A Genetic Approach to Understanding the Plant Circadian Clock

Somers, David E.,

Department of Plant Biology, The Ohio State University, Columbus, OH

Close coordination of developmental and physiological processes with the environment is essential to successful plant growth. Many of the key events in plant development depend on receiving the appropriate environmental signals at the right time, and plants possess internal timekeeping mechanisms that allow them to keep pace with and anticipate cyclic events in their environment. By alternating between restrictive and permissive phases, such an internal timekeeper can act to both gate the effect of an environmental stimulus, and to modulate the extent to which physiological processes occur. The best understood of these endogenous clocks is the one based on a 24 hour (circadian) pacemaker (1).

Using a low-light video imaging system, we are exploiting transgenic Arabidopsis expressing firefly luciferase to obtain a novel bioluminescent phenotype. The promoter of the gene coding for the chlorophyll a/b binding protein 2 (CAB2), driving luciferase expression (CAB2-luc), can accurately report the temporal and spatial expression patterns of the endogenous CAB2 gene to obtain accurate and reproducible estimates of the period, phase and amplitude of cyclic transcription. We have applied this technique to genetically dissect and characterize components affecting the circadian clock in Arabidopsis (2-6).

A simple model of the clock posits an input (entrainment) pathway, a central oscillator and an output pathway (1). Our focus on identifying mutations that alter period length has led to the characterization of the TOC7 locus. Mutations in this gene lengthen the normally 24.5 h period to ca. 27-28 h. The severity of the change in period is strongly dependent on light intensity, suggesting its importance in modulating light input to the clock. TOC7 cloning is nearly complete and future work will focus on characterizing the role of the protein within the circadian system, as well as identifying genetic and biochemical interactors. The uniquely circadian phenotype of these mutants, which look morphologically wild-type, suggests they will serve as a excellent entry point towards the elucidation of the molecular basis of the clock in plants.

1. Somers, D.E. 1999. The physiology and molecular bases of the plant circadian clock. Plant Physiol. 121: 9-20.
   
   
2. Park D.H., Somers D.E., Kim Y.S., Choy Y.H., Lim H.K., Soh M.S., Kim H.J., Kay S.A., Nam H.G. 1999. Control of circadian rhythms and photoperiodic flowering by the Arabidopsis GIGANTEA gene. Science. 285: 1579-1582.
   
   
3. Somers, D.E. Devlin, P.A. and Kay, S.A. 1998. Phytochromes and cryptochromes in the entrainment of the Arabidopsis circadian clock. Science 282: 1488-1490.
   
   
4. Somers, D.E., Webb, A., Pearson, M., and Kay, S.A. 1998. The short-period mutant, toc1-1, alters circadian clock regulation of multiple outputs throughout development in Arabidopsis thaliana. Development 125: 485-494.
   
   
5. Anderson, S.L., Somers, D.E., Millar, A.J., Hanson, K., Chory, J. and Kay, S.A. 1997. Attenuation of phytochrome A and B signaling pathways by the Arabidopsis circadian clock. Plant Cell 9: 1727-1743.
   
   
6. Hicks, K.A., Millar, A.J., Carre, I.A., Somers, D.E., Straume, M., Meeks-Wagner, D.R. and Kay, S.A. 1996. Conditional circadian dysfunction of the Arabidopsis early-flowering 3 mutant. Science 274: 790-792.