Molecular basis of signaling network involved in biological clock and circadian rhythms in higher plants

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Circadian rhythms are driven by an endogenous biological clock(s) that regulates many biochemical, physiological and behavioral processes in a wide variety of organisms. In higher plants too, there are a wide range of biological processes that are controlled through the circadian clock In these respects, recent intensive studies on the model plant Arabidopsis thaliana have begun to shed light on the molecular mechanisms underlying these circadian-controlled biological events. In Arabidopsis thaliana, a number of circadian-associated factors have been identified. Among those, TOC1 is believed to be a component of the central oscillator. TOC1 is a member of a small family of proteins, designated as ARABIDOPSIS PSEUDO-RESPONSE REGULATORS (APRR1/TOC1, APRR3, APRR5, APRR7, and APRR9). Nonetheless, it is not very clear whether or not the APRR family members other than APRR1(TOC1 are also implicated in the mechanisms underlying the circadian rhythm. To address this issue further, here we characterized a set of T-DNA insertion mutants, each of which is assumed to have a severe lesion in each one of the quintet genes. We found that a given mutation singly, if not directly, affects the circadian-associated biological events simultaneously : (i) the flowering time in the long-day photopertod conditions, and (ii) the red light sensitivity of seedlings during the early photomorphogenesis, (iii) the period of free-wnning rhythms of certain clock-controlled genes including CCA1 and APRR1/TOC1 in constant white light. These results suggest that, although the quintet members other than APRR1/TOC1 may not be directly integrated into the framework of the central oscillator, they are crucial for a better understanding of the molecular mechanisms underlying the Arabidopsis circadian clock.