Symbiotic and commensal root microbes are key determinants of plant responses to rising CO2
Rising atmospheric CO2 stimulates plant growth, but this effect is often constrained by nutrient availability. While beneficial soil-derived microbial symbionts such as nitrogen-fixing rhizobia and mycorrhizal fungi are known to mitigate this limitation, the role of plant commensal microbiota remains unclear. Here, using a gnotobiotic growth system and the model legume Lotus japonicus, we show that the magnitude of plant responses to elevated CO2 is largely explained by the composition of its root-associated microbial community. Axenic plants exhibited limited physiological responses to increasing CO2 but inoculation with symbiotic and commensal microbes significantly amplified this effect. Commensal bacteria alone, while not increasing overall biomass, shifted plant carbon allocation belowground. Microbial abundances in the root increased with atmospheric CO2, even when normalized to plant DNA. The strongest plant and microbial responses required the simultaneous presence of rhizobia, arbuscular mycorrhizal fungi, and the commensal bacterial community. These findings demonstrate that multi-partner microbial associations will be critical for maximizing plant growth and carbon sequestration as atmospheric CO2 concentrations continue to rise.