Rewiring a plant pathogen into a legume symbiont by lab-evolution
C. MASSON-BOIVIN (1), D. Capela (1), M. Marchetti (1), C. Clerissi (2), S. Cruveiller (3), E. Rocha (2), P. Remigi (4) (1) INRA, France; (2) CNRS, France; (3) CEA, France; (4) Massey University, France

Rhizobia, the nitrogen-fixing symbionts of legumes, arose via horizontal gene transfer of essential symbiotic genes in different genetic backgrounds. They currently belong to hundreds of species distributed in 14 saprophyte- and pathogen-containing genera of alpha- and beta-proteobacteria. Successful transfer over large phylogenetic distances has been rare, likely because the full expression of the symbiotic potential acquired by HGT requires subsequent genomic readjustments. To get insights into the molecular and evolutionary mechanisms that facilitated the long distance spread of symbiotic genes, we experimentally replayed the evolution of rhizobia. Following introduction of the symbiotic plasmid of Cupriavidus taiwanensis, the Mimosa symbiont, into the pathogenic Ralstonia solanacearum we challenged transconjugants to become Mimosa symbionts through serial ex planta-in planta passages, recapitulating the shifts between free-living and symbiotic lives that have shaped the natural evolution of rhizobia. The first major symbiotic steps, nodulation and nodule cell infection, were activated and improved over 17 cycles in most lineages. Evolution was very fast for i) the legume is a highly selective environment for these two traits and ii) symbiotic genes were co-transferred with mutagenesis genes that accelerates symbiotic adaptation. Evolution relied on rewiring of regulatory circuitries leading to inactivation of some virulence functions and recruitment of local functions.

Abstract Number: C7-2
Session Type: Concurrent