Ryosuke Hayama

Adaptation of photoperiodic control pathways produces short-day flowering in rice

The photoperiodic control of flowering is one of the important developmental processes of plants because it is directly related to successful reproduction. Although the molecular genetic analysis of Arabidopsis thaliana, a long-day (LD) plant, has provided models to explain the control of flowering time in this species, very little is known about its molecular mechanisms for short-day (SD) plants. Here we show how the photoperiodic control of flowering is regulated in rice, a SD plant. Overexpression of OsGI, an orthologue of the Arabidopsis GIGANTEA (GI) gene in transgenic rice, caused late flowering under both SD and LD conditions. Expression of the rice orthologue of the Arabidopsis CONSTANS (CO) gene was increased in the transgenic rice, whereas expression of the rice orthologue of FLOWERING LOCUS T (FT) was suppressed. Our results indicate that three key regulatory genes for the photoperiodic control of flowering are conserved between Arabidopsis, a LD plant, and rice, a SD plant, but regulation of the FT gene by CO was reversed, resulting in the suppression of flowering in rice under LD conditions.

The molecular basis of diversity in the photoperiodic flowering responses of Arabidopsis and rice

Fluctuations in the length of the day affect developmental processes and behaviors of many organisms. Mammals and birds reproduce in spring in response to lengthening days and insects pupate in autumn when daylength shortens. These phenomena, called photoperiodism, allow detection of seasonal changes and anticipation of environmental conditions such as low temperatures and desiccation. Photoperiodism was first described in detail by Garner and Allard in 1920 through the demonstration that many plants flower in response to changes in daylength (Garner and Allard, 1920). Subsequently, they showed that some plant species promote flowering when daylength falls below a critical daylength, whereas other plants accelerate flowering in response to daylengths longer than a critical daylength. These plants are called short-day (SD) and long-day (LD) plants, respectively. During the last decade, molecular-genetic approaches were applied to understanding the control of flowering time, mainly in the LD plant Arabidopsis, and notable progress has been made in identifying the molecular mechanisms by which Arabidopsis recognizes daylength and promotes flowering specifically under LDs. Also, recent genetic studies in rice enabled the mechanisms of the daylength response in this SD plant to be compared with those of Arabidopsis. Here we review the recent advances in understanding the regulatory mechanisms for daylength response of flowering in Arabidopsis and compare them with those of rice. MODEL OF DAYLENGTH MEASUREMENT FOR CONTROL OF FLOWERING TIME

Shedding light on the circadian clock and the photoperiodic control of flowering.

Recently, notable progress has been made towards understanding the genetic interactions that underlie the function of the circadian clock in plants, and how these functions are related to the seasonal control of flowering time. The LHY/CCA1 and TOC1 genes have been proposed to participate in a negative feedback loop that is part of the central oscillator of the circadian clock. Furthermore, analysis of a flowering-time pathway has suggested how transcriptional regulation by the circadian clock, combined with post-transcriptional regulation by light, could activate proteins that control flowering time in response to appropriate daylengths.

A Circadian Rhythm Set by Dusk Determines the Expression of FT Homologs and the Short-Day Photoperiodic Flowering Response in Pharbitis

Seasonal control of flowering through responsiveness to daylength shows extreme variation. Different species flower in response to long days or short days (SDs), and this difference evolved several times. The molecular mechanisms conferring these responses have been compared in detail only in Arabidopsis thaliana and rice (Oryza sativa) and suggest that a conserved pathway confers daylength responses through regulation of FLOWERING LOCUS T (FT) transcription by CONSTANS (CO). We studied Pharbitis (Ipomoea nil; formerly, Pharbitis nil), a widely used SD model species and a member of the Convolvulaceae, and showed using transgenic plants together with detailed expression analysis that two putative orthologs of FT (Pn FT1 and Pn FT2) promote flowering specifically under SDs. These genes are expressed only under SDs, and light flashes given during the night reduce their expression and prevent flowering. We demonstrate that in Pharbitis a circadian rhythm set by the light-to-dark transition at dusk regulates Pn FT expression, which rises only when the night is longer than 11 h. Furthermore, Pharbitis accessions that differ in their critical night-length responses express Pn FT at different times after dusk, demonstrating that natural genetic variation influencing the clock regulating Pn FT expression alters the flowering response. In these assays, Pn FT mRNA abundance was not related to Pn CO expression, suggesting that Pn FT may be regulated by a different transcription factor in Pharbitis. We conclude that SD response in Pharbitis is controlled by a dedicated light sensitive clock, set by dusk, that activates Pn FT transcription in darkness, a different mechanism for measuring daylength than described for Arabidopsis and rice.

Phosphorylation of CONSTANS and its COP1-dependent degradation during photoperiodic flowering of Arabidopsis.

Seasonal flowering involves responses to changes in day length. In Arabidopsis thaliana, the CONSTANS (CO) transcription factor promotes flowering in the long days of spring and summer. Late flowering in short days is due to instability of CO, which is efficiently ubiquitinated in the dark by the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) E3 ligase complex. Here we show that CO is also phosphorylated. Phosphorylated and unphosphorylated forms are detected throughout the diurnal cycle but their ratio varies, with the relative abundance of the phosphorylated form being higher in the light and lower in the dark. These changes in relative abundance require COP1, because in the cop1 mutant the phosphorylated form is always more abundant. Inactivation of the PHYTOCHROME A (PHYA), CRYPTOCHROME 1 (CRY1) and CRYPTOCHROME 2 (CRY2) photoreceptors in the phyA cry1 cry2 triple mutant most strongly reduces the amount of the phosphorylated form so that unphosphorylated CO is more abundant. This effect is caused by increased COP1 activity, as it is overcome by introduction of the cop1 mutation in the cop1 phyA cry1 cry2 quadruple mutant. Degradation of CO is also triggered in red light, and as in darkness this increases the relative abundance of unphosphorylated CO. Finally, a fusion protein containing truncated CO protein including only the carboxy-terminal region was phosphorylated in transgenic plants, locating at least one site of phosphorylation in this region. We propose that CO phosphorylation contributes to the photoperiodic flowering response by enhancing the rate of CO turnover via activity of the COP1 ubiquitin ligase.

Pseudo response regulators stabilize constans protein to promote flowering in response to day length

Seasonal reproduction in many organisms requires detection of day length. This is achieved by integrating information on the light environment with an internal photoperiodic time‐keeping mechanism. Arabidopsis thaliana promotes flowering in response to long days (LDs), and CONSTANS (CO) transcription factor represents a photoperiodic timer whose stability is higher when plants are exposed to light under LDs. Here, we show that PSEUDO RESPONSE REGULATOR (PRR) proteins directly mediate this stabilization. PRRs interact with and stabilize CO at specific times during the day, thereby mediating its accumulation under LDs. PRR‐mediated stabilization increases binding of CO to the promoter of FLOWERING LOCUS T (FT), leading to enhanced FT transcription and early flowering under these conditions. PRRs were previously reported to contribute to timekeeping by regulating CO transcription through their roles in the circadian clock. We propose an additional role for PRRs in which they act upon CO protein to promote flowering, directly coupling information on light exposure to the timekeeper and allowing recognition of LDs.

Differential effects of light-to-dark transitions on phase setting in circadian expression among clock-controlled genes in Pharbitis nil

ABSTRACT The circadian clock is synchronized by the day-night cycle to allow plants to anticipate daily environmental changes and to recognize annual changes in day length enabling seasonal flowering. This clock system has been extensively studied in Arabidopsis thaliana and was found to be reset by the dark to light transition at dawn. By contrast, studies on photoperiodic flowering of Pharbitis nil revealed the presence of a clock system reset by the transition from light to dark at dusk to measure the duration of the night. However, a Pharbitis photosynthetic gene was also shown to be insensitive to this dusk transition and to be set by dawn. Thus Pharbitis appeared to have two clock systems, one set by dusk that controls photoperiodic flowering and a second controlling photosynthetic gene expression similar to that of Arabidopsis. Here, we show that circadian mRNA expression of Pharbitis homologs of a series of Arabidopsis clock or clock-controlled genes are insensitive to the dusk transition. These data further define the presence in Pharbitis of a clock system that is analogous to the Arabidopsis system, which co-exists and functions with the dusk-set system dedicated to the control of photoperiodic flowering.