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.


CONCLUSIONS
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.
Dipartimento di Biologia, Università di Padova, Via U, Bassi 58/B, 35131 Padova, Italy. roberto.moscatiello@unipd.it
Roberto
R
Moscatiello
Sara
S
Alberghini
Andrea
A
Squartini
Paola
P
Mariani
Lorella
L
Navazio
D000331:Aequorin / Q000235:genetics / Q000378:metabolism; D020561:Alphaproteobacteria / Q000235:genetics / Q000378:metabolism*; D001059:Apoproteins / Q000235:genetics / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D000070116:Lotus / Q000382:microbiology*; D018517:Plant Roots / Q000378:metabolism / Q000382:microbiology; D012329:RNA, Bacterial / Q000235:genetics; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013559:Symbiosis
D001059:Apoproteins; D012329:RNA, Bacterial; D011994:Recombinant Proteins; C060014:apoaequorin; D000331:Aequorin; D002118:Calcium