title: Experimental evolution of a plant pathogen into a legume symbiont.
abstract: Rhizobia are phylogenetically disparate alpha- and beta-proteobacteria that have achieved the environmentally essential function of fixing atmospheric nitrogen in symbiosis with legumes. Ample evidence indicates that horizontal transfer of symbiotic plasmids/islands has played a crucial role in rhizobia evolution. However, adaptive mechanisms that allow the recipient genomes to express symbiotic traits are unknown. Here, we report on the experimental evolution of a pathogenic Ralstonia solanacearum chimera carrying the symbiotic plasmid of the rhizobium Cupriavidus taiwanensis into Mimosa nodulating and infecting symbionts. Two types of adaptive mutations in the hrpG-controlled virulence pathway of R. solanacearum were identified that are crucial for the transition from pathogenicity towards mutualism. Inactivation of the hrcV structural gene of the type III secretion system allowed nodulation and early infection to take place, whereas inactivation of the master virulence regulator hrpG allowed intracellular infection of nodule cells. Our findings predict that natural selection of adaptive changes in the legume environment following horizontal transfer has been a major driving force in rhizobia evolution and diversification and show the potential of experimental evolution to decipher the mechanisms leading to symbiosis.
authors.affiliation: Laboratoire des Interactions Plantes Micro-organismes (LIPM), UMR CNRS-INRA 2594/441, Castanet-Tolosan, France.
authors.forename: Marta
authors.initials: M
authors.lastname: Marchetti
authors.forename: Delphine
authors.initials: D
authors.lastname: Capela
authors.forename: Michelle
authors.initials: M
authors.lastname: Glew
authors.forename: Stéphane
authors.initials: S
authors.lastname: Cruveiller
authors.forename: Béatrice
authors.initials: B
authors.lastname: Chane-Woon-Ming
authors.forename: Carine
authors.initials: C
authors.lastname: Gris
authors.forename: Ton
authors.initials: T
authors.lastname: Timmers
authors.forename: Véréna
authors.initials: V
authors.lastname: Poinsot
authors.forename: Luz B
authors.initials: LB
authors.lastname: Gilbert
authors.forename: Philipp
authors.initials: P
authors.lastname: Heeb
authors.forename: Claudine
authors.initials: C
authors.lastname: Médigue
authors.forename: Jacques
authors.initials: J
authors.lastname: Batut
authors.forename: Catherine
authors.initials: C
authors.lastname: Masson-Boivin
mesh_terms: D000220:Adaptation, Biological; D002678:Chimera; D019020:Directed Molecular Evolution; D007887:Fabaceae / Q000382:microbiology*; D022761:Gene Transfer, Horizontal; D009586:Nitrogen Fixation; D055170:Plant Root Nodulation / Q000235:genetics; D020641:Polymorphism, Single Nucleotide; D012231:Rhizobium / Q000235:genetics* / Q000502:physiology; D013559:Symbiosis / Q000235:genetics*