Evidence for calcium-mediated perception of plant symbiotic signals in aequorin-expressing Mesorhizobium loti

A plasmid encoding the bioluminescent Ca2+ probe aequorin was introduced into Mesorhizobium loti USDA 3147T strain to investigate whether a Ca2+ 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 Ca2+ concentration. Only root exudates from the homologous host Lotus japonicus induce Ca2+ signalling and downstream activation of nodulation genes. The extracellular Ca2+ chelator EGTA inhibits both transient intracellular Ca2+ increase and inducible nod gene expression, while not affecting the expression of other genes, either constitutively expressed or inducible.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.Rhizobia are Gram-negative soil bacteria which can engage in a mutualistic association with leguminous plants. Under nitrogen-limiting conditions, rhizobia colonize plant roots and highly specialized plant organs, the nodules, are generated de novo on host roots (for a recent review see [1]). When living symbiotically, rhizobia are able to fix atmospheric nitrogen into forms usable by the plant. In return, they receive dicarboxylic acids as a carbon and energy source for their metabolism. Nitrogen is the most frequent limiting macronutrient in many soils, and it is generally supplied as fertilizer. The rhizobium-legume mutualistic association can reduce or eliminate nitrogen fertilizer requirements, resulting also in a benefit to the environment [2].A successful symbiosis is the result of an elaborate developmental program, regulated by the exchange of molecular signals between the two partners [3]. During growth in the rhizosphere of the host plant, rhizobia sense compounds secreted by the host root and respond by inducing bacterial nodul