Biological clocks in Arabidopsis thaliana

Abstract

This review is dedicated to the memory of Colin Pittendrigh, whose outstanding contribution to research on circadian rhythms includes the term ' circadian '. Biological rhythms are ubiquitous in eukaryotes, and the best understood of these occur with a period of approximately a day -circadian rhythms. Such rhythms persist even when the organism is placed under constant conditions, with a period that is close, but not exactly equal, to 24 h, and are driven by an endogenous timerone of the many ' biological clocks '. In plants, research into circadian rhythms has been driven forward by genetic experiments using Arabidopsis. Higher plant genomes include a particularly large number of genes involved in Abbreviations : bHLH, transcription factor with basic helix-loop-helix DNA-binding domain ; CAB, chlorophyll a\b-binding

Natural allelic variation identifies new genes in the Arabidopsis circadian system

“…Such co-localization at 10 cM resolution could arise by chance linkage, because about 80 loci with some effect on¯owering time have been described, throughout the Arabidopsis genome (reviewed by Koornneef et al, 1998;Levy and Dean, 1998). Many of the mutations that affect photoperiodic control are thought to have functions speci®c to photoperiodism (Coupland, 1998;Koornneef et al, 1998;Millar, 1999). Such genes are not expected to affect the circadian system, although feedback from the photoperiodic system to the circadian oscillator remains possible.…”

“…Two types of mutation have been shown to affect both circadian period and¯owering time. Simple models of photoperiodic control suggest that the measurement of day length that underlies photoperiodism requires both photoreceptors and circadian timing (Coupland, 1998;Koornneef et al, 1998;Millar, 1999;Thomas and Vince-Prue, 1996). A QTL that affects the circadian clock could alter period directly, and¯owering time indirectly, via the photoperiodic mechanism.…”

“…The circadian system is an endogenous biological timer controlling a wide range of rhythmic processes, all of which maintain rhythmic periods close to 24 h under constant environmental conditions (Lumsden and Millar, 1998;Millar, 1999;Sweeney, 1987). Circadian rhythms in Arabidopsis thaliana include rhythmic leaf movements (nyctinasty, Engelmann et al, 1992) and hypocotyl elongation (Dowson-Day and Millar, 1999); the rhythmic opening of stomata (Somers et al, 1998b;Webb, 1998); and the transcription of a number of genes including chlorophyll a/ b-binding protein (CAB or LHC) genes (reviewed by Fejes and Nagy, 1998).…”

“…A further arhythmic mutant was identi®ed as a transgenic line that overexpressed the gene circadian clock-associated 1, CCA1 (Wang and Tobin, 1998). Circadian rhythm defects are expected in some aphotoperiodic mutants, because a circadian system is thought to be required for the measurement of day length that underlies photoperiodism (Coupland, 1998;Koornneef et al, 1998;Millar, 1999;Thomas and Vince-Prue, 1996). It is not yet clear whether any of the circadian rhythm mutations identi®es components of the central oscillator in Arabidopsis; ELF3 appears to affect the oscillator indirectly via interaction with a phototransduction pathway (Anderson et al, 1997;Hicks et al, 1996;Millar, 1998).…”

Picking out parallels: plant circadian clocks in context

“…There are many such rhythms in plants; for example, leaf and petal movements, photosynthetic capacity, fragrance emissions, cell division, stomatal movements and gene expression (Millar 1999;McClung 2000). A great aid to the measurement of circadian rhythms in plants has been the development of reporter gene fusions, most notably to the ¢re£y luciferase (luc) gene.…”