Cold stress has adverse effects on plant growth and development. Plants respond and acclimate to cold stress through various biochemical and physiological processes, thereby acquiring stress tolerance. To better understand the basis for tolerance, we carried out a proteomic study in the model moss, Physcomitrella patens, characterizing gametophore proteins with 2-DE and mass spectroscopy. Following exposure to 0 degrees C for up to 3 days, out of the more than 1000 protein spots reproducibly resolved, only 45 changed in abundance by at least 1.5-fold. Of these, 35 were identified by tryptic digestion and mass spectroscopy. Photosynthetic proteins decreased, whereas many catabolic proteins increased. In addition, cold stress up-regulated a variety of signaling, cytoskeleton, and defense proteins and few proteins in these classes were down-regulated. Up-regulated proteins include the 14-3-3-like protein, actin, HSP70s, lipoxygenases, and cytochrome P450 proteins. These results point to pathways that are important for the mechanism of cold stress response in P. patens and by extension to the entire plant kingdom.