P-422 - Microbial community composition shapes plant responses to increasing atmospheric CO2 levels

In nature, plants interact with diverse soil-derived microbes, which provide benefits to the host in exchange for organic carbon, forming complex communities (root microbiota). Elevated atmospheric CO₂ (eCO₂) levels are likely to enhance organic carbon release into the rhizosphere, while exacerbating limitations in essential nutrients (e.g. nitrogen and phosphorus). Despite the significance of eCO₂ levels in these interactions, their precise impact is poorly understood. To explore this, we developed a new gnotobiotic system, allowing dynamic control and monitoring of CO₂ levels. Using this system, we grew the model legume Lotus japonicus under contrasting CO₂ levels in associations with different microbial synthetic communities (SynCom), including root microbiota and adapted symbionts (nitrogen-fixing rhizobia and AMF). Precise monitoring of plant growth, CO₂ fluxes and microbiome composition revealed a strong interaction between the three variables. Specifically, the positive impact of eCO₂ on plant growth was highly influenced by the composition of the SynCom, with the most diverse SynComs leading to the strongest CO₂ fertilization effects. Our results suggest an increasing importance of interactions between plants and beneficial microbes, symbionts and the root microbiota, as atmospheric CO₂ levels rise, highlighting the pivotal role of microbial community composition in shaping plant responses to eCO₂ for sustainable agriculture strategies under future climate scenarios.