Evidence for Calcium-mediated Perception of Plant Symbiotic Signals in Aequorin-expressing Mesorhizobium Loti

Background: During the interaction between rhizobia and leguminous plants the two partners engage in a molecular conversation that leads to reciprocal recognition and ensures the beginning of a successful symbiotic integration. In host plants, intracellular Ca(2+) changes are an integral part of the signalling mechanism. In rhizobia it is not yet known whether Ca(2+) can act as a transducer of symbiotic signals.

Results: A plasmid encoding the bioluminescent Ca(2+) probe aequorin was introduced into Mesorhizobium loti USDA 3147(T) strain to investigate whether a Ca(2+) response is activated in rhizobia upon perception of plant root exudates. We find that M. loti cells respond to environmental and symbiotic cues through transient elevations in intracellular free Ca(2+) concentration. Only root exudates from the homologous host Lotus japonicus induce Ca(2+) signalling and downstream activation of nodulation genes. The extracellular Ca(2+) chelator EGTA inhibits both transient intracellular Ca(2+) increase and inducible nod gene expression, while not affecting the expression of other genes, either constitutively expressed or inducible.

Conclusion: These findings indicate a newly described early event in the molecular dialogue between plants and rhizobia and highlight the use of aequorin-expressing bacterial strains as a promising novel approach for research in legume symbiosis.