Symbiotic and commensal root microbes are key determinants of plant responses to rising CO2

Rising levels of atmospheric carbon dioxide (CO2) trigger plant growth, yet this effect can be limited by nutrient availability. Symbiotic soil microbes, like nitrogen-fixing bacteria and mycorrhizal fungi, are known to alleviate this constraint. However, the influence of plant commensal microbes has yet to be fully understood. In this study, researchers utilized a controlled, gnotobiotic growth system with the model legume Lotus japonicus to investigate the impact of root-associated microbial communities on plant responses to elevated CO2.

The results reveal that the size of plant responses to increased CO2 levels is predominantly determined by the makeup of the root microbiome. In the absence of microbes, plants showed minimal physiological changes when CO2 levels rose. Nevertheless, inoculation with symbiotic and commensal microbes significantly enhanced these responses.

Interestingly, even when commensal bacteria alone were present, they redirected plant carbon allocation towards belowground tissues, although they did not boost overall biomass. Moreover, microbial populations in the roots increased with rising atmospheric CO2, even when adjusted for plant DNA content. The most pronounced plant and microbial responses were observed when symbiotic bacteria (rhizobia) and mycorrhizal fungi, along with the commensal bacterial community, coexisted.

These findings suggest that complex microbial partnerships play a crucial role in both plant growth and the redirection of carbon allocation belowground as CO2 concentrations continue to climb.