cAMP signalling in the Sinorhizobium meliloti – Medicago sativa symbiosis.
Whereas cAMP signalling in animal and more recently in plant pathogenesis is increasingly well documented, cAMP contribution to symbiosis has been poorly investigated so far. Yet the high number of (type III, universal) adenylate-guanylate cyclases in many rhizobia (26 in S. meliloti) suggest these enzymes may play a prominent role in the rhizospheric and/or symbiotic lives of these bacteria.
We have recently found that a set of three adenylate cyclases CyaD1D2K as well as a putative cAMP-binding transcriptional regulator contribute to the negative control of infection in S.meliloti, preventing superfluous epidermal infection of Medicago roots. This system may be part of a larger system that keeps infection under control hence preserving mutualism. Interestingly, activation of the cyclases depends on a plant signal that is present in nodules but not in roots.
Our present aim is to biochemically characterize the plant signal and its mode of perception and to elucidate the mechanisms that link cAMP signalling to infection and possibly nodulation.
Experimental evolution of the plant pathogen Ralstonia solanacearum into a legume symbiont
Rhizobia do not form a homogenous group but are phylogenetically disparate α- and β- proteobacteria that have achieved a nitrogen fixing symbiosis with legume. How rhizobia have emerged is a fascinating, but only partly documented, question. Ample evidence supports the view that rhizobia have evolved through horizontal transfer of key symbiotic functions into diverse soil bacteria followed by subsequent recipient genome adaptation under selection in the host plant environment. To experience this evolutionary scenario we have launched the experimental evolution of the plant pathogen Ralstonia solanacearum into a legume symbiont. A chimeric Ralstonia strain was engineered by introducing the symbiotic plasmid of the β-rhizobium model Cupriavidus taiwanensis into R. solanacearum. Large-scale and long-term evolution experiments under legume selection pressure are being performed to activate and/or improve competitiveness in nodulation, infection, bacterial maintenance in infected cells and tentatively nitrogen fixation. This project exploits the recent advances in fast genome re-sequencing technology to track genome modifications, both R. solanacearum and C. taiwanensis being completely sequenced. Genomic, cellular and molecular genetics approaches are combined to elucidate the underlying genetic basis of symbiotic adaptation. This project is at the interface of symbiosis and pathogenesis.
Our collaborators on the cAMP project are Dr JV Cullimore (LIPM), Dr V. Poinsot (IMRCP) and Prof. JC. Portais (MetaToul platform).
Our collaborators on the experimental evolution project are Dr. S. Cruveiller and Dr. C. Medigue (Genoscope Evry), Dr. E. Rocha (Pasteur Institute Paris) and Dr. A. Jauneau (Imaging platform Toulouse)
ANR Bioadapt (2013-2016) (12-ADAP-0014-01).Shaping new rhizobia by experimental evolution: insights into bacterial adaptation to the plant environment . C. Masson-Boivin coordinator, 308 K€.
TULIP innovative project (2012-2014). Environmental and genetic factors triggering hypermutability during experimental evolution of legume symbionts. C. Masson-Boivin coordinator, 77 K€.
ANR Blanc (2011-2013) (RhizoCamp-2010-BLAN-1719-01) Cyclic AMP signalling and the control of infection in the Medicago symbiont, Sinorhizobiummeliloti.: J. Batut coordinator, 300 K€.