Putative Components of the Arabidopsis Circadian Clock

Abstract

Although plant circadian rhythms have been described as far back as Antiquity, the molecular basis of the plant biological clock is still unknown. The reasons for this lag behind Neurospora, Drosophila and now mammalian systems has primarily been the lack of suitable genetic systems and of easily screenable phenotypes for the isolation of mutants with defective circadian rhythms. In the past few years, the small, fast growing weed Arabidopsis thaliana has become a widely used system for plant molecular genetics, and extensive genome mapping and sequencing programs have greatly facilitated gene isolation. New, non-invasive assays for plant circadian rhythms have been developed, that made the isolation of clock mutants possible. Defective circadian rhythms have been shown to correlate with visible phenotypes, such as aberrant flowering times, and these phenotypes have been used successfully in the isolation of mutants with altered clock function. As a consequence plant circadian biology is now experiencing a renewed interest, due to the isolation of a number of putative components of the circadian regulatory system. The first break-through was from Steve Kay and colleagues, who fused the promoter of the circadian-regulated CAB (chlorophyll a/b-binding protein) gene to a firefly luciferase reporter gene. Transgenic Arabidopsis plants carrying this transgene exhibited rhythmic luminescence when supplied with the substrate luciferin. The luminescence rhythm was monitored using a photon-counting camera. This non-invasive assay was used to isolate mutants with altered rhythmicity (Millar et al., 1995a). One mutant isolated by this method, toc1 (Timing of CAB-1) has been extensively characterized. The toc1 mutation was shown to shorten the period of a variety of circadian rhythms, including CAB (Millar et al., 1995a) and