Agrobacterium-host attachment and biofilm formation

Abstract

Physical association with host plant tissue is a prerequisite to Agrobacterium tumefaciens infection and subsequent disease. Mechanisms of tissue adherence have been extensively studied in mammalian pathogens, but less so in plant-associated bacteria. Cells of A. tumefaciens often attach to plant tissue by a single pole. In the appropriate environment, these attached bacteria eventually develop into multicellular assemblies called biofilms, enmeshed within exopolymeric material produced by the bacteria and possibly the plant host. It remains unclear whether all modes of plant attachment can lead to interkingdom gene transfer, or whether the conformation of the infecting agrobacterial population influences this process. A two-step model was proposed in which the bacterium initially attaches reversibly by way of interactions between a bacterial adhesin structure(s) and a plant receptor(s), followed by a more tenacious attachment coincident with production of cellulose fibrils. This adherence model, while potentially still valid, remains largely untested. Possible A. tumefaciens adherence functions, including lipopolysaccharides and cyclic β-1,2-glucans have been identified, but none has been definitively shown to mediate productive attachment to plants. Similarly, despite some promising leads, no confirmed plant receptor candidates have been identified. A. tumefaciens forms biofilms on a variety of surfaces including but not restricted to plant tissues. Studies of biofilm formation by A. tumefaciens on model surfaces have revealed a degree of structural and functional overlap with plant association, including several common cell surface structures and key regulatory pathways. © 2008 Springer-Verlag New York.