TE7, AN INEFFICIENT SYMBIOTIC MUTANT OF MEDICAGO-TRUNCATULA GAERTN CVJEMALONG

A mutagenesis program using ethylmethane sulfonate on Medicago truncat ula Gaertn cv Jemalong, an annual, autogamous and diploid lucerne, per mitted the isolation of a mutant (TE7) unable to establish an effectiv e nitrogen-fixing symbiosis, [Nod(+)Fix(-)], with Rhizobium meliloti w ild-type strains. The mutant phenotype is characterized by an altered infection process that leads to the formation of two kinds of ineffici ent nodules on the same root system. A certain proportion of the nodul es are small, round, and uninfected, with infection threads limited to the outer root cortical cells. Others develop to a normal elongated s hape and are infected; bacterial release occurs but the bacteria do no t differentiate into bacteroids. The ratio of invaded to uninvaded nod ules depends on the bacterial strain used. Throughout the infection pr ocess, certain events correlated with the plant defense response again st pathogens can be observed: (a) the presence of polyphenolic compoun ds associated with the walls of infected cells and also with some part s of infection threads in the root cortex; (b) appositions on infectio n thread walls during the early stage of infection and also within the central tissue of infected nodules; and (c) autophagy of the plant ce lls that contain released bacteria. Genetic data suggest that the phen otype of TE7 is under monogenic and recessive control; this gene has b een designated Mtsym1.