A biovar-specific signal of Rhizobium leguminosarum bv. viciae induces increased nodulation gene-inducing activity in root exudate of Vicia sativa subsp. nigra.

Flavonoids in root exudate of leguminous plants activate the transcription of Rhizobium genes involved in the formation of root nodules (nod genes). We report that inoculation with the homologous symbiont R. leguminosarum bv. viciae results in an increased nod gene-inducing activity (Ini) in root exudate of V. sativa subsp. nigra, whereas inoculation with heterologous Rhizobium strains results in exudates with nod gene-inducing activity comparable to that of uninfected plants. Ini can be demonstrated by using either of the isogenic indicator strains containing an inducible nod promoter fused to the Escherichia coli lacZ reporter gene and the regulatory nodD gene of R. leguminosarum bv. viciae, R. leguminosarum bv. trifolii, or R. meliloti. The presence of genes nodDABCEL of R. leguminosarum bv. viciae appeared to be essential for induction of Ini. Mutation of the genes nodI and nodJ causes a delay of Ini, whereas gene nodF appears to be required for both the timely appearance and the maximum level of Ini activity. The nodE gene is responsible for the biovar specificity of induction of Ini by Rhizobium spp. Ini is caused by a soluble heat-stable factor of rhizobial origin. This Rhizobium-produced Ini factor has an apparent molecular weight between 1,000 and 10,000 and does not originate from flavonoid precursors.