Physcomitrium patens response to elevated CO2 is flexible and determined by an interaction between sugar and nitrogen availability

• Mosses hold a unique position in plant evolution and are crucial for protecting natural, long-term carbon storage systems such as permafrost and bogs. Due to small stature, mosses grow close to the soil surface and are exposed to high levels of CO₂, produced by soil respiration. However, the impact of elevated CO₂ (eCO₂) levels on mosses remains underexplored.
• We determined the growth responses of the moss Physcomitrium patens to eCO₂ in combination with different nitrogen levels and characterized the underlying physiological and metabolic changes.
• Three distinct growth characteristics, an early transition to caulonema, the development of longer, highly pigmented rhizoids, and increased biomass, define the phenotypic responses of P. patens to eCO₂. Elevated CO₂ impacts growth by enhancing the level of a sugar signaling metabolite, T6P. The quantity and form of nitrogen source influences these metabolic and phenotypic changes. Under eCO₂, P. patens exhibits a diffused growth pattern in the presence of nitrate, but ammonium supplementation results in dense growth with tall gametophores, demonstrating high phenotypic plasticity under different environments.
• These results provide a framework for comparing the eCO₂ responses of P. patens with other plant groups and provide crucial insights into moss growth that may benefit climate change models.