SALICYLIC-ACID ACTIVATES A 48-KD MAP KINASE IN TOBACCO

The involvement of phosphorylation/dephosphorylation in the salicylic acid (SA) signal transduction pathway leading to pathogenesis-related gene induction has previously been demonstrated using kinase and phosp hatase inhibitors. Here, we show that in tobacco suspension cells, SA induced a rapid and transient activation of a 48-kD kinase that uses m yelin basic protein as a substrate. This kinase is called the p48 SIP kinase (for SA-induced protein kinase). Biologically active analogs of SA, which induce pathogenesis-related genes and enhanced resistance, also activated this kinase, whereas inactive analogs did not. Phosphor ylation of a tyrosine residue(s) in the SIP kinase was associated with its activation. The SIP kinase was purified to homogeneity from SA-tr eated tobacco suspension culture cells. The purified SIP kinase is str ongly phosphorylated on a tyrosine residue(s), and treatment with eith er protein tyrosine or serine/threonine phosphatases abolished its act ivity. Using primers corresponding to the sequences of internal trypti c peptides, we cloned the SIP kinase gene. Analysis of the SIP kinase sequence indicates that it belongs to the MAP kinase family and that i t is distinct from the other plant MAP kinases previously implicated i n stress responses, suggesting that different members of the MAP kinas e family are activated by different stresses.