cAMP signalling and autoregulation in the Sinorhizobium meliloti – Medicago symbiosis.
In contrast to the situation in animal pathogens, cAMP signaling in symbionts such as the legume symbiont S. meliloti has received little attention so far. Yet the large number of adenylate cyclases in rhizobia (27 in S. meliloti) suggest they play a prominent role in the rhizospheric and/or symbiotic life of rhizobia.
We focus our research on three adenylate cyclases and their cognate transcriptional regulator called Clr that regulate secondary infection of Medicago roots by S. meliloti. These genes belong to a pathway called AOI (for autoregulation of infection) by which endosymbiotic bacteria inhibit secondary infection by their rhizospheric peers. We aim to characterize the mechanism underlying AOI in both the plant and microbial partners, with a special emphasis on the signals and pathways involved.
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 an evolution experiment aiming at converting a soil bacterium into a legume symbiont. A chimeric Ralstonia strain was engineered by introducing the symbiotic plasmid of the β-rhizobium model Cupriavidus taiwanensis into the plant pathogen Ralstonia-solanacearum. This chimeric strain is being evolved under legume (M. pudica, the host plant of C. taiwanensis) selection pressure 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 "signalization through cAMP-AOI" project are C. Gough and F. Maillet (LIPM), V. Poinsot (IMRCP) and V. Morales/P. Polard (LMGM).
Our main collaborators on the "experimental evolution" project are D. Roche (Genoscope-CEA Evry), E. Rocha (Pasteur Institute Paris) and A. Jauneau (Imaging platform Toulouse)
ANR REPLAY (2017-2020) Replaying the evolution of rhizobia: towards a conceptual and practical framework for the design of new nitrogen-fixing plant symbionts. Partners: Genoscope (D. Roche), IP Paris (E. Rocha), 460 k€.
ANR AOI (2016-2019) Autoregulation of infection in the rhizobium-legume symbiosis. Partners LIPM (C. Gough) IMRCP UMR 5326 CNRS Univ Toulouse III (V. Poinsot). 434 k€