Two protein kinases displaying mitogen-activated protein kinase (MAPK) properties are activated both by an hypoosmotic stress and by oligogalacturonides in tobacco cell suspensions [Cazalé et al. (1999) Plant J. 19, 297-307]. Using specific antibodies, they were identified as the salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK). The SIPK was also activated by an hyperosmotic stress, indicating that the same kinase may play a role both in hypo- and hyperosmotic signalling pathways, in addition to its involvement in the transduction of elicitor signals. Using immunoprecipitation followed by two-dimensional in-gel kinase assay, three molecular forms of the SIPK were observed, suggesting that additional modifications of the activated kinase may occur. In contrast to WIPK and SIPK, which are located at the crossroad of several transduction pathways initiated by elicitor or osmotic stimuli, a 44 kDa kinase, that would not belong to the MAPK family, appeared more specific to osmotic stress.