Characterization of calreticulin as a phosphoprotein interacting with cold-induced protein kinase in rice

Calreticulin is an abundant endo/sarcoplasmic reticulum Ca2+-binding protein. To investigate whether calreticulin (CRO1) is involved in the cold-stress response in rice, a transgenic plant was constructed. The transcriptional level was decreased within 30 min and recovered within 2 h of a cold treatment. The calreticulin protein was shifted from a soluble fraction to an insoluble fraction by cold stress. Endogenous abscisic acid (ABA) is an important factor in cold response, and the synthesis of ABA was strongly induced in CRO1-sense transgenic rice, the same as in cold-sensitive rice. The phosphorylation of calreticulin increased after cold treatment. Over-expression of calreticulin enhanced the activities of 47 kDa Ca2+-dependent protein kinase (CDPK) that had been induced by cold treatment. The 47-kDa CDPK activity increases more in the cold sensitive variety IR36 and the sense transgenic rice than it does in other varieties. The synthesis of ABA, phosphorylation of calreticulin and 47-kDa CDPK activity induced in sense transgenic rice were the same as in cold-sensitive rice and the phosphorylation of antisense transgenic rice was similar to that of cold-tolerant rice. These results suggest that the calreticulin is involved in the signaling pathway leading to response to cold stress.