A mutagenesis program using ethylmethane sulfonate on Medicago fruncafula Caertn cv Jemalong, an annual, autogamous and diploid lucetne, permitted the isolation of a mutant (TE7) unable to establish an effective nitrogen-fixing symbiosis, [Nod+Fix-], with Rhizobium melilofi wild-type strains. The mutant phenotype is characterized by an altered infection process that leads to the formation of two kinds of inefficient nodules on the same root system. A certain proportion of the nodules are small, round, and uninfected, with infection threads limited to the outer root cortical cells. Others develop to a normal elongated shape and are infected; bacterial release occurs but the bacteria do not differentiate into bacteroids. l h e ratio of invaded to uninvaded nodules depends on the bacterial strain used. Throughout the infection process, certain events correlated with the plant defense response against pathogens can be observed: (a) the presence of polyphenolic compounds associated with the walls of infected cells and also with some parts of infection threads in the root cortex; (b) appositions on infection thread walls during the early stage of infection and also within the central tissue of infected nodules; and (c) autophagy of the plant cells that contain released bacteria. Cenetic data suggest that the phenotype of TE7 is under monogenic and recessive control; this gene has been designated Mtsyml.