Shoresh, M., Department of Plant Sciences, Weizmann Institute of Science, Rehovot 76100, Israel
Gal-On, A., Department of Virology, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel
Leibman, D., Department of Virology, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel
Chet, I., Department of Plant Sciences, Weizmann Institute of Science, Rehovot 76100, Israel

The fungal biocontrol agent Trichoderma asperellum has been recently shown to induce systemic resistance in plants through a mechanism that employs jasmonic acid and ethylene signal transduction pathways. Mitogen-activated protein kinase (MAPK) proteins have been implicated in the signal transduction of a wide variety of plant stress responses. Here we report the identification and characterization of a Trichoderma-induced MAPK (TIPK) gene function in cucumber (Cucumis sativus). Similar to its homologs, wound-induced protein kinase, MPK3, and MPK3a, TIPK is also induced by wounding. Normally, preinoculation of roots with Trichoderma activates plant defense mechanisms, which result in resistance to the leaf pathogen Pseudomonas syringae pv lachrymans. We used a unique attenuated virus vector, Zucchini yellow mosaic virus (ZYMV-AGII), to overexpress TIPK protein and antisense (AS) RNA. Plants overexpressing TIPK were more resistant to pathogenic bacterial attack than control plants, even in the absence of Trichoderma preinoculation. On the other hand, plants expressing TiPiC-AS revealed increased sensitivity to pathogen attack. Moreover, Trichoderma preinoculation could not protect these AS plants against sub-sequent pathogen attack. We therefore demonstrate that Trichoderma exerts its protective effect on plants through activation of the TIPK gene, a MAPK that is involved in signal transduction pathways of defense responses. © 2006 American Society of Plant Biologists.