Activation of salicylic acid-induced protein kinase, a mitogen-activated protein kinase, induces multiple defense responses in tobacco

The activation of mitogen-activated protein kinases (MAPKs) is one of the e arliest responses in plants challenged by avirulent pathogens or cells trea ted with pathogen-derived elicitors. Expression of a constitutively active MAPK kinase, NtMEK2(DD), in tobacco induces the expression of defense genes and hypersensitive response-like cell death, which are preceded by the act ivation of two endogenous MAPKs, salicylic acid-induced protein kinase (SIP K) and wounding-induced protein kinase (WIPK). However, the roles that SIPK and WIPK each play in the process are unknown. Here we report that SIPK al one is sufficient to activate these defense responses. In tobacco leaves tr ansiently transformed with SIPK under the control of a steroid-inducible pr omoter, the induction of SIPK expression after the application of dexametha sone, a steroid, leads to an increase of SIPK activity. The increase of SIP K activity is dependent on the phosphorylation of newly synthesized SIPK by its endogenous upstream kinase. In contrast, the expression of WIPK under the same conditions fails to increase its activity, even though the protein accumulates to a similar level. Studies using chimeras of SIPK and WIPK de monstrated that the C terminus of SIPK contains the molecular determinant f or its activation, which is rather surprising because the N termini of SIPK and WIPK are more divergent. SIPK has been implicated previously in the re gulation of both plant defense gene activation and hypersensitive response- like cell death based on evidence from pharmacological studies using kinase inhibitors. This gain-of-function study provided more direct evidence for its role in the signaling of multiple defense responses in tobacco.