The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. ; Title from title screen of research.pdf file (viewed on July 18, 2008) ; Vita. ; Thesis (Ph. D.) University of Missouri-Columbia 2006. ; [ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Protein phosphorylation and dephosphorylation are fundamental mechanisms involved in many cell-signaling events. Coordinated actions of protein kinases and protein phosphatases help maintain the signaling through reversible protein phosphorylation and dephosphorylation. In Arabidopsis, both protein kinases and protein phosphatases exist as large protein families. Here the recent progress in understanding the roles of plant protein phosphatases and mitogen-activated protein kinases (MAPKs) is reviewed. To further understand the regulatory functions of protein phosphatases and kinases, I carried out detailed functional analyses of kinase associated protein phosphatase (KAPP) and a MAPK cascade involving MAPK3 and MAPK6. KAPP is the only protein phosphatase that has been shown to physically interact with multiple receptor like protein kinases (RLKs). To understand the role of KAPP in coordinating RLK signal transduction, we carried out detailed functional analyses of KAPP in Arabidopsis. We demonstrated the KAPP protein phosphatase 2C (PP2C) domain is functional in vivo; overexpressing KAPP with a mutation in the PP2C domain (D346G) creates a dominant negative effect and the transgenic plants have a pleiotropic phenotype. Fork-head associated (FHA) domain-mediated protein-protein interaction is responsible for this dominant negative effect. Multiple null alleles of KAPP were isolated and no obvious growth or developmental defects were observed. However, double mutants of kapp3/bri1-5 show increased sensitivity to brassinolide treatment. In vivo and in ...