CAPELA, D. | bioRxiv Science

bioRxiv2026

Transitive interactions among rhizobia determine their symbiotic fitness

During host-microbe symbioses, the fitness of mutualistic microbes is determined by the interactions that concurrently occur, throughout their life cycle, with their host and other members of the surrounding microbial community. Disentangling how these multiple interactions shape the fitness of microbial symbionts is challenging, but is essential to understand the diversity and functioning of mutualisms. Here we examined the different fitness components of rhizobial symbionts of the legume plant Mimosa pudica across the multiple stages of their symbiotic life cycle. By comparing rhizobial symbiotic fitness in single and pairwise inoculations, we found that inter-bacterial interactions causing significant fitness effects are common, transitive and can have major consequences, sometimes leading to the extinction of a strain. These interactions predominantly occur at the root infection (nodulation) step, but smaller post-infection interaction effects, involving yet uncharacterized mechanisms, were also detected. Furthermore, considering pairwise interactions was sufficient to predict fitness ranks in more complex rhizobial communities consisting of 6 or 8 strains, indicating that higher-order interaction effects do not play a significant role in these communities. Overall, our results provide a quantitative framework to describe the main drivers of rhizobial symbiotic fitness in a simple community context.

microbiology↗

bioRxiv2025

Disruption of putrescine export in experimentally evolved Ralstonia pseudosolanacearum enhances symbiosis with Mimosa pudica

Polyamines are essential molecules across all domains of life, but their role as signaling molecules in host-microbe interactions is increasingly recognized. However, because they are produced by both the host and the microbe, their dual origin makes their functional dissection challenging. The plant pathogen Ralstonia pseudosolanacearum GMI1000 secretes large amounts of putrescine both in vitro and in the xylem sap of host plants. In this study, we investigated the genetic changes underlying its experimental evolution into a legume symbiont. We showed that the paeA gene (RSc2277), which was repeatedly mutated during this process, encodes a putrescine exporter. Mutations in paeA completely abolished putrescine excretion in vitro and enhanced bacterial proliferation within nodules during interaction with the legume Mimosa pudica. When these mutations occurred in symbionts already capable of intracellular infection, it further increased bacterial load in nodules and allowed the detection of nitrogenase activity. In addition, paeA-mutated symbionts modulated host gene expression towards a more functional symbiotic state by repressing defense-related genes and inducing nodule development genes. These nodule development genes include genes encoding leghemoglobins and an arginine decarboxylase, a key enzyme in plant putrescine biosynthesis. These results indicate that bacterial and plant putrescine have distinct functions in legume symbiosis and highlight the complex role of polyamines in plant-microbe interactions. ImportanceRhizobia, the nitrogen-fixing symbionts of legumes, emerged through repeated and independent horizontal transfers of some essential symbiotic genes. However, these transfers alone are often insufficient to convert the recipient bacterium into a functional legume symbiont. In a laboratory experiment, we evolved the plant pathogen Ralstonia pseudosolanacearum into a nodulating and intracellularly infecting symbiont of Mimosa pudica. This transition required genomic modifications in the recipient bacterium to activate its acquired symbiotic potential. Here, we demonstrated that one of these key adaptive modifications is the inactivation of bacterial putrescine export. This polyamine, when produced by the microsymbiont, appears to act as a negative signal for the plant. This study provides new insights into the distinct roles of bacterial- and plant-derived putrescine in plant-microbe interactions, highlighting their functional divergence despite being produced by both organisms.

microbiology↗