75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

185 DAWDLE, a Forkhead-Associated Domain Gene, Regulates Plant Development
David Chevalier, Erin Morris, John Walker
University of Missouri, Columbia, MO
Phosphoprotein-binding domains are found in many different proteins and specify protein-protein interactions critical
for signal transduction pathways. Fork-head associated (FHA) domains bind phosphothreonine containing peptides and
control many aspects of cell proliferation in yeast and animal cells. The Arabidopsis thaliana protein, Kinase Associated
Protein Phosphatase includes a FHA domain that mediates interactions with receptor-like kinases, which in turn regulate
a variety of signaling pathways involved plant growth and pathogen responses. Screens for insertional mutations in
other Arabidopsis FHA domain containing genes identified a mutant with pleiotropic defects. dawdle (ddl) plants are
developmentally delayed, produce defective roots, shoots, flowers, and have reduced seed set. DDL is expressed in the
root and shoot meristems and the reduced size of the root apical meristem in ddl plants suggest a role early in organ
development.
186 The Circadian Clock Regulates Auxin Signaling and Responses in Arabidopsis
Michael Covington, Stacey Harmer
Section of Plant Biology, College of Biological Sciences, University of California, Davis, California, USA
The circadian clock plays a pervasive role in the temporal regulation of plant physiology, environmental
responsiveness, and development. In contrast, the phytohormone auxin plays a similarly far-reaching role in the spatial
regulation of plant growth and development. Seventy years ago, Went and Thimann noted that plant sensitivity to auxin
varied according to the time of day, an observation which they could not explain. Here we present work that explains this
puzzle, demonstrating that the circadian clock regulates auxin signal transduction. Using genome-wide transcriptional
profiling, we found many auxin-induced genes are under clock regulation. We verified that endogenous auxin signaling
is clock regulated with a luciferase-based assay. Exogenous auxin has only modest effects on the plant clock, but the
clock controls plant sensitivity to applied auxin. Significantly, we found both transcriptional and growth responses to
exogenous auxin are gated by the clock. Thus the circadian clock regulates some, and perhaps all, auxin responses. As
a consequence, many aspects of plant physiology not previously thought to be under circadian control may show timeof-day
specific sensitivity, with likely important consequences for plant growth and environmental responses.
The project was supported by the NRI of the USDA CSREES (2004-35100-14903 to MFC) and the NIH (5R01GM069418-02 to SLH).