Mitogen-activated protein kinase (MPK) cascades are conserved mechanisms of signal transduction across eukaryotes. Despite the importance of MPK proteins in signaling events, specific roles for many Arabidopsis MPK proteins remain unknown. Multiple studies have suggested roles for MPK signaling in a variety of auxin-related processes. To identify MPK proteins with roles in auxin response, we screened mpk insertional alleles and identified mpk1-1 as a mutant that displays hypersensitivity in auxin-responsive cell expansion assays. Further, mutants defective in the upstream MAP kinase kinase MKK3 also display hypersensitivity in auxin-responsive cell expansion assays, suggesting that this MPK cascade affects auxin-influenced cell expansion. We found that MPK1 interacts with and phosphorylates ROP BINDING PROTEIN KINASE 1 (RBK1), a protein kinase that interacts with members of the Rho-like GTPases from Plants (ROP) small GTPase family. Similar to mpk1-1 and mkk3-1 mutants, rbk1 insertional mutants display auxin hypersensitivity, consistent with a possible role for RBK1 downstream of MPK1 in influencing auxin-responsive cell expansion. We found that RBK1 directly phosphorylates ROP4 and ROP6, supporting the possibility that RBK1 effects on auxin-responsive cell expansion are mediated through phosphorylation-dependent modulation of ROP activity. Our data suggest a MKK3 • MPK1 • RBK1 phosphorylation cascade that may provide a dynamic module for altering cell expansion.
Bibliographical noteFunding Information:
The authors would like to thank Scott Peck for helpful discussion, Zhenbiao Yang for rop2i rop4-1 seed and helpful discussion, the ABRC for providing mkk3, mpk1 and rbk1 SALK insertional alleles, the ABRC for providing RBK1, ROP4 and ROP6 cDNAs, RIKEN for MPK1 cDNA, and Hongwei Jing, Sam Powers and Ashley Sherp for critical comments on the manuscript. This research was supported by the National Science Foundation (DGE-1143954 to T.A.E. and IOS-1453750 to L.C.S.), the United States Department of Agriculture-National Institute of Food and Agriculture (2016-67011-25096 to E.M.F.) and the National Institutes of Health (R01 GM112898 to L.C.S.).
- Arabidopsis thaliana
- MAP kinase
- RHO-like GTPase from plants
- cell expansion