Infections of wound sites in dicotyledonous plants by the gram-negative bacterium Agrobacterium tumefaciens result in the formation of crown gall tumors. The tumors result from the transfer of a piece of DNA (the T-DNA) from the bacterium to the plant host cell. This DNA becomes integrated into the plant DNA, where it codes for the uncontrolled synthesis of plant growth hormones, thus resulting in the formation of a tumor (26). One of the early steps in bacterial pathogenesis is the attachment of the bacteria to plant host cells (13). This attachment is a two-step process. In the first step, the bacteria bind loosely to the plant surface. In the second step, substances released from the plant stimulate the bacteria to elaborate cellulose fibrils. These cellulose fibrils cause the bacteria to bind very tightly to the plant cell surface (15). The fibrils also entrap additional bacteria, resulting in the formation of bacterial aggregates. The majority of the bacteria in these aggregates are bound only indirectly to the plant cell via the network of cellulose fibrils (18).Bacterial mutants which are unable to carry out the first step of loose binding to host cells are avirulent (5,16,31). Bacterial outer membrane proteins have been implicated in this initial attachment to the host (16). However, the identity of the host cell receptor to which the bacteria bind is unknown. The receptor can be removed from the surface of carrot cells by treatment with trypsin or other proteases or by extraction with dilute detergent (9, 23). The host range of biotype 1 strains ofA. tumefaciens is very broad, suggesting that the receptor must be found on the cell surface of many species of plants (4). These observations suggest that the receptor may be a cell surface protein which is highly conserved in evolution.Vitronectin (S protein) is a serum spreading factor found in animals (32). It is an important constituent of the extracellular matrix and is localized at focal adhesions or contact sites in cultured mammalian cells (1). The properties of vitronectin and its role in adhesion and in the stabilization of the cytoskeleton have been discussed in several recent reviews, for example, that by Tomasini and Mosher (32).
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