The circadian system of Arabidopsis thaliana: forward and reverse genetic approaches.

It is now widely accepted that autoregulatory circuits involving transcription/translation of clock genes form the molecular basis of the endogenous circadian clock in different organisms. In Arabidopsis thaliana, the RNA-binding protein AtGRP7 (Arabidopsis thaliana glycine-rich protein) has been identified as part of a negative-feedback loop through which AtGRP7 regulates the circadian oscillations of its own transcript. Experimental evidence indicates that this feedback loop also is influenced by another oscillator. Support for this hypothesis comes from the characterization of the clock mutant toc1 (timing of cab expression) and the recent isolation of two candidate clock molecules, LHY (late elongated hypocotyl) and CCA1 (circadian clock associated). TOC1, as well as the LHY and CCA1 oscillatory feedback loops, influence several rhythmic physiological and molecular processes in Arabidopsis, including cyclic Atgrp7 gene expression. We discuss the features of these feedback loops with relation to the organization of the circadian system in Arabidopsis.