Expansins are a highly conserved group of cell wall-localized proteins that appear to mediate changes in cell wall plasticity during cell expansion or differentiation. The accumulation of expansin protein or the mRNA for specific expansin gene family members has been correlated with the growth of various plant organs. Because expansin proteins are closely associated with plant cell wall expansion, and as part of a larger study to determine the role of different gene products in the legume-Rhizobium spp. symbiosis, we investigated whether a Melilotus alba (white sweetclover) expansin gene is expressed during nodule development. A cDNA fragment encoding an expansin gene (EXP) was isolated from Sinorhizobium meliloti-inoculated sweetclover root RNA by reverse-transcriptase polymerase chain reaction using degenerate primers, and a full-length sweetclover expansin sequence (MaEXP1) was obtained using 5' and 3' rapid amplification of cDNA end cloning. The predicted amino acid of the sweetclover expansin is highly conserved with the various alpha-expansins in the GenBank database. MaEXP1 contains a series of eight cysteines and four tryptophans that are conserved in the alpha-expansin protein family. Northern analysis and whole-mount in situ hybridization analyses indicate that MaEXP1 mRNA expression is enhanced in roots within hours after inoculation with S. meliloti and in nodules. Western and immunolocalization studies using a cucumber expansin antibody demonstrated that a cross-reacting protein accumulated in the expanding cells of the nodule.