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.