In vivo and in vitro activation of temperature-responsive plant map kinases

Alfalfa cells possess two temperature-responsive Mitogen-Activated Protein Kinases (MAPKs), SAMK (Stress-Activated MAP Kinase) activated at 4°C and HAMK (Heat shock-Activated MAP Kinase) activated at 37°C. Both are inactive at 25°C. We show here that SAMK is activated when cells are transferred from 37°C to 25°C, and HAMK is activated when cells are transferred from 4°C to 25°C. Moreover, we show that heat activation of HAMK also occurs in cell-free extracts. We conclude that (i) SAMK or HAMK activation does not require a particular temperature but a relative temperature shift, and (ii) that either HAMK itself or one or more of its upstream activators can sense temperature change directly.