When a plant cell is challenged by a well-defined stimulus, complex signal transduction pathways are activated to promote the modulation of specific sets of genes and in fine to develop adaptive responses. In this context, protein phosphorylation plays a fundamental role through the activation of multiple protein kinase families. While the involvement of protein kinases at the plasma membrane and cytosolic levels are now well documented, their nuclear counterparts are still poorly investigated. In the field of plant defense reactions, no known study has yet reported the activation of a nuclear protein kinase and / or its nuclear activity in plant cells, although some protein kinases, e.g. MAPK, are known to be translocated into the nucleus. In the present report we investigated the ability of cryptogein, a proteinaceous elicitor of tobacco defense reactions, to induce different nuclear protein kinase activities. We found that at least four nuclear protein kinases are activated in response to cryptogein treatment in a time-dependent manner, some of them exhibiting Ca2+-dependent activity. Our study focused on one 47-kDa protein kinase with a Ca2+-independent activity, closely related to the MAPK family. After purification and microsequencing, this protein kinase was formally identified as SIPK, a biotic and abiotic stress-activated MAPK of tobacco. We also showed that cytosolic activation of SIPK is not sufficient to promote a nuclear SIPK activity, the latter being correlated with cell death. In that way, this study provides evidences of a functional nuclear MAPK activity involved in response to an elicitor treatment.