Shaping Bacterial Symbiosis With Legumes by Experimental Evolution

• Published in: Molecular plant-microbe interactions Vol. 27; no. 9; pp. 956 - 964
• Main Authors: Marchetti, Marta, Jauneau, Alain, Capela, Delphine, Remigi, Philippe, Gris, Carine, Batut, Jacques, Masson-Boivin, Catherine
• Format: Journal Article
• Language: English
• Published: American Phytopathological Society 01-09-2014
• Subjects: Life Sciences; Vegetal Biology; NITROGEN-FIXATION; ARABIDOPSIS-THALIANA; RALSTONIA-SOLANACEARUM; REACTIVE OXYGEN; COMPARATIVE GENOMICS; ADAPTATION; RHIZOBIA; INFECTION; FIX NITROGEN; HOST SANCTIONS
• ISSN: 0894-0282
• DOI: 10.1094/MPMI-03-14-0083-R

Nitrogen-fixing symbionts of legumes have appeared after the emergence of legumes on earth, approximately 70 to 130 million years ago. Since then, symbiotic proficiency has spread to distant genera of α-and β-proteobacteria, via horizontal transfer of essential symbiotic genes and subsequent recipient genome remodeling under plant selection pressure. To tentatively replay rhizobium evolution in laboratory conditions, we previously transferred the sym-biotic plasmid of the Mimosa symbiont Cupriavidus taiwan-ensis in the plant pathogen Ralstonia solanacearum, and selected spontaneous nodulating variants of the chimeric Ralstonia sp. using Mimosa pudica as a trap. Here, we pursued the evolution experiment by submitting two of the rhizobial drafts to serial ex planta-in planta (M. pudica) passages that may mimic alternating of saprophytic and symbiotic lives of rhizobia. Phenotyping 16 cycle-evolved clones showed strong and parallel evolution of several sym-biotic traits (i.e., nodulation competitiveness, intracellular infection, and bacteroid persistence). Simultaneously, plant defense reactions decreased within nodules, suggesting that the expression of symbiotic competence requires the capacity to limit plant immunity. Nitrogen fixation was not acquired in the frame of this evolutionarily short experiment, likely due to the still poor persistence of final clones within nodules compared with the reference rhizobium C. taiwan-ensis. Our results highlight the potential of experimental evolution in improving symbiotic proficiency and for the elucidation of relationship between symbiotic capacities and elicitation of immune responses.