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The rotation of earth around its axis leads to environmental changes in light intensity and temperature that predictably define day-night cycle. Most organisms coordinate their life cycle in anticipation of these environmental fluctuations. The measurement of time and synchronization with the environment are achieved by an internal time-keeping mechanism biological clock. The circadian clock is an endogenous mechanism that generates rhythms with an approximately 24-h period and enables plants to adapt to daily and seasonal changes in their environment. The circadian clock can be considered as a mechanism that translates environmental signals into temporal information in order to rhythmically coordinate metabolism and physiology. Circadian rhythms enable biological processes to occur at the most appropriate times during the day-night cycle, which confers a advantage to organisms. Circadian clock regulates many processes in plants, including leaf and stomatal movements, photosynthesis and growth and contributes to photoperiodism. Circadian rhythms are generated by molecular oscillators that in Arabidopsis thaliana have been shown to consist of network of transcription factors arranged in interlocking negative-feedback loops involving a number of elements. Circadian clock is based on the feedback loop involving the genes TOC1 (TIMING OF CAB EXPRESSION1), CCA1 (CIRCADIAN CLOCK ASSOCIATED1) and LHY (LATE ELONGATED HYPOCOTYL). Protein TOC1 induces the transcription of genes LHY1 and CCA1, which are translated into proteins that repress the expression of TOC1. An important characteristic of circadian oscillators is that they can be entrained by cues from the environment, such as daily changes in light and temperature, transduced via input pathways. The last components of circadian clock model are the output pathways that provide a link between the oscillator and the various biological processes and their rhythms, which are controlled by circadian clock. A unique property of circadian oscillator's mechanism is temperature compensation the stability of the period of the clock over a wide range of physiological temperatures and the persistence of rhythms in the absence of environmental signals. Bidirectional, mutual regulation is known between plant hormones' (auxin, abscisic acid, cytokinin) signaling pathways and grid of interactions of central oscillator's compounds. Plant hormones and central oscillator together control many physiological processes, and additionally, parallel regulation of common pool of genes was reported. Mechanism of central oscillator is regulated not only during initiation of transcription of genes encoding oscillator's compounds. It was shown that influence on the expression of oscillator's genes encoding transcription factors is exerted at the level of transcript stability, alternative splicing or miRNA as well. Control of degradation of oscillator's proteins, through their phosphorylation as a first step, is yet another level of genes expression regulation. Distinct and still not fully elucidated aspect of expression of central oscillator's genes is its regulation through alterations of chromatin architecture.
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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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