Legume plants are able to fix nitrogen in symbiotic association with rhizobia and, like many crops, are sensitive to high salt conditions. However, very few molecular markers can be associated to stress tolerance in legume crops. A Kruppel-like transcription factor, <i>Mtzpt2-1</i>, required for the formation of the nitrogen-fixing region, confers salt tolerance to yeast cells. Here, legume responses to salt stresses were studied using alfalfa and its close relative <i>Medicago truncatula</i>, a model legume species. Salt stress induces the <i>Mszpt2-1</i> gene both in roots and root harbouring nodules. In addition, <i>Sinorhizobium meliloti</i> strains tolerating up to 700 mM NaCl, were used in nodulation assays to assess salt tolerance of the symbiotic response of <i>M. truncatula</i>. Few nodules, mainly in the upper part of the root, could be detected in plants treated with 200 mM NaCl, suggesting that nodule initiation was particularly sensitive to salt stress. We have also defined for <i>M. truncatula</i> the threshold of NaCl tolerance after which recovery of stressed plants is irreversible under laboratory conditions. After analysing several times of salt treatment (150 mM NaCl), M. truncatula 108R plants stressed for 7 days could not recover (less than 5%), whereas a 4-day treatment allowed at least 75% recovery. Transgenic <i>M. truncatula</i> plants expressing <i>Mtzpt2-1</i> in antisense configuration are more sensitive to `recover' from salt stress than the wild type. These results identify <i>Mtzpt2-1</i> as a molecular marker potentially linked to stress tolerance in <i>M. truncatula</i> and suggest its participation in a transcriptional program induced in these plants to cope with salt stress.Facultad de Ciencias ExactasInstituto de Biotecnología y Biología Molecular