Legume lectins have been proposed to have important symbiotic roles during Rhizobium-legume symbioses. To test this hypothesis, the symbiotic responses of transgenic alfalfa plants that express a portion of the putative alfalfa lectin gene MsLEC1 or MsLEC2 in either the antisense or sense orientation were analyzed following inoculation with wild-type Sinorhizobium meliloti 1021. MsLEC1-antisense (LEC1AS) plants were stunted, exhibited hypernodulation, and developed not only abnormally large nodules but also numerous small nodules, both of which senesced prematurely. MsLEC2-antisense plants were intermediate in growth and nodule number compared with LEC1AS and vector control plants. The symbiotic abnormalities of MsLEC1-sense transgene plants were similar to but milder than the responses shown by the LEC1AS plants, whereas MsLEC2-sense transgene plants exhibited symbiotic responses that were identical to those of vector and nontransgenic control plants. MsLEC1 mRNA accumulation was not detected in nodule RNA by Northern blot analysis but was localized to alfalfa nodule meristems and the adjacent cells of the invasion zone by in situ hybridization; transcripts were also detected in root meristems. A similar spatial pattern of MsLEC2 expression was found by using a whole-mount in situ hybridization procedure. Moreover, mRNAs for an orthologous lectin gene (MaLEC) were detected in white sweetclover (Melilotus alba) nodules and root tips.