Circadian Clock in Arabidopsis
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Circadian Clock in Arabidopsis

Photoreceptors and circadian clocks are universal mechanisms for sensing and responding to the light environment. In addition to regulating daily activities, photoreceptors and circadian clocks are also involved in the seasonal regulation of processes such as flowering. Circadian rhythms govern many plant processes, including movements of organs such as leaves and petals, stomata opening, stem elongation, sensitivity to light of floral induction, metabolic processes such as respiration and photosynthesis and expression of a large number of different genes. In plants, phototransduction not only sets the phase, but also affects the amplitude and period of circadian rhythms. Members of the phytochrome family of plant photoreceptors, which can exist in two photochemically interconversible forms: the inactive Pr form (red light absorbing) that phototransforms into the active Pfr form (Far-red light absorbing) upon absorption of red light. Both Pr and Pfr and are involved in regulation of plant development and growth, play important roles in regulating clock activities (Ref.1).

In Arabidopsis, the interaction between light and the circadian clock is dependent on both the fluency rate and the quality of light. The circadian clock phases the peak mRNA abundance of many genes to distinct times of day. Furthermore, the phase angle of specific circadian rhythms with environmental light-dark (LD) cycle controls seasonal behavior such as flowering (Ref.2). The Arabidopsis cryptochromes CCT1 or CCT2 (Cry1 and Cry2) fused to Gus (Glucuronidase) mediates a variety of blue light-induced responses, including hypocotyl shortening, cotyledon expansion, and anthocyanin production. Direct targeting of light signals through Phy (Phytochrome) molecules to a promoter-bound basic helix-loop-helix factor PIF3 (Phytochrome-Interacting Factor-3) regulates both photomorphogenic and clock genes through a short, branched transcriptional cascade. PhyB (Phytochrome-B) translocates to the nucleus following light-induced conversion of the Pr (PrB) to the Pfr (PfrB) form where it binds to G-box-bound PIF3 and induces the expression of the primary target genes CCA1 (Circadian Clock-Associated Protein-1) and LHY (Late Elongated Hypocotyl) either directly by functioning as coregulators in recruiting or modifying components of the core transcriptional machinery, PIC (Preinitiation Complex), or indirectly by modifying the potential transcriptional regulatory activity of PIF3. CCA1 is regulated at different levels in the light and circadian pathways and can be phosphorylated by the CSNK2 (Casein Kinase-2) (Ref.3). The distinct phosphorylation states of CCA1 also affect its regulatory role in light and circadian control of gene expression. The encoded Myb-related transcription factors bind in turn to their cognate binding sites CBS (CCA1-Binding Site), where they either induce expression of genes such as CAB (Chlorophyll A/B Binding Protein) or repress expression of TOC1 (Timing of CAB Expression-1). TOC1 regulates CCA1, creating a feedback loop that constitutes the circadian clock. The ELF3 (Early Flowering-3) gene also acts in the input pathway and is a component of a PhyB signaling complex that controls hypocotyl elongation (Ref.4).

The circadian clock plays a number of different roles. One is in the regulation of photoperiodism, which controls reproduction in many organisms, including flowering time in many plants. This photoperiodic response ensures that plants flower during the appropriate season. Arabidopsis is a facultative long-day plant; in which flowering is initiated earlier in response to long days, but Arabidopsis will eventually flower, albeit much later, even under short days. An additional advantage of photoperiodic control is that, by ensuring coincident flowering of a population of plants, the chances of outcrossing are increased (Ref.5). Another function that has been suggested for the clock is to allow organisms to program activities so that they occur at a specific part of the diurnal cycle. Circadian clocks may also allow the organism to anticipate night/day changes. For example, some molecular processes important for photosynthesis are initiated before dawn so that by sunrise the plants are ready to take maximum advantage of available light for photosynthesis. In addition, organisms may produce screening pigments before the sun rises to avoid damage by visible and UV light (Ref.6).