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Adaptation to elevated temperatures is of major importance for the survival of plants. The role of kinases in heat stress response was studied in tomato by in gel and in solution kinase assays using myelin basic protein as substrate. The application of heat stress in a naturally occurring temperature range resulted in a fast and transient activation of a 50 kDa mitogen-activated protein (MAP) kinase both in a photoautotrophic cell suspension culture and in leaves of mature plants. The heat activation of the MAP kinase was shown to be calcium-dependent. The specific phosphorylation of tomato heat stress transcription factor HsfA3 by a partially purified preparation of the heat-activated MAP kinase supports a physiological role of the identified kinase activity in transducing the heat stress signal. [Copyright &y& Elsevier]