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Current Research (PhD Thesis):

Fundamental key components of early plant defense signaling are mitogen-activated protein kinase (MAPK) cascades. In Arabidopsis at least 60 MAPKKKs, 10 MAPKKs and 20 MAPKs were found. One complete MAPK cascade has been identified, which composes of AtMEKK1, the MAPK kinase kinase, AtMKK4/AtMKK5, the MAPK kinases, and finally AtMPK3 and AtMPK6, the MAPKs. Recently the AtMKK9, another MAPK kinase, has also been shown to activate AtMPK3 and AtMPK6. AtMKK4 and AtMKK5 are activated by both bacterial and fungal pathogens; AtMKK9 is also involved in stress response and salt sensitivity. This suggests that various MAPKKs activate multiple MAPKs and that cross-talk between various signaling pathways intersect at this level. In Nicotiana attenuata, SIPK and WIPK, two MAPKs, are important regulators of plants’ resistance to herbivores: activation of SIPK and WIPK is necessary to induce plants’ phytohormone biosynthesis, transcriptional changes, and defensive secondary metabolites. We use virus-induced gene silencing (VIGS) to study the function of MAPKKs in plants’ resistance to herbivores. We silenced NaMEK2 and NaMKK1, which are homologues of AtMKK4/AtMKK4 and AtMKK9 in N. attenuata, respectively. In these plants, we measured phytohormones after mimicked herbivory treatment (applying Manduca sexta oral secretions to wounds). Silencing individual MAPKK did not result in significant decrease in jasmonic acid (JA) or ethylene (ET) level. Furthermore, these phytohormone levels were not different from those in empty vector plants after knocking down both NaMKK1 and NaMEK2. These data strongly suggest the great redundancy of MAPKKs which have overlapping functions in activating herbivory-induced phytohormone biosynthesis. Future work includes preparing VIGS constructs which silence several other MAPKKs together with NaMEK2 and NaMKK1 to further identify the MAPKK signaling network that plays critical role in activating plant’s defense against herbivores.