TAB2, a nucleoside diphosphate protein kinase, is a component of the tMEK2 disease resistance pathway in tomato

Signal transduction is used by plants to coordinate their development and to sense and respond to fluctuations in their surroundings. With previous proteomics approaches, we specifically studied activation events downstream of tMEK2, a mitogen-activated protein kinase kinase (MAPKK), in tomato. LC-MS/MS revealed a group of phosphoproteins in tMEK2MUT-transgenic tomato plants, where tMEK2 was constitutively activated. Of particular interest is TAB2, a nucleoside diphosphate protein kinase (NDPK), which has long been considered as a nonregulatory housekeeping enzyme. Overexpression of tMEK2MUT in tomato protoplasts and tomato plants up-regulated TAB2. Overexpression of TAB2 also enhanced resistance to virulent Pseudomonas syringae pv. tomato. Mutation at the kinase interaction motif (KIM) of TAB2 significantly reduced tMEK2MUT-induced up-regulation of PR1b1, β-1,3-glucanase and chitinase genes. Pull-down assays suggest that TAB2 interacts with LeMPK3, but not other members of tomato MAPKs or tMEK2. Our work suggests that TAB2 is downstream of LeMPK3 and is directly involved in tMEK2-mediated disease resistance.