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Meet the Faculty

John C. Walker

John Walker
Adjunct Professor
Division of Plant Sciences

Education
B.S., Biology, Arizona State University
Ph.D., Biochemistry, University of Georgia

Description

Floral Position Diagram

The receptor-like protein kinases HAE and HSL2 regulate abscission. Phenotypes of representative siliques from wildtype and hae hsl2 plants at different flower positions. The floral organs of of wildtype plants abscise by position 8, but the flowers from the hae hsl2 double mutant plants display an abscission defective phenotype.

Multicellular organisms need to carry out many processes in a coordinated manner to sense and respond to both external and internal signals in an intricate and precise way. Multi-step signal transduction creates the necessary complexity for refined regulation of a cell's response to developmental signals and its environment. A common way cells relay molecular messages is by reversible protein phosphorylation; protein kinases add phosphates to their target protein(s) and protein phosphatases remove them. Cells can begin this process with receptor protein kinases, using phosphorylation status to transduce external messages into the cell. A plethora of candidate receptor protein kinases have been found in plants, but only a few have been investigated. Function for the plant receptor-like kinases (RLKs) has been shown in various biological processes such as development, disease resistance and self-incompatibility.

There are several hundred RLK genes and they represent the largest group of cell surface receptors in plants. The 1999 report of the NSF-Sponsored Workshop: New Directions in Plant Biological Research points out the RLKs represent unexpected discoveries derived from the Arabidopsis genomic sequence. The report states, "What are the roles of the hundreds of these proteins? Their existence implies a massive network of cell-cell and environment-plant communication, via a series of ligands yet to be discovered. Understanding this network will give us an entirely new view of plant development, environmental response, and organismal integration”. Our research is directed toward understanding the function of the RLK genes.

We are using the approaches of functional genomics and proteomics to understand RLK function. We have ongoing projects to 1) Isolate and characterize loss-of-function and gain-of-function mutations; 2) Determine the patterns of expression of the mRNAs, and establish the cellular and subcellular locations of these receptors by use of green fluorescent protein fusions; 3) Define the regulatory networks that mediate signaling by these protein kinases by screening for genetic modifiers and interaction partners. Although we are not yet able to describe an entire signal transduction cascade for any one RLK, these approaches promise to provide important insights into the molecular mechanisms by which the receptor protein kinases control development and adaptive responses in plants.

Selected publications

Larue, C.T., J. Wen and J.C. Walker 2008 A microRNA-transcription factor module regulates lateral organ size and patterning in Arabidopsis, Plant J, in press Cho, S.K, C.T. Larue, D. Chevalier, H. Wang, T.L. Jinn, S. Zhang and J.C. Walker 2008 Regulation of Floral Organ Abscission in Arabidopsis thaliana Proceedings of the National Academy of Sciences of the USA, 105:15629-15634.

Yu, B., L. Bi, B. Zheng, L. Ji, D. Chevalier, M. Agarwal, V. Ramachandran, W. Li, T. Lagrange, J.C. Walker and X. Chen 2008 The FHA domain proteins DAWDLE in Arabidopsis and SNIP1 in humans act in small RNA biogenesis, Proceedings of the National Academy of Sciences of the USA, 105:10073-10078.

Wang, H., Y. Liu, K. Bruffett, J. Lee, G. Hause, J.C. Walker and S. Zhang 2008 Haplo-insufficiency of MPK3 in MPK6 Mutant Background Uncovers a Novel Function of these two MAPKs in Arabidopsis Ovule Development, Plant Cell, 20:602-613.

Ding, Z., H. Wang, X. Liang, E. R. Morris, F. Gallazzi, S. Pandit, J. Skolnick, J.C. Walker and S. R. Van Doren 2007 Phosphoprotein and Phosphopeptide Interactions with the FHA Domain from Kinase-Associated Protein Phosphatase, Biochemistry, 46:2684-2696.

Wang, H., Y. Liu, N. Ngwenyama, J.C. Walker and S. Zhang 2007 Stomatal development and patterning are regulated by environmentally responsive MAP kinases, Plant Cell, 19:63-73.