Rhizobia are soil bacteria that can establish a mutualistic nitrogen-fixing symbiosis with leguminous plants. Their impact on agro-ecosystems is fundamental, since leguminous plants do not rely on synthetic nitrogen fertilizers for their growth. A better understanding of the functioning and evolution of these symbioses may thus help improve strategies favouring sustainable plant nitrogen nutrition.
In bacteria, the ability to perform symbiotic nitrogen fixation evolved several times independently via the acquisition of essential nodulation and nitrogen fixation genes by horizontal gene transfer (HGT), sometimes occurring between distantly related bacteria. The symbiotic capacity conferred by these genes may need to be optimized through genomic remodeling of the recipient ‘proto-rhizobia’ strain. This optimisation occurs under plant-mediated selection and allows bacterial adaptation to their new ecological niche via the modification of metabolic or signalling pathways. To better understand the dynamics of genome remodeling in proto-rhizobia, an evolution experiment was launched in the host team to replay the evolution of a new rhizobium from a pathogenic ancestor. The symbiotic plasmid from the β-rhizobium Cupriavidus taiwanensis was transferred to the plant pathogen Ralstonia solanacearum. The resulting chimeric strain was evolved during successive inoculation-nodulation cycles on the plant Mimosa pudica, the natural host of C. taiwanensis. This protocol mimicked –on an accelerated basis– the alternation of ex planta and in planta periods that bacteria encounter in the wild, and provided an ecological framework to select bacteria with improved symbiotic phenotypes. Evolved bacteria thus constitute an ideal collection of strains to dissect the genetic bases of the stepwise acquisition of symbiotic properties.
Research carried on in this project aims at harnessing this evolution experiment in order to understand how bacteria adapt to life within plant cells, a defining but poorly understood aspect of nitrogen-fixing symbioses. Three distinct work-packages were designed to address the following questions. First, what are the genetic routes to the acquisition and amelioration of intracellularity (WP1)? Second, what are the bacterial functions involved in intracellular infection and persistence within plant cells (WP2)? Third, how do plant transcriptional responses adjust to accommodate bacterial strains that differ in their intracellular properties (WP3)?
