Background: During the interaction between rhizobia and leguminous plants the two partners\udengage in a molecular conversation that leads to reciprocal recognition and ensures the beginning\udof a successful symbiotic integration. In host plants, intracellular Ca2+ changes are an integral part\udof the signalling mechanism. In rhizobia it is not yet known whether Ca2+ can act as a transducer of\udsymbiotic signals.\udResults: A plasmid encoding the bioluminescent Ca2+ probe aequorin was introduced into\udMesorhizobium loti USDA 3147T strain to investigate whether a Ca2+ response is activated in\udrhizobia upon perception of plant root exudates. We find that M. loti cells respond to\udenvironmental and symbiotic cues through transient elevations in intracellular free Ca2+\udconcentration. Only root exudates from the homologous host Lotus japonicus induce Ca2+ signalling\udand downstream activation of nodulation genes. The extracellular Ca2+ chelator EGTA inhibits both\udtransient intracellular Ca2+ increase and inducible nod gene expression, while not affecting the\udexpression of other genes, either constitutively expressed or inducible.\udConclusion: These findings indicate a newly described early event in the molecular dialogue\udbetween plants and rhizobia and highlight the use of aequorin-expressing bacterial strains as a\udpromising novel approach for research in legume symbiosis.
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