Physcomitrium patens response to elevated CO 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 CO , produced by soil respiration. However, the impact of elevated CO (eCO ) levels on mosses remains underexplored. We determined the growth responses of the moss Physcomitrium patens to eCO 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 eCO . Elevated CO 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 eCO , 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 eCO responses of P. patens with other plant groups and provide crucial insights into moss growth that may benefit climate change models.