated by carbohydrate- binding lectins, exopolysaccharides, and their interactions in the extracellular matrix (ECM). During tomato infectionRalstonia pseudosolanacearum(Rps) GMI1000 highly expresses three lectins: LecM, LecF, and LecX. The latter two are uncharacterized. We evaluated the roles in bacterial wilt disease of LecF, a fucose-binding lectin, LecX, a xylose-binding lectin, and theRpsexopolysaccharide EPS I. Interestingly, single and double lectin mutants attached to tomato roots better and formed more biofilm under static conditionsin vitro. Consistent with this finding, static bacterial aggregation was suppressed by heterologous expression oflecFGMI1000andlecXGMI1000in otherRalstoniastrains that naturally lack these lectins. Crude ECM from a ΔlecF/Xdouble mutant was more adhesive than the wild-type ECM, and LecF and LecX increasedRpsattachment to ECM. The enhanced adhesiveness of the ΔlecF/XECM could explain the double mutant’s hyper-attachment in static conditions. Unexpectedly, mutating lectins decreasedRpsattachment and biofilm viscosity under shear stress, which this pathogen experiences in plant xylem. LecF, LecX, and EPS I were all essential for biofilm development in xylem fluid flowing through cellulose-coated microfluidic channels. These results suggest that under shear stress, LecF and LecX increaseRpsattachment by interacting with the ECM and plant cell wall components like cellulose. In static conditions such as on root surfaces and in clogged xylem vessels, the same lectins suppress attachment to facilitate pathogen dispersal. Thus,Rpslectins have a dual biological function that depends on the physical environment.

Author Summary

Bacterial wilt diseases caused byRalstoniaspecies inflict significant losses on diverse, globally important agricultural plants. The pathogen first colonizes roots and ultimately the water-transporting xylem. There it attaches to host cell walls and other bacterial cells to form biofilms that eventually block xylem vessels and disrupt sap flow. It is not well known howRalstoniaspp. modulate attachment, but precise control of both attachment and dispersal is critical for successful host colonization over the disease cycle. Excessive adhesion could trap bacteria in a toxic or nutrient-depleted environment. Conversely, insufficient adhesion in a flowing environment could displace bacteria from an optimal niche. We provide evidence of dual, environment-specific roles of carbohydrate-binding lectins and exopolysaccharide EPS I inRalstonia pseudosolanacearum(Rps) attachment. In static conditions, whichRpsexperiences on a host root, two lectins suppress bacterial aggregation and adhesion to roots. However, in flowing conditions, whichRpsexperiences in healthy xylem vessels, the same two lectins and EPS I are essential for biofilm development. The lectins increase the biofilm viscosity and support colony structural integrity, likely by interacting with polysaccharides in the biofilm matrix. This novel multifunctionality of bacterial lectins reveals how pathogens adapt to a physically dynamic host environment.