Plants can respond to a variety of biotic and abiotic signals or factors that affect their growth and development. Although the responses to these signals and factors in plants have been extensively studied at physiological and biochemical levels, the perception and intracellular transmission mechanisms remain largely unknown. Under natural growth conditions, plants do encounter various stress conditions such as drought, salinity, and high or low temperature, which have profound effects on

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... growth and development. Rice seedlings can respond to cold treatment with major changes in gene expression. 1) It is generally recognized that patterns of protein synthesis and mRNA level change when plants are exposed to cold. 2) Various stress conditions have been shown to influence protein phosphorylation. 3,4) During the early stages of cold acclimation, phosphorylation of cellular proteins and activation of protein kinases have been detected. 5) In our earlier study, the 60 kDa protein was independently phosphorylated in cold-tolerant Japonica varieties in cold treatment, but this phosphorylation is induced in cold-sensitive Indica varieties. 6) Recently, it has been reported that rice seedlings respond to cold stress of 5°C by definite changes in protein phosphorylation. 7) This suggests that subjecting rice seedlings to low temperatures results in a variety of biochemical changes at the cellular level. Low temperature treatment has been shown to induce an increase in cytosolic Ca 2ϩ levels, 8, 9) and Ca 2ϩ could play an essential role in the cold-acclimation process. 10,11) Ca 2ϩ plays important roles in numerous physiological processes and the presence of Ca 2ϩ -regulated protein kinases will have wider implications in cellular signal transduction. 12,13) Ca 2ϩ -dependent protein kinases (CDPKs) play a key role in stress and Ca 2ϩ -mediated signal transduction. As a specific group of kinases CDPKs require only micromolar concentrations of Ca 2ϩ for their activity, do not require calmodulin or phospholipids in plants and have been identified as Ca 2ϩ -dependent and calmodulin-independent serine/threonine kinases. 14,15) The Ca 2؉ -dependent protein kinases (CDPKs) and abscisic acid (ABA) are known to be involved in low-temperature stress response. The focus of this study was to characterize the 45 kDa protein kinase identified in the crude extract of rice (Oryza sativa L.) seedling roots in response to cold (5°C) stress. The activity of the 45 kDa protein kinase decreased at low temperature as evident by an in-gel kinase assay using histone III-S as a substrate. Also, the Ca 2؉ -dependent activity of this protein kinase was suppressed by cold in the membrane fractions of the root. A general protein kinase inhibitor and Ca 2؉ chelator inhibited the activity of the 45 kDa protein kinase, suggesting that it was a plant CDPK. The 45 kDa CDPK identified was found to be independent of photosynthetic tissues such as the leaf and leaf sheath of rice seedlings, supporting a direct sensing mechanism in the roots of rice seedlings to cold stress. The suppressed activity of the 45 kDa CDPK was reverted by supplementing with 5 m mM ABA under cold stress. The 45 kDa CDPK activity was stronger in the cold-tolerant variety of the 4 types tested than it was in the cold-sensitive one. These results suggest the involvement of endogenous ABA in regulating the activity of the 45 kDa CDPK in response to cold stress.