Transgenic alfalfa (Medicago sativa L. cv Regen) roots carrying genes encoding soybean lectin or pea (Pisum sativum) seed lectin (PSL) were inoculated with Bradyrhizobium japonicum or Rhizobium leguminosarum bv viciae, respectively, and their responses were compared with those of comparably inoculated control plants. We found that nodule-like structures formed on alfalfa roots only when the rhizobial strains produced Nod factor from the alfalfa-nodulating strain, Sinorhizobium meliloti. Uninfected nodule-like structures developed on the soybean lectin-transgenic plant roots at very low inoculum concentrations, but bona fide infection threads were not detected even when B. japonicum produced the appropriate S. meliloti Nod factor. In contrast, the PSL-transgenic plants were not only well nodulated but also exhibited infection thread formation in response to R. leguminosarum bv viciae, but only when the bacteria expressed the complete set of S. meliloti nod genes. A few nodules from the PSL-transgenic plant roots were even found to be colonized by R. leguminosarum bv viciae expressing S. meliloti nod genes, but the plants were yellow and senescent, indicating that nitrogen fixation did not take place. Exopolysaccharide appears to be absolutely required for both nodule development and infection thread formation because neither occurred in PSL-transgenic plant roots following inoculation with an Exo Ϫ R. leguminosarum bv viciae strain that produced S. meliloti Nod factor. Bacteria belonging to the family Rhizobiaceae (Rhizobium, Bradyrhizobium, Azorhizobium, Mesorhizobium, and Sinorhizobium) induce the formation of nitrogenfixing nodules on their leguminous hosts. This symbiotic interaction, which is governed by sequential signal exchange between rhizobia and their symbiotic partners, exhibits a high degree of specificity, and a number of signal molecules involved in the initial stages of this specificity have been extensively studied. Nod factors are synthesized by the products of rhizobial nod genes, which are induced by plantsecreted molecules such as flavonoids (Hirsch, 1992; Long, 1996). The Sinorhizobium meliloti Nod factor consists of a variable-length N-acetylglucosamine oligomer with a C-16 acyl tail at the non-reducing end and a sulfate at the reducing end, whereas the Rhizobium leguminosarum bv viciae Nod factor has a C-18 fatty acyl residue and no sulfate (for review, see Schultze and Kondorosi, 1998). Nod factors are considered the main rhizobial inducËer molecules for nodulation because the purified molecules elicit, in a host-specific way, many of the plant responses observed in the early stages of nodule formation. These responses include changes in free calcium levels and ion balance, alterations in cytoskeletal organization and morphology of root hairs, the initiation of cortical cell divisions (Ccd), and the triggering of nodule development (Spaink et al., 1991; Truchet et al., 1991;