The circadian clock controls the expression pattern of the circadian input photoreceptor, phytochrome B

Abstract

Developmental and physiological responses are regulated by light throughout the entire life cycle of higher plants. To sense changes in the light environment, plants have developed various photoreceptors, including the red͞far-red light-absorbing phytochromes and blue light-absorbing cryptochromes. A wide variety of physiological responses, including most light responses, also are modulated by circadian rhythms that are generated by an endogenous oscillator, the circadian clock. To provide information on local time, circadian clocks are synchronized and entrained by environmental time cues, of which light is among the most important. Light-driven entrainment of the Arabidopsis circadian clock has been shown to be mediated by phytochrome A (phyA), phytochrome B (phyB), and cryptochromes 1 and 2, thus affirming the roles of these photoreceptors as input regulators to the plant circadian clock. Here we show that the expression of PHYB::LUC reporter genes containing the promoter and 5 untranslated region of the tobacco NtPHYB1 or Arabidopsis AtPHYB genes fused to the luciferase (LUC) gene exhibit robust circadian oscillations in transgenic plants. We demonstrate that the abundance of PHYB RNA retains this circadian regulation and use a PHYB::Luc fusion protein to show that the rate of PHYB synthesis is also rhythmic. The abundance of bulk PHYB protein, however, exhibits only weak circadian rhythmicity, if any. These data suggest that photoreceptor gene expression patterns may be significant in the daily regulation of plant physiology and indicate an unexpectedly intimate relationship between the components of the input pathway and the putative circadian clock mechanism in higher plants.Arabidopsis ͉ tobacco ͉ oscillation ͉ luciferase ͉ transgenic plants

DRACULA2, a dynamic nucleoporin with a role in the regulation of the shade avoidance syndrome in Arabidopsis

“…To identify new regulatory components of the SAS, a highthroughput genetic screen was performed after EMS mutagenesis of a shade-responsive luciferase reporter line, PHYB:LUC (hereafter PBL), which expresses the Luciferase (LUC) gene under the control of the Arabidopsis PHYB promoter in the Ws-2 genetic background (Kozma Bognar et al, 1999). As a result we identified dracula (dra) mutants, which exhibit an attenuated luciferase response to low R: FR light.…”

A Constitutively Active Allele of Phytochrome B Maintains Circadian Robustness in the Absence of Light

“…Outputs are easily defined as processes under circadian control that do not feed back into the circadian system; however, the distinction between input and core components of the circadian system has become increasingly blurred with our expanding knowledge of the circadian system. For example, phys transduce light signals from the red and far-red portion of the spectrum into the circadian system, but the transcription of this photoreceptor family is concurrently regulated by the clock (Somers et al, 1998;Bognár et al, 1999;Devlin and Kay, 2000;Tóth et al, 2001;Wenden et al, 2011). Despite this complication, the core oscillator is generally considered to consist of a complex web of interacting feedback loops that are sufficient to generate a selfsustaining oscillation of transcripts and proteins (Fogelmark and Troein, 2014;Hsu and Harmer, 2014).…”

Picking out parallels: plant circadian clocks in context

“…Their data demonstrated that the plant circadian clock controls the expression of the input receptors, adding a further regulatory loop on the lightinput pathway. However, PHYB protein, the most abundant Arabidopsis PHY, does not show circadian variation in its absolute level, although new synthesis is rhythmic (Bognar et al 1999).…”

“…The circadian clock regulates expression of the PHYB gene in tobacco and Arabidopsis (Bognar et al 1999). These authors used luciferase^reporter constructs to demonstrate that PHYA, PHYB, PHYC and CRY1 but not CRY2, had clear circadian rhythms of expression, and the regulation is maintained at the level of messenger RNA accumulation (Bognar et al 2000).…”

“…This CAB2::luc marker provides us with another circadian phenotype that has aided in the isolation of mutants in the circadian clock, such as the toc (timing of CAB) mutants (Millar et al 1995). Since then, other clock-related gene promoter^luciferase fusions have been made so that the expression of other genes can be inferred (Bognar et al 1999;I. Carre¨and J.-Y.…”