Circadian Clock-Regulated Expression of Phytochrome and Cryptochrome Genes in Arabidopsis

Abstract

Many physiological and biochemical processes in plants exhibit endogenous rhythms with a period of about 24 h. Endogenous oscillators called circadian clocks regulate these rhythms. The circadian clocks are synchronized to the periodic environmental changes (e.g. day/night cycles) by specific stimuli; among these, the most important is the light. Photoreceptors, phytochromes, and cryptochromes are involved in setting the clock by transducing the light signal to the central oscillator. In this work, we analyzed the spatial, temporal, and long-term light-regulated expression patterns of the Arabidopsis phytochrome (PHYA to PHYE) and cryptochrome (CRY1 and CRY2) promoters fused to the luciferase (LUC ؉ ) reporter gene. The results revealed new details of the tissue-specific expression and light regulation of the PHYC and CRY1 and 2 promoters. More importantly, the data obtained demonstrate that the activities of the promoter::LUC ؉ constructs, with the exception of PHYC::LUC ؉ , display circadian oscillations under constant conditions. In addition, it is shown by measuring the mRNA abundance of PHY and CRY genes under constant light conditions that the circadian control is also maintained at the level of mRNA accumulation. These observations indicate that the plant circadian clock controls the expression of these photoreceptors, revealing the formation of a new regulatory loop that could modulate gating and resetting of the circadian clock.The periodic succession of days and nights is an eternally recurring environmental factor ever since life has appeared on the Earth. It is postulated that organisms possessing the ability to adapt to the predictable changes of the environment have an evolutionary advantage and that this benefit has promoted the development of timekeeping mechanisms (endogenous clocks). The biological clocks that generate and maintain oscillations of many physiological and molecular processes with a period length close to 24 h are also referred to as circadian clocks. Circadian rhythms persist under constant conditions; however, to function reliably and to be useful for the organisms, the clocks must operate in harmony with the periodic changes of the outer environment. To achieve this synchrony, the circadian clock is reset to the local time by specific stimuli perceived at dawn and dusk. The most important entraining factors are light and temperature. Light signals are perceived and transduced to the central oscillator via specialized photoreceptors. In plants, the photoreceptor phytochrome and cryptochrome have been shown to be involved in this process (Somers et al., 1998;Devlin and Kay, 2000).Phytochromes are chromoproteins that contain a covalently linked linear tetrapyrrole chromophore per molecule and exist as homodimers. These photoreceptor molecules absorb red and far-red light, which activates or inactivates them, respectively . In higher plants, small multigene families encode these molecules. In Arabidopsis, five genes (PHYA-E) have been isolated (Sharrock and Quail, 1989; Clack et al...

Regulation of flowering time: all roads lead to Rome

“…Common to all of these hypotheses is that they require internal oscillators, i.e., genes regulated by the circadian clock, and environmental changes such as the day-night cycle to synchronize these rhythms. Interestingly, phytochromes and cryptochromes themselves have been shown to be regulated by the circadian clock, indicating the existence of a regulatory loop that modulates gating and resetting of the circadian clock [32].…”

Cryptochrome photoreceptors cry1 and cry2 antagonistically regulate primary root elongation in Arabidopsis thaliana

“…Thus, the relatively dim light gradients that may penetrate soil as well as light guiding (Mandoli et al 1990) could be suYcient to activate the three major plant photosensory receptors (phytochromes, cryptochromes, and phototropins) in roots. Studies involving GFP fusions with phytochromes have demonstrated that both phyA and phyB are expressed in Arabidopsis root tissues (Somers and Quail 1995), as are phototropins (Sakamoto and Briggs 2002) and cryptochromes (Toth et al 2001). The sites for signal perception of phytochrome-mediated graviperception and for blue-light induced (phototropin mediated) phototropic curvature have both been shown to be localized to the root, although inhibition of growth by phytochrome may involve a signal transmitted from the shoot (Correll and Kiss 2005).…”

Photoreceptors in Arabidopsis thaliana : light perception, signal transduction and entrainment of the endogenous clock

“…Rather, the promoter activity of phytochromes and cryptochromes is diurnally regulated (Bognar et al 1999;Toth et al 2001). Transcripts encoding the light-stable proteins phyB, phyC, phyD, phyE and cry1 peak during the early hours of the daily light phase whereas those encoding light-labile phyA and cry2 reach their highest level close to dusk (Fig.…”

“…zt Zeitgeber time, hours after lights on. Modified from Toth et al (2001) Reduced synthesis of photoreceptive molecules during the night may serve to buffer the circadian clock against moonlight, for example, which does not signal dawn or dusk and thus should not reset the clock (Bu¨nning and Moser 1969).…”