Harnessing experimental evolution of rhizobia for an integrative view of endosymbiosis

ended

Microbiota shape growth and survival of eukaryotes through countless symbiotic associations. A prominent example for agriculture is the mutualistic nitrogen-fixing symbiosis between legume plants and rhizobial bacteria. Understanding the evolution and functioning of these symbioses offers promises to optimise their beneficial use in agro-ecosystems and, potentially, to extend it to non-legume plants. An ambitious project initiated several years ago in the host team aims at recapulating the evolution of new nitrogen-fixing bacteria from a pathogenic ancestor. A symbiotic plasmid was artificially transferred into the plant pathogen Ralstonia solancearum and the resulting chimera was selected for improved in planta symbiotic performance by experimental evolution. This experiment offers a unique opportunity to witness the ‘real-time’ adaptation of chimeric bacteria to their new host plant. In this proposal, I will leverage the biological material generated during this experiment to progress towards an integrative understanding of the evolutionary events underpinning the transition to symbiosis. In particular, I will focus on the acquisition of intracellular uptake and accommodation of bacteria by plant cells, a defining and poorly understood aspect of these symbiotic associations. First, I will use whole-population sequencing to track allelic frequencies in evolving populations and identify mutations improving bacterial intracellular fitness. Functional genetics analyses will then uncover the bacterial functions that are required for endosymbiotic life. Finally, plant transcriptional responses to bacteria showing different infectious abilities will be analysed by RNA-sequencing. Altogether, this work will use a combination of approaches (experimental evolution, genetics and high-throughput sequencing) to advance our understanding of the genetic and selective processes underpinning the evolution of nitrogen-fixing symbioses.