Dénarié et al., 1992). Nodulation is regulated by a two-way exchange of plant and bacterial signal molecules (Fisher and Long, 1992;Spaink et al., 1993). Signal exchange begins in the rhizosphere. Rhizobia sense flavonoids secreted by seeds and by the host plant root, which activate the constitutively expressed nodD gene product (reviewed by Schlaman et al., 1992). This step is a determinant of host plant specificity in some Rhizobium-host plant combinations (Horvath et al., 1987; Spaink et al., 1987). Transcription of structural nod genes is under the control of NodD and results in production and secretion of specific lipochitin oligosaccharides (Nod factors), which act as host plant-specific inducers of nodule morphogenesis (reviewed by Dénarié and Cullimore, 1993;Spaink et al., 1993). Most probably, Nod factors constitute the primary determinants of host-plant specificity of nodulation.Root lectins are plant factors involved in nodulation. Lectins are nonenzymatic sugar-binding (g1yco)proteins characterized by sugar-binding specificity. For example, root PSL is Glc/Man specific (Díaz et al., 1990) and white clover root lectin is 2-deoxyglucose specific (Sherwood et al., 1984). In general, the interaction between RI uiciae and white clover does not proceed beyond root hair curling, and these bacteria are not able to penetrate host plant cells by infection thread formation (Yao and Vincent, 1969;Djordjevic et al., 1986;Huang et al., 1993). However, RI viciae is able to induce infection thread formation and nodulation in a significant percentage of white clover plants with roots transformed with the psl gene, as reported by Díaz et al. (1989Díaz et al. ( , 1995. This heterologous nodulation does not take place when a psl mutant, which encodes a non-sugar-binding protein, is used for transformation of white clover roots (Van Eijsden et al., 1992. These results strongly suggest that (a) PSL is functionally expressed in transgenic white clover roots, (b) conservation of the sugar-binding activity of PSL is necessary for infection of white clover by RI viciae, and (c) root lectin helps to break a host plant-specific barrier for nodulation. The relationship between Nod factor recognition by host plants and Rhizobium recognition by host lectin is not yet clear.PSL is an ellipsoidal, nonglycosylated dimeric lectin with two sugar-binding sites, each located near the top of two identical monomers (Einspahr et al., 1988). PSL is encoded by a single gene (Gatehouse et al., 1987;Kaminski et al., 1987) and is synthesized as a single translation product, a prolectin with a molecular mass of 28 kD. Posttranslational modifications result in a protein that dissociates into two p subunits (18 kD each) and two 01 subunits (6 kD each, Higgins et al., 1983)
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