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Involvement of Salicylic Acid in the Establishment of the Rhizobium meliloti-Alfalfa Symbiosis

¹Departamento de Microbiología del Suelo y Sistemas Simbióticos; ²Departamento de Bioquímica y Biología Molecular y Celular de Plantas, Estación Experimental del Zaidín, CSIC, Profesor Albareda, 1. 18008 Granada, Spain; ³Department of Molecular Biology, Wageningen Agricultural University, 67034A Wageningen, The Netherlands

Inoculation of alfalfa plants with either incompatible Rhizobium or a Rhizobium mutant blocked in Nod factor synthesis led to an accumulation of salicylic acid in roots, in contrast to plants inoculated with a wild-type (compatible) R. meliloti strain. When salicylic acid was exogenously applied prior to inoculation of alfalfa plants with either purified Nod factor or compatible Rhizobium strains, a significant inhibition of nodule primordia formation and a reduction of the number of emerging nodules, respectively, as well as a delay in nodule visualization, were observed. These results suggest an involvement of Rhizobium-synthesized Nod factors in the inhibition of salicylic acid-mediated defense in legumes.

Additional keywords: hypersensitive response, plant defense, systemic acquired resistance.