Attachment of bacteria to plant cells is one of the earliest steps in many plant-bacterium interactions. This review covers the current knowledge on one of the best-studied examples of bacterium-plant attachment, namely the molecular mechanism by which Rhizobium bacteria adhere to plant roots. Despite differences in several studies with regard to growth conditions of bacteria and plants and to methods used for measuring attachment, an overall consensus can be drawn from the available data. Rhizobial attachment to plant root hairs appears to be a two-step process. A bacterial Ca(2+)-binding protein, designated as rhicadhesin, is involved in direct attachment of bacteria to the surface of the root hair cell. Besides this step, there is another step which results mainly in accumulation and anchoring of the bacteria to the surface of the root hair. This leads to so-called firm attachment. Depending on the growth conditions of the bacteria, the latter step is mediated by plant lectins and/or by bacterial appendages such as cellulose fibrils and fimbriae. The possible role of these adhesions in root nodule formation is discussed.

Attachment of Rhizobium and Agrobacterium bacteria to cells of their host plants is a two-step process. The first step, direct attachment of bacteria to the plant cell wall, is mediated by the bacterial protein rhicadhesin. A putative plant receptor molecule for rhicadhesin was purified from cell walls of pea roots using a bioassay based on suppression of rhicadhesin activity. This molecule appeared to be sensitive to treatments with pronase or glycosidase. Its isoelectric point is 6.4, and its apparent molecular mass was estimated to be 32 kDa before and 29 kDa after glycosidase treatment, as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and ultrafiltration. The sequence of the first 29 N-terminal amino acids was determined: A-D-A-D-A-L-Q-D-L-C(?)-V-A-D-Y-A-S-V-I-L- V-N-G-F-A-S-K(Q)-(P/Q)-(L)-(I). No homology with known proteins was found. In the course of this research project the extracellular matrix protein vitronectin was reported to inhibit attachment of A. tumefaciens to carrot cells [29]. A variety of adhesive proteins, including vitronectin, contain a common cell attachment determinant with the sequence R-G-D. Since we could not detect other cell wall components able to suppress rhicadhesin activity, and since an R-G-D containing hexapeptide was also active as a receptor, we speculate that the plant receptor for rhicadhesin is a glycoprotein containing an R-G-D attachment site.

Cyclic 1-1,2-glucan is considered to play a role in osmoadaptation of members of the family Rhizobiaceae in hypotonic media. Agrobacterium tumefaciens chvB mutants, lacking j0-1,2-glucan, exhibit a pleiotropic phenotype, including nonmotility, attachment deficiency, and avirulence. Here we report that by growth of chvB mutant cells in tryptone-yeast extract medium supplemented with 7 mM CaCl2 and 100 mM NaCi, the mutant cells become motile, attach to pea root hair tips, and are virulent on Kalanchoi leaves. Moreover, whereas chvB mutants grown in tryptone-yeast extract medium containing 7 mM CaCl2 do not produce active rhicadhesin, addition of 100 mM NaCl to this medium resulted in restoration of rhicadhesin activity. The presence of CaCl2 appeared to be required for attachment, virulence, and activity of rhicadhesin. The results support a role for cyclic j0-1,2-glucan in osmoadaptation and strengthen the notion that rhicadhesin is required for attachment and virulence of A. tumefaciens.Attachment to plant cells is one of the early steps in the infection process of the plant pathogen Agrobacterium tumefaciens. Attachment of A. tumefaciens is a two-step process. The first step, direct attachment of bacteria to plant cells, is mediated by a bacterial Ca2"-binding protein called rhicadhesin (15). The second step, anchoring of bacteria to each other and to the plant cell surface, is established by bacterial cellulose fibrils (8). Another component reported to be required for agrobacterial virulence and suggested to be involved in attachment is cyclic ,B-1,2-glucan (12).Mutants defective in synthesis or export of cyclic ,B-1,2-glucan, chvB and chvA mutants, respectively, were found to be avirulent (4). The chvB gene encodes a protein in the cytoplasmic membrane that catalyzes the synthesis of cyclic ,-1,2-glucan (19). chvB mutants show a pleiotropic phenotype. They are avirulent, are attachment deficient, show reduced motility (3), and show some additional alterations, probably all linked to changes in the cell envelope (17). Moreover, chvB mutants lack active rhicadhesin (18). Miller and coworkers (9) proposed a role for cyclic 3-1,2-glucan in osmoadaptation. These authors observed an accumulation of cyclic P-1,2-glucan in the periplasmic space when wild-type bacteria were grown under hypo-osmotic conditions. If lack of cyclic P-1,2-glucan results in inadequate osmoadaptation, this may be the cause of the various cell envelope deficiencies in chvB mutants. Those deficiencies, especially lack of active rhicadhesin, could yield the avirulence and attachment deficiency phenotypes of chvB mutants. If so, growth under high-osmotic-strength conditions may restore attachment ability and virulence of chvB mutants.We therefore investigated the effect of osmotic strength on growth, motility, attachment, rhicadhesin activity, and virulence of chvB mutants.Growth and motility. Bacteria were grown at 28°C in 100-ml Erlenmeyer flasks containing 50 ml of medium on a rotary shaker (180 rpm

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In contrast to wild-type Agrobacterium tumefaciens strains, beta-1,2-glucan-deficient chvB mutants were found to be unable to attach to pea root hair tips. The mutants appeared to produce rhicadhesin, the protein that mediates the first step in attachment of Rhizobiaceae cells to plant root hairs, but the protein was inactive. Both attachment to root hairs and virulence of the chvB mutants could be restored by treatment of the plants with active rhicadhesin, whereas treatment of plants with beta-1,2-glucan had no effect on attachment or virulence. Moreover, nodulation ability of a chvB mutant carrying a Sym plasmid could be restored by pretreatment of the host plant with rhicadhesin. Apparently the attachment-minus and avirulence phenotype of chvB mutants is caused by lack of active rhicadhesin, rather than directly being caused by a deficiency in beta-1,2-glucan synthesis. The results strongly suggest that rhicadhesin is essential for attachment and virulence of A. tumefaciens cells. They also indicate that the mechanisms of binding of Agrobacterium and Rhizobium bacteria to plant target cells are similar, despite differences between these target cells.