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Identification and characterization of hemagglutinins at different stages of bacterial wilt disease

Devanshi Khokhani: University of Wisconsin Madison

<div>Attachment to host surfaces is a critical early step in plant pathogenesis. The wide host range bacterial wilt pathogen <em>Ralstonia solanacearum (Rs) </em>must attach to roots before it invades and colonizes host plant xylem vessels. The <em>Rs</em> genome encodes 18 predicted non-fimbrial adhesins (hemagglutinins). Interestingly, many of these hemagglutinin genes are upregulated in a quorum sensing mutant which is locked in the pathogen’s low cell density mode, presumably corresponding to early disease. However, a distinct set of hemagglutinin genes are upregulated only at high cell density. Using scanning electron microscopy, tomato root attachment assays, and plant colonization experiments, we found that quorum sensing mutants attach to roots, xylem walls, and other <em>Rs</em> cells much better than wild-type, but are defective in spreading in the plant. These results suggest that <em>Rs</em> attaches to host surfaces avidly early in the infection process, but eventually becomes less sticky to facilitate dispersal within the plant. Phylogenetic analysis revealed a cluster of hemagglutinins with novel bacterial collagen-like domains. <em>Rs</em> mutants lacking one or more of these collagen-domain adhesins did not colonize xylem effectively. Together the study suggests that different kinds of attachment are adaptive at various points in bacterial wilt disease development and also provides novel targets for disease control.</div>