Production ofSinorhizobium meliloti nodGene Activator and Repressor Flavonoids fromMedicago sativaRoots

Abstract

During symbiosis between leguminous plants and rhizobia, flavonoids exuded by the plants act as chemoattractants and nodulation (nod) gene regulators in the other partner. To better understand the role of these compounds during the early steps of the alfalfa-Sinorhizobium meliloti symbiosis and the regulation of their production we have isolated nod gene inducers from alfalfa roots. All the compounds that we identified in this study as nod gene inducers in the root are flavonoids, indicating that other compounds with nod gene activator capacity may have little contribution, if any, to nod gene activation. Most of the intermediates of the flavonoid pathway were found in Medicago sativa roots and nodules, but only end products of the flavonoid pathway were identified in the root exudate. We have also studied flavonoid production in different parts of the root and found that it is developmentally regulated during root growth. Finally, we have shown that coumestrol and medicarpin, present in the exudates and previously described as phytoalexins, possess nod gene repressing activity, indicating that the in vivo nod gene inducing activity of the root exudate results from positive as well as negative controls of nod gene expression by the flavonoids.

Flavonoids and Auxin Transport Inhibitors Rescue Symbiotic Nodulation in the Medicago truncatula Cytokinin Perception Mutant cre1

“…As the cre1 mutant was partially defective in the induction of isoliquiritigenin, which can act as a nod gene inducer in S. meliloti (Zuanazzi et al, 1998), this may affect the ability of rhizobia to induce nodules in the cre1 mutant. However, in our study, we used the A2102 strain harboring the pE65 plasmid, which expresses the NodD3 gene from a constitutive promoter (Barnett et al, 2004).…”

Phenolic acids act as signaling molecules in plant-microbe symbioses

“…Flavonoid production is also induced by rhizobia in roots and nodules 81 which regulate Nod (nodulation) factor production prior to and during infection. 82 It is possible that flavonoids like phenolic acids protect dividing cells from oxidative damage because of their activity as antioxidants. 83 Leguminous plants secrete a variety of phenolic compounds from roots such as flavones and flavonols from Vicia faba, 20 isoflavonoids from soybean, 84 and vanillin from peanut 85 to mention a few.…”

Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

“…Glycosides, however, are usually less active in inducing nod genes (199). Flavonoids are released in their greatest amounts near root tips (180,198,382), and optimal concentrations occur near the emerging root hair zone, which is the most favorable site for rhizobium infection (553).…”