Flavonoid-induced calcium signalling in Rhizobium leguminosarum bv. viciae

• Legume–rhizobium symbiosis requires a complex dialogue based on the exchange of diffusible signals between the partners. Compatible rhizobia express key nodulation (nod) genes in response to plant signals – flavonoids – before infection. Host plants sense counterpart rhizobial signalling molecules – Nod factors – through transient changes in intracellular free-calcium. Here we investigate the potential involvement of Ca2+ in the symbiotic signalling pathway activated by flavonoids in Rhizobium leguminosarum bv. viciae. • By using aequorin-expressing rhizobial strains, we monitored intracellular Ca2+ dynamics and the Ca2+ dependence of nod gene transcriptional activation. • Flavonoid inducers triggered, in R. leguminosarum, transient increases in the concentration of intracellular Ca2+ that were essential for the induction of nod genes. Signalling molecules not specifically related to rhizobia, such as strigolactones, were not perceived by rhizobia through Ca2+ variations. A Rhizobium strain cured of the symbiotic plasmid responded to inducers with an unchanged Ca2+ signature, showing that the transcriptional regulator NodD is not directly involved in this stage of flavonoid perception and plays its role downstream of the Ca2+ signalling event. • These findings demonstrate a key role played by Ca2+ in sensing and transducing plant-specific flavonoid signals in rhizobia and open up a new perspective in the flavonoid–NodD paradigm of nod gene regulation.