Alfalfa nuclei contain cold-responsive phosphoproteins and accumulate heat-stable proteins during cold treatment of seedlings

We have examined whether low temperature, as a pervasive thermodynamic stimulus, is sensed independently in different parts of the cell by studying low temperature responses of phosphoproteins in isolated nuclei. The isolated alfalfa (Medicago sativa) nuclei respond to cold by rapid and reversible changes in phosphorylation level of their proteins. The population of such cold-regulated phosphoproteins and the cold-stimulation of their phosphorylation are greater in a freezing-tolerant cultivar Apica than in a sensitive cultivar Trek. With a 4-day cold treatment of the seedlings, additional proteins showing cold-stimulated phosphorylation appear in the nucleus of Apica while there is little change in the case of Trek. Furthermore, nuclei from cold-treated seedlings of Apica, but not of Trek, show a large accumulation of heat-stable proteins. These results support the view that the low temperature sensing and acclimation occur in all vital parts of the cell and that accumulation of heat-stable nuclear proteins may be related to freezing tolerance.