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UBC Theses and Dissertations

Regulation of MAP kinase cascades in plant immunity Nitta, Yukino

Abstract

In eukaryotes, MAP kinase cascades are important players of signal transduction in many biological processes. In plant immunity, two main MAP kinase pathways, MKK4/MKK5-MPK3/MPK6 and MEKK1-MKK1/MKK2-MPK4, are activated upon elicitor treatments. On the other hand, the YDA-MKK4/MKK5-MPK3/MPK6 cascade is involved in the negative regulation of stomatal differentiation. Plants have evolved different mechanisms to regulate the activity of these cascades. For example, brassinosteroid (BR) signaling has been reported to promote the YDA pathway. This study revealed that the mapkkk3 mapkkk5 double mutant showed reduced phosphorylation of MPK3/MPK6 upon treatment with conserved microbial molecules called PAMPs, suggesting that MAPKKK3/MAPKKK5 function upstream of MKK4/MKK5 to form a cascade in plant immunity. The YDA cascade shares the same MAPKK and MAPK as the MKK4/MKK5-MPK3/MPK6 cascade in plant immunity. Interestingly, loss of YDA or blocking BR signaling leads to increased elicitor-induced activation of MPK3/MPK6. On the other hand, development defects caused by silencing of YDA are suppressed in the mapkkk3 mapkkk5 double mutant. These data suggest that there are antagonistic interactions between the two MAP kinase cascades in stomatal development and plant immunity. Disruption of the MEKK1-MKK1/MKK2-MPK4 cascade leads to immune receptor SUMM2-mediated immune responses. MEKK2, a close paralog of MEKK1, was previously shown to act as a positive regulator of SUMM2-mediated immunity. I discovered that this occurred due to the inhibition of the activity of MPK4 by MEKK2 as MEKK2 is able to inhibit the phosphorylation of MPK4 by MKK2 in vitro. Interestingly, activation of SUMM2-mediated defense responses in the mekk1, mkk1 mkk2 and mpk4 mutant plants leads to increased MEKK2 transcript levels, which contributes to positive feedback regulation of SUMM2-mediated immunity. MEKK2 arose from a recent duplication event resulting in the tandem gene repeat consisting of MEKK1, MEKK2 and MEKK3. My data suggest that MEKK2 underwent dramatic functional divergence from other MAPKKKs to gain the function as a negative regulator of MAP kinases, which is an interesting evolutionary event conferring novel biochemical mechanisms of the paralog. Taken together, the studies in my Ph.D. thesis provide new insights into the regulation of the two main MAP kinase cascades activated during plant immunity.

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