Dataset Information

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

ABSTRACT:

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.

SUBMITTER: Moscatiello R

PROVIDER: S-EPMC2759959 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Rhizobia are symbiotic nitrogen-fixing soil bacteria that show a symbiotic relationship with their host legume. Rhizobia have 2 different physiological conditions: a free-living condition in soil, and a symbiotic nitrogen-fixing condition in the nodule. The lifestyle of rhizobia remains largely unknown, although genome and transcriptome analyses have been carried out. To clarify the lifestyle of bacteria, proteome analysis is necessary because the protein profile directly reflects in vivo reactions of the organisms. In proteome analysis, high separation performance is required to analyze complex biological samples. Therefore, we used a liquid chromatography-tandem mass spectrometry system, equipped with a long monolithic silica capillary column, which is superior to conventional columns. In this study, we compared the protein profile of Mesorhizobium loti MAFF303099 under free-living condition to that of symbiotic conditions by using small amounts of crude extracts.We identified 1,533 and 847 proteins for M. loti under free-living and symbiotic conditions, respectively. Pathway analysis by Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed that many of the enzymes involved in the central carbon metabolic pathway were commonly detected under both conditions. The proteins encoded in the symbiosis island, the transmissible chromosomal region that includes the genes that are highly upregulated under the symbiotic condition, were uniquely detected under the symbiotic condition. The features of the symbiotic condition that have been reported by transcriptome analysis were confirmed at the protein level by proteome analysis. In addition, the genes of the proteins involved in cell surface structure were repressed under the symbiotic nitrogen-fixing condition. Furthermore, farnesyl pyrophosphate (FPP) was found to be biosynthesized only in rhizobia under the symbiotic condition.The obtained protein profile appeared to reflect the difference in phenotypes under the free-living and symbiotic conditions. In addition, KEGG pathway analysis revealed that the cell surface structure of rhizobia was largely different under each condition, and surprisingly, rhizobia might provided FPP to the host as a source of secondary metabolism. M. loti changed its metabolism and cell surface structure in accordance with the surrounding conditions.