Genetic Approaches to Photoperiodism

212

183

A wide range of processes in plants, including expression of certain genes, is regulated by endogenous circadian rhythms. The circadian clock-associated 1 (CCA1) and the late elongated hypocotyl (LHY) proteins have been shown to be closely associated with clock function in Arabidopsis thaliana. The protein kinase CK2 can interact with and phosphorylate CCA1, but its role in the regulation of the circadian clock remains unknown. Here we show that plants overexpressing CKB3, a regulatory subunit of CK2, display increased CK2 activity and shorter periods of rhythmic expression of CCA1 and LHY. CK2 is also able to interact with and phosphorylate LHY in vitro. Additionally, overexpression of CKB3 shortened the periods of four known circadian clock-controlled genes with different phase angles, demonstrating that many clock outputs are affected. This overexpression also reduced phytochrome induction of an Lhcb gene. Finally, we found that the photoperiodic flowering response, which is influenced by circadian rhythms, was diminished in the transgenic lines, and that the plants flowered earlier on both long-day and short-day photoperiods. These data demonstrate that CK2 is involved in regulation of the circadian clock in Arabidopsis. C ircadian rhythms are driven by endogenous biological clocks that regulate many biochemical, physiological, and behavioral processes in a wide variety of organisms. According to current models, circadian clocks regulating these rhythms consist of input pathways, a central oscillator, and output pathways (1-3). Oscillators are thought to generate rhythms by a transcription-translation negative feedback loop (4-8). Studies in cyanobacteria, Neurospora, Drosophila, and mouse have found that both positive and negative elements that activate and inhibit the transcription of clock genes are required to maintain the loop. In addition, posttranscriptional and posttranslational regulation play important roles in circadian clocks in Drosophila and Neurospora (7-10). The oscillator can be entrained by input pathways from environmental cues such as light and temperature, and, in turn, regulates specific cellular events such as expression of clock-controlled genes (1-3).Until recently, little was known about the molecular mechanisms of circadian clocks in plants (11-13). In Arabidopsis thaliana, the toc1 mutation affects the period of many circadian rhythms (14,15). Although the corresponding gene has not yet been cloned, it is thought that TOC1 encodes a component of the oscillator. The ELF3 gene is proposed to act in the input pathway (16). Two Myb-related genes, circadian clock-associated 1 (CCA1) and late elongated hypocotyl (LHY), have also been identified as potential clock genes (17, 18), and CCA1 was found to act as a transcription factor for Lhcb gene expression (19). Expression of CCA1 and LHY oscillates with a circadian rhythm. Constitutive expression of CCA1 was shown to abolish several distinct circadian rhythms, suppress its own expression as well as that of LHY, and delay flowering substa...

“…A developmental response in which circadian rhythms are involved is photoperiodic flowering (30). We found that CKB3 overexpression did affect the photoperiodic induction of flow- ering.…”

Section: Ck2 Can Interact With and Phosphorylate Lhy In Vitromentioning

confidence: 72%

The protein kinase CK2 is involved in regulation of circadian rhythms in Arabidopsis

Sugano

Andronis²,

Ong³

et al. 1999

212

183

“…Although photoperiodic control of flowering time in soybean was exten- sively studied in the early 20 th century, there is surprisingly little information concerning latitudinal cline in photoperiodic flowering of soybean examined in defined photoperiod and temperature conditions (7). To further understand the role of cryptochrome in soybean photoperiodic f lowering, we analyzed photoperiodic responses of flowering time of soybean cultivars collected from areas in China that range from Ϸ25°N to Ϸ50°N (Fig.…”

Section: -G)mentioning

confidence: 99%

“…Plants have at least two types of cryptochromes: cryptochrome 1 (CRY1) and cryptochrome 2 (CRY2) (4,5). In Arabidopsis, CRY1 mediates mainly blue light control of deetiolation, whereas CRY2 regulates primarily photoperiodic flowering, defined here as the reaction to change flowering time in response to altered photoperiods (4,6,7). In addition to Arabidopsis, cryptochromes have also been studied in other plants, including algae (8), moss (9), fern (10), tomato (11,12), rapeseed (13), pea (14), and rice (15,16).…”

mentioning

confidence: 99%

Association of the circadian rhythmic expression of GmCRY1a with a latitudinal cline in photoperiodic flowering of soybean

Zhang

et al. 2008

124

Photoperiodic control of flowering time is believed to affect latitudinal distribution of plants. The blue light receptor CRY2 regulates photoperiodic flowering in the experimental model plant Arabidopsis thaliana. However, it is unclear whether genetic variations affecting cryptochrome activity or expression is broadly associated with latitudinal distribution of plants. We report here an investigation of the function and expression of two cryptochromes in soybean, GmCRY1a and GmCRY2a. Soybean is a short-day (SD) crop commonly cultivated according to the photoperiodic sensitivity of cultivars. Both cultivated soybean (Glycine max) and its wild relative (G. soja) exhibit a strong latitudinal cline in photoperiodic flowering. Similar to their Arabidopsis counterparts, both GmCRY1a and GmCRY2a affected blue light inhibition of cell elongation, but only GmCRY2a underwent blue light-and 26S proteasome-dependent degradation. However, in contrast to Arabidopsis cryptochromes, soybean GmCRY1a, but not GmCRY2a, exhibited a strong activity promoting floral initiation, and the level of protein expression of GmCRY1a, but not GmCRY2a, oscillated with a circadian rhythm that has different phase characteristics in different photoperiods. Consistent with the hypothesis that GmCRY1a is a major regulator of photoperiodic flowering in soybean, the photoperiod-dependent circadian rhythmic expression of the GmCRY1a protein correlates with photoperiodic flowering and latitudinal distribution of soybean cultivars. We propose that genes affecting protein expression of the GmCRY1a protein play an important role in determining latitudinal distribution of soybeans.blue light ͉ cryptochrome ͉ photoperiodism ͉ photoreceptor

“…Arabidopsis is a facultative LD plant for which flowering-time regulation has been extensively studied (2)(3)(4)(5). Although the detailed mechanism underlying photoperiodism is not well understood, extensive plant physiological studies support a hypothesis referred to as the external coincidence model (6)(7)(8). According to this hypothesis, the light signal must interact at the appropriate time of the day (or ''coincide'') with the photoperiodic response rhythm (PRR) of a cellular activity to confer photoperiodic responsiveness.…”

mentioning

confidence: 99%

Regulation of photoperiodic flowering byArabidopsisphotoreceptors

Mockler

Yang

et al. 2003

273

213

Photoperiodism is a day-length-dependent seasonal change of physiological or developmental activities that is widely found in plants and animals. Photoperiodic flowering in plants is regulated by photosensory receptors including the red͞far-red light-receptor phytochromes and the blue͞UV-A light-receptor cryptochromes. However, the molecular mechanisms underlying the specific roles of individual photoreceptors have remained poorly understood. Here, we report a study of the day-length-dependent response of cryptochrome 2 (cry2) and phytochrome A (phyA) and their role as day-length sensors in Arabidopsis. The protein abundance of cry2 and phyA showed a diurnal rhythm in plants grown in short-day but not in plants grown in long-day. The short-day-specific diurnal rhythm of cry2 is determined primarily by blue light-dependent cry2 turnover. Consistent with a proposition that cry2 and phyA are the major day-length sensors in Arabidopsis, we show that phyA mediates far-red light promotion of flowering with modes of action similar to that of cry2. Based on these results and a finding that the photoperiodic responsiveness of plants depends on light quality, a model is proposed to explain how individual phytochromes and cryptochromes work together to confer photoperiodic responsiveness in Arabidopsis.P hotoperiodic flowering in plants was the first photoperiodism phenomenon documented (1). The flowering of longday (LD) or short-day (SD) plants occurs or is accelerated in the LD or SD condition, respectively. Arabidopsis is a facultative LD plant for which flowering-time regulation has been extensively studied (2-5). Although the detailed mechanism underlying photoperiodism is not well understood, extensive plant physiological studies support a hypothesis referred to as the external coincidence model (6-8). According to this hypothesis, the light signal must interact at the appropriate time of the day (or ''coincide'') with the photoperiodic response rhythm (PRR) of a cellular activity to confer photoperiodic responsiveness. It has been found that mRNA expression of flowering-time genes in Arabidopsis, including CO, GI, and FT, exhibited circadian rhythms, which have different phase shapes in plants grown in LD compared with plants grown in SD (9-12). Therefore, the day-length-dependent circadian expression of one or more flowering-time genes may represent the PRR.Arabidopsis relies on at least nine photosensory receptors, including five phytochromes (phyA-phyE), two cryptochromes (cry1 and cry2), and two phototropins (phot1 and phot2), to regulate most of its light responses (13-16). Among these photoreceptors, phytochromes and cryptochromes are known to regulate flowering time (5). It has also been found that phyA and cry2 protein abundance is regulated by light (17, 18) and that cry2 expression changes in response to photoperiod (19). These studies indicate that cry2 and phyA may act as major day-length sensors. Indeed, it has been found that the coincidence of light perception by cry2 and phyA with the peak circadian ...