INVOLVEMENT OF SALICYLIC-ACID IN THE ESTABLISHMENT OF THE RHIZOBIUM-MELILOTI - ALFALFA SYMBIOSIS

Inoculation of alfalfa plants with either incompatible Rhizobium or a Rhizobium mutant blocked in Nod factor synthesis led to an accumulatio n of salicylic acid in roots, in contrast to plants inoculated with a wild-type (compatible) R. meliloti strain, When salicylic acid was exo genonsly applied prior to inoculation of alfalfa plants with either pu rified Nod factor or compatible Rhizobium strains, a significant inhib ition of nodule primordia formation and a reduction of the number of e merging nodules, respectively, as well as a delay in nodule visualizat ion, were observed. These results suggest an involvement of Rhizobium- synthesized Nod factors in the inhibition of salicylic acid-mediated d efense in legumes.