A mutation in the <i>ilv</i>C gene of <i>Sinorhizobium meliloti</i> 1021 determines a symbiotically defective phenotype, <i>ilv</i>C mutants obtained from different <i>S. meliloti</i> wild-type strains are able to induce root hair deformation on alfalfa roots and show variable activation of the common nodulation genes <i>nod</i>ABC. All of these mutants are noninfective. The presence of extra copies of <i>nod</i>D3-<i>syr</i>M in an IlvC<SUP>‒</SUP> background does not promote <i>nod</i> expression but allows the detection of low levels of Nod factor production. The sulphation of the Nod factor metabolites, however, is not affected. Furthermore, IlvC<SUP>‒</SUP> strains induce a specific pattern of starch accumulation on alfalfa roots as well as of early nodulin expression. Hence, the pleiotropic action of the <i>ilv</i>C gene in <i>S. meliloti</i> may reveal novel complexities involved in the symbiotic interaction.Instituto de Biotecnologia y Biologia MolecularFacultad de Ciencias Exactas