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 their 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 plant CDPKs have been shown to phosphorylate a number of exogenous substrates, including histone III-S.
16)CDPKs exist as multiple isoforms 17) and isolation of cDNA further revealed that these enzymes are encoded by a multigene family. 18,19) As for their localization, CDPKs are either membrane-associated 20,21) or cytosolic in nature. 22,23) A 32 kDa CDPK was detected during regeneration of rice cultured suspension cells by an in-gel kinase assay.24) Also a 45 kDa Ca 2ϩ -and phospholipid-dependent protein kinases in rice embryos and leaves were identified in previous studies.25) A 45 kDa CDPK, which can phosphorylate histone III-S in gel, was found in rice leaf under darkness in the presence of Ca 2ϩ and phosphatidylserine 26) ; their functions, however, remain to be clarified. Rice seedlings under cold stress induce a 47 kDa CDPK in an increasing manner in leaf sheath tissues. 27) However, the functi...