The E3 ubiquitin ligase HOS1 regulates Arabidopsis flowering by mediating CONSTANS degradation under cold stress

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.

INDUCER OF CBF EXPRESSION 1 integrates cold signals into FLOWERING LOCUS C-mediated flowering pathways in Arabidopsis

Plants constantly monitor changes in photoperiod and temperature throughout the year to synchronize flowering with optimal environmental conditions. In the temperate zones, both photoperiod and temperature fluctuate in a somewhat predictable manner through the seasons, although a transient shift to low temperature is also encountered during changing seasons, such as early spring. Although low temperatures are known to delay flowering by inducing the floral repressor FLOWERING LOCUS C (FLC), it is not fully understood how temperature signals are coordinated with photoperiodic signals in the timing of seasonal flowering. Here, we show that the cold signaling activator INDUCER OF CBF EXPRESSION 1 (ICE1), FLC and the floral promoter SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborate signaling network that integrates cold signals into flowering pathways. The cold-activated ICE1 directly induces the gene encoding FLC, which represses SOC1 expression, resulting in delayed flowering. In contrast, under floral promotive conditions, SOC1 inhibits the binding of ICE1 to the promoters of the FLC gene, inducing flowering with a reduction of freezing tolerance. These observations indicate that the ICE1-FLC-SOC1 signaling network contributes to the fine-tuning of flowering during changing seasons.

Red Light-Mediated Degradation of CONSTANS by the E3 Ubiquitin Ligase HOS1 Regulates Photoperiodic Flowering in Arabidopsis

The regulation of CONSTANS (CO) gene expression is crucial to accurately measure changes in daylength, which influences flowering time in Arabidopsis thaliana. CO expression is under both transcriptional and posttranslational control mechanisms. We previously showed that the E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 (HOS1) physically interacts with CO in Arabidopsis. This interaction is required to precisely modulate the timing of CO accumulation and, consequently, to maintain low levels of FLOWERING LOCUS T expression during the first part of the day. The data presented here demonstrate that HOS1 is involved in the red light-mediated degradation of CO that takes place in the early stages of the daylight period. Our results show that phytochrome B (phyB) is able to regulate flowering time, acting in the phloem companion cells, as previously described for CO and HOS1. Moreover, we reveal that phyB physically interacts with HOS1 and CO, indicating that the three proteins may be present in a complex in planta that is required to coordinate a correct photoperiodic response in Arabidopsis.

The unified ICE-CBF pathway provides a transcriptional feedback control of freezing tolerance during cold acclimation in Arabidopsis

During cold acclimation, C-repeat binding factors (CBFs) activate downstream targets, such as cold-regulated genes, leading to the acquisition of freezing tolerance in plants. Inducer of CBF expression 1 (ICE1) plays a key role by activating CBF3 expression in shaping the cold-induced transcriptome. While the ICE1-CBF3 regulon constitutes a major cold acclimation pathway, gene regulatory networks governing the CBF signaling are poorly understood. Here, we demonstrated that ICE1 and its paralog ICE2 induce CBF1, CBF2, and CBF3 by binding to the gene promoters. ICE2, like ICE1, was ubiquitinated by the high expression of osmotically responsive gene 1 (HOS1) E3 ubiquitin ligase. Whereas ICE2-defective ice2-2 mutant did not exhibit any discernible freezing-sensitive phenotypes, ice1-2 ice2-2/+ plant, which is defective in ICE1 and has a heterozygotic ice2 mutation, exhibited significantly reduced freezing tolerance. Accordingly, all three CBF genes were markedly down-regulated in the ice1-2 ice2-2/+ plant, indicating that ICE1 and ICE2 are functionally redundant with different implementations in inducing CBF genes. Together with the negative regulation of CBF3 by CBF2, we propose that the unified ICE-CBF pathway provides a transcriptional feedback of freezing tolerance to sustain plant development and survival during cold acclimation.

The E3 Ubiquitin Ligase COP1 Regulates Thermosensory Flowering by Triggering GI Degradation in Arabidopsis

Floral transition is influenced by environmental factors such as light and temperature. Plants are capable of integrating photoperiod and ambient temperature signaling into their developmental program. Despite extensive investigations on individual genetic pathways, little is known about the molecular components that integrate both pathways. Here, we demonstrate that the RING finger–containing E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) acts as an integrator of photoperiod and ambient temperature signaling. In addition to the role in photoperiodic destabilization of CONSTANS (CO), COP1 also regulates temperature sensitivity by controlling the degradation of GIGANTEA (GI). COP1-impaired mutants showed reduced sensitivity to low ambient temperature. Notably, COP1 is more stabilized at low temperature and accelerates GI turnover in a 26S proteasome-dependent manner. The direct association of GI with the promoter of FLOWERING LOCUS T (FT) was reduced because of its ambient temperature-dependent protein stability control, and thus COP1-triggered GI turnover delays flowering at low temperatures via a CO-independent pathway. Taken together, our findings indicate that environmental conditions regulate the stability of COP1, and conditional specificity of its target selection stimulates proper developmental responses and ensures reproductive success.

Arabidopsis TOE proteins convey a photoperiodic signal to antagonize CONSTANS and regulate flowering time

Plants flower in an appropriate season to allow sufficient vegetative development and position flower development in favorable environments. In Arabidopsis, CONSTANS (CO) and FLAVIN-BINDING KELCH REPEAT F-BOX1 (FKF1) promote flowering by inducing FLOWER LOCUS T (FT) expression in the long-day afternoon. The CO protein is present in the morning but could not activate FT expression due to unknown negative mechanisms, which prevent premature flowering before the day length reaches a threshold. Here, we report that TARGET OF EAT1 (TOE1) and related proteins interact with the activation domain of CO and CO-like (COL) proteins and inhibit CO activity. TOE1 binds to the FT promoter near the CO-binding site, and reducing TOE function results in a morning peak of the FT mRNA. In addition, TOE1 interacts with the LOV domain of FKF1 and likely interferes with the FKF1-CO interaction, resulting in partial degradation of the CO protein in the afternoon to prevent premature flowering.

The RING finger protein NtRCP1 is involved in the floral transition in tobacco (Nicotiana tabacum)

The transition from the vegetative phase to the reproductive phase is a major developmental process in flowering plants. The underlying mechanism controlling this cellular process remains a research focus in the field of plant molecular biology. In the present work, we identified a gene encoding the C3H2C3-type RING finger protein NtRCP1 from tobacco BY-2 cells. Enzymatic analysis demonstrated that NtRCP1 is a functional E3 ubiquitin ligase. In tobacco plants, expression level of NtRCP1 was higher in the reproductive shoot apices than in the vegetative ones. NtRCP1-overexpressing plants underwent a more rapid transition from the vegetative to the reproductive phase and flowered markedly earlier than the wild-type control. Histological analysis revealed that the shoot apical meristem of NtRCP1-overexpressing plants initiated inflorescence primordia precociously compared to the wild-type plant due to accelerated cell division. Overexpression of NtRCP1 in BY-2 suspension cells promoted cell division, which was a consequence of the shortened G2 phase in the cell cycle. Together, our data suggest that NtRCP1 may act as a regulator of the phase transition, possibly through its role in cell cycle regulation, during vegetative/reproductive development in tobacco plant.

HOS1 regulates Argonaute1 by promoting transcription of the microRNA gene MIR168b in Arabidopsis

Proper accumulation and function of miRNAs is essential for plant growth and development. While core components of the miRNA biogenesis pathway and miRNA-induced silencing complex have been well characterized, cellular regulators of miRNAs remain to be fully explored. Here we report that High Expression Of Osmotically Responsive Genes1 (HOS1) is a regulator of an important miRNA, mi168a/b, that targets the Argonaute1 (AGO1) gene in Arabidopsis. HOS1 functions as an ubiquitin E3 ligase to regulate plant cold-stress responses, associates with the nuclear pores to regulate mRNA export, and regulates the circadian clock and flowering time by binding to chromatin of the flowering regulator gene Flowering Locus C (FLC). In a genetic screen for enhancers of sic-1, we isolated a loss-of-function Arabidopsis mutant of HOS1 that is defective in miRNA biogenesis. Like other hos1 mutant alleles, the hos1-7 mutant flowered early and was smaller in stature than the wild-type. Dysfunction in HOS1 reduced the abundance of miR168a/b but not of other miRNAs. In hos1 mutants, pri-MIR168b and pre-MIR168b levels were decreased, and RNA polymerase II occupancy was reduced at the promoter of MIR168b but not that of MIR168a. Chromatin immunoprecipitation assays revealed that HOS1 protein is enriched at the chromatin of the MIR168b promoter. The reduced miR168a/b level in hos1 mutants results in an increase in the mRNA and protein levels of its target gene, AGO1. Our results reveal that HOS1 regulates miR168a/b and AGO1 levels in Arabidopsis by maintaining proper transcription of MIR168b.

Exploring the pleiotropy of hos1

Understanding of the roles that HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 1 (HOS1) plays in the plant's ability to sense and respond to environmental signals has grown dramatically. Mechanisms through which HOS1 affects plant development have been uncovered, and the broader consequences of hos1 on the plant's ability to perceive and respond to its environment have been investigated. As such, it has been possible to place HOS1 as a key integrator of temperature information in response to both acute signals and cues that indicate time of year into developmental processes that are essential for plant survival. This review summarizes knowledge of HOS1's form and function, and contextualizes this information so that it is relevant for better understanding the processes of cold signalling, flowering time, and nuclear pore complex function more broadly.

Beyond ubiquitination: proteolytic and nonproteolytic roles of HOS1

The E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 (HOS1) functions as a cold signaling attenuator by degrading the INDUCER OF CBF EXPRESSION 1 transcription factor, which is a key regulator of the cold-induced transcriptome and freezing tolerance in plants. Recent studies demonstrate that HOS1 also plays nonproteolytic roles in gene expression regulation. HOS1 acts as a chromatin remodeling factor that modulates FLOWERING LOCUS C chromatin in cold regulation of flowering time. It associates with the nuclear pore complex to facilitate nucleocytoplasmic mRNA export to maintain circadian periodicity over a range of light and temperature conditions. In this review, we summarize recent advances in molecular mechanisms underlying HOS1 function during plant development in response to fluctuating environmental conditions.

Quantitative phosphoproteomic profiling of fiber differentiation and initiation in a fiberless mutant of cotton

Background

The cotton (Gossypium spp.) fiber cell is an important unicellular model for studying cell differentiation. There is evidence suggesting that phosphorylation is a critical post-translational modification involved in regulation of a wide range of cell activities. Nevertheless, the sites of phosphorylation in G. hirsutum and their regulatory roles in fiber cell initiation are largely unknown. In this study, we employed a mass spectrometry-based phosphoproteomics to conduct a global and site-specific phosphoproteome profiling between ovules of a fuzzless-lintless (fl) Upland cotton (G. hirsutum) mutant and its isogenic parental wild type (WT) at -3 and 0 days post-anthesis (DPA).

Results

A total of 830 phosphopeptides and 1,592 phosphorylation sites from 619 phosphoproteins were identified by iTRAQ (isobaric tags for relative and absolute quantitation). Of these, 76 phosphoproteins and 1,100 phosphorylation sites were identified for the first time after searching the P³DB public database using the BLAST program. Among the detected phosphopeptides, 69 were differentially expressed between the fl mutant and its WT in ovules at -3 and 0 DPA. An analysis using the Motif-X program uncovered 19 phosphorylation motifs, 8 of which were unique to cotton. A further metabolic pathway analysis revealed that the differentially phosphorylated proteins were involved in signal transduction, protein modification, carbohydrate metabolic processes, and cell cycle and cell proliferation.

Conclusions

Our phosphoproteomics-based research provides the first global overview of phosphorylation during cotton fiber initiation, and also offers a helpful dataset for elucidation of signaling networks in fiber development of G. hirsutum.

Electronic supplementary material

The online version of this article (doi: 10.1186/1471-2164-15-466) contains supplementary material, which is available to authorized users.

The miR172 target TOE3 represses AGAMOUS expression during Arabidopsis floral patterning

microRNA172 (miR172) regulates phase transition and floral patterning in Arabidopsis by repressing targets that encode the APETALA2 (AP2) and AP2-like transcription factors. The miR172-mediated repression of the AP2 gene restricts AGAMOUS (AG) expression. In addition, most miR172 targets, including AP2, redundantly act as floral repressors, and the overexpression of the target genes causes delayed flowering. However, how miR172 targets other than AP2 regulate both of the developmental processes remains unclear. Here, we demonstrate that miR172-mediated repression of the TARGET OF EAT 3 (TOE3) gene is critical for floral patterning in Arabidopsis. Transgenic plants that overexpress a miR172-resistant TOE3 gene (rTOE3-ox) exhibit indeterminate flowers with numerous stamens and carpelloid organs, which is consistent with previous observations in transgenic plants that overexpress a miR172-resistant AP2 gene. TOE3 binds to the second intron of the AG gene. Accordingly, AG expression is significantly reduced in rTOE3-ox plants. TOE3 also interacts with AP2 in the nucleus. Given the major role of AP2 in floral patterning, miR172 likely regulates TOE3 in floral patterning, at least in part via AP2. In addition, a miR156 target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 directly activates TOE3 expression, revealing a novel signaling interaction between miR156 and miR172 in floral patterning.

The Arabidopsis floral repressor BFT delays flowering by competing with FT for FD binding under high salinity

Soil salinity is one of the most serious agricultural problems that significantly reduce crop yields in the arid and semi-arid regions. It influences various phases of plant growth and developmental processes, such as seed germination, leaf and stem growth, and reproductive propagation. Salt stress delays the onset of flowering in many plant species. We have previously reported that the Arabidopsis BROTHER OF FT AND TFL1 (BFT) acts as a floral repressor under salt stress. However, the molecular mechanisms underlying the BFT function in the salt regulation of flowering induction is unknown. In this work, we found that BFT delays flowering under high salinity by competing with FLOWERING LOCUS T (FT) for binding to the FD transcription factor. The flowering time of FD-deficient fd-2 mutant was insensitive to high salinity. BFT interacts with FD in the nucleus via the C-terminal domain of FD, which is also required for the interaction of FD with FT, and interferes with the FT-FD interaction. These observations indicate that BFT constitutes a distinct salt stress signaling pathway that modulates the function of the FT-FD module and possibly provides an adaptation strategy that fine-tunes photoperiodic flowering under high salinity.

The role of ubiquitin and the 26S proteasome in plant abiotic stress signaling

Ubiquitin is a small, highly conserved, ubiquitously expressed eukaryotic protein with immensely important and diverse regulatory functions. A well-studied function of ubiquitin is its role in selective proteolysis by the ubiquitin-proteasome system (UPS). The UPS has emerged as an integral player in plant response and adaptation to environmental stresses such as drought, salinity, cold and nutrient deprivation. The UPS has also been shown to influence the production and signal transduction of stress-related hormones such as abscisic acid. Understanding UPS function has centered mainly on defining the role of E3 ubiquitin ligases, which are the substrate-recruiting component of the ubiquitination pathway. The recent identification of stress signaling/regulatory proteins that are the subject of ubiquitin-dependent degradation has increased our knowledge of how the UPS facilitates responses to adverse environmental conditions. A brief overview is provided on role of the UPS in modulating protein stability during abiotic stress signaling. E3 ubiquitin ligases for which stress-related substrate proteins have been identified are discussed.

Environmental stress and flowering time: the photoperiodic connection

Plants maximize their chances to survive adversities by reprogramming their development according to environmental conditions. Adaptive variations in the timing to flowering reflect the need for plants to set seeds under the most favorable conditions. A complex network of genetic pathways allows plants to detect and integrate external (e.g., photoperiod and temperature) and/or internal (e.g., age) information to initiate the floral transition. Furthermore different types of environmental stresses play an important role in the floral transition. The emerging picture is that stress conditions often affect flowering through modulation of the photoperiodic pathway. In this review we will discuss different modes of cross talk between stress signaling and photoperiodic flowering, highlighting the central role of the florigen genes in this process.

The cold signaling attenuator HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE1 activates FLOWERING LOCUS C transcription via chromatin remodeling under short-term cold stress in Arabidopsis

Exposure to short-term cold stress delays flowering by activating the floral repressor FLOWERING LOCUS C (FLC) in Arabidopsis thaliana. The cold signaling attenuator HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE1 (HOS1) negatively regulates cold responses. Notably, HOS1-deficient mutants exhibit early flowering, and FLC expression is suppressed in the mutants. However, it remains unknown how HOS1 regulates FLC expression. Here, we show that HOS1 induces FLC expression by antagonizing the actions of FVE and its interacting partner histone deacetylase 6 (HDA6) under short-term cold stress. HOS1 binds to FLC chromatin in an FVE-dependent manner, and FVE is essential for the HOS1-mediated activation of FLC transcription. HOS1 also interacts with HDA6 and inhibits the binding of HDA6 to FLC chromatin. Intermittent cold treatments induce FLC expression by activating HOS1, which attenuates the activity of HDA6 in silencing FLC chromatin, and the effects of intermittent cold are diminished in hos1 and fve mutants. These observations indicate that HOS1 acts as a chromatin remodeling factor for FLC regulation under short-term cold stress.

HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 is required for circadian periodicity through the promotion of nucleo-cytoplasmic mRNA export in Arabidopsis

Cold acclimation has been shown to be attenuated by the degradation of the INDUCER OF CBF EXPRESSION1 protein by the E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 (HOS1). However, recent work has suggested that HOS1 may have a wider range of roles in plants than previously appreciated. Here, we show that hos1 mutants are affected in circadian clock function, exhibiting a long-period phenotype in a wide range of temperature and light environments. We demonstrate that hos1 mutants accumulate polyadenylated mRNA in the nucleus and that the circadian defect in hos1 is shared by multiple mutants with aberrant mRNA export, but not in a mutant attenuated in nucleo-cytoplasmic transport of microRNAs. As revealed by RNA sequencing, hos1 exhibits gross changes to the transcriptome with genes in multiple functional categories being affected. In addition, we show that hos1 and other previously described mutants with altered mRNA export affect cold signaling in a similar manner. Our data support a model in which altered mRNA export is important for the manifestation of hos1 circadian clock defects and suggest that HOS1 may indirectly affect cold signaling through disruption of the circadian clock.

Flowering time regulation: photoperiod- and temperature-sensing in leaves

Plants monitor changes in photoperiod and temperature to synchronize their flowering with seasonal changes to maximize fitness. In the Arabidopsis photoperiodic flowering pathway, the circadian clock-regulated components, such as FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 and CONSTANS, both of which have light-controlled functions, are crucial to induce the day-length specific expression of the FLOWERING LOCUS T (FT) gene in leaves. Recent advances indicate that FT transcriptional regulation is central for integrating the information derived from other important internal and external factors, such as developmental age, amount of gibberellic acid, and the ambient temperature. In this review, we describe how these factors interactively regulate the expression of FT, the main component of florigen, in leaves.

Controlled turnover of CONSTANS protein by the HOS1 E3 ligase regulates floral transition at low temperatures

The timing of flowering is coordinately regulated by complex gene regulatory networks that integrate developmental and environmental cues. Light and temperature are major environmental determinants in flowering time control. Temperature signals include two major categories: ambient temperature signals and cold nonfreezing temperature signals. Notably, the effects of cold temperatures on flowering timing are profoundly differentiated, depending on the duration of cold exposure. Whereas long-term exposure to cold temperatures, designated vernalization, promotes flowering, short-term cold exposure delays flowering. Genes constituting the vernalization pathway and underlying molecular mechanisms have been extensively studied. However, how cold stress signals delay flowering is largely unknown. We have recently reported that the HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 1 (HOS1)-CONSTANS (CO) module is at least partly responsible for the daily sensing of cold stress signals in flowering time control. Intermittent cold stress triggers the degradation of CO, a central activator of photoperiodic flowering, via a ubiquitination pathway that involves the HOS1 E3 ubiquitin ligase, leading to suppression of FLOWERING LOCUS T (FT) gene and delayed flowering. It is proposed that CO serves as a molecular knot that integrates photoperiod and temperature signals into the flowering pathways, fine-tuning photoperiodic flowering under short-term temperature fluctuations.

Cold signaling and cold response in plants

Plants are constantly exposed to a variety of environmental stresses. Freezing or extremely low temperature constitutes a key factor influencing plant growth, development and crop productivity. Plants have evolved a mechanism to enhance tolerance to freezing during exposure to periods of low, but non-freezing temperatures. This phenomenon is called cold acclimation. During cold acclimation, plants develop several mechanisms to minimize potential damages caused by low temperature. Cold response is highly complex process that involves an array of physiological and biochemical modifications. Furthermore, alterations of the expression patterns of many genes, proteins and metabolites in response to cold stress have been reported. Recent studies demonstrate that post-transcriptional and post-translational regulations play a role in the regulation of cold signaling. In this review article, recent advances in cold stress signaling and tolerance are highlighted.

HOS1-mediated activation of FLC via chromatin remodeling under cold stress

The Arabidopsis E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 1 (HOS1) has been shown to act as a negative regulator of cold responses by degrading the INDUCER OF CBF EXPRESSION 1 (ICE1) transcription factor through the ubiquitin/proteasome pathway. Notably, loss-of-function hos1 mutants exhibit early flowering, and the transcript level of the floral repressor FLOWERING LOCUS C (FLC) is downregulated in the mutants. However, it is largely unknown how HOS1 regulates FLC transcription. We found that HOS1 activates FLC transcription by inhibiting the activity of histone deacetylase 6 (HDA6) under cold stress. Cold temperatures induce the binding of HOS1 to FLC chromatin in an FVE-dependent manner. Cold-activated HOS1 promotes the dissociation of HDA6 from FLC chromatin, and the cold effects disappear in both hos1 and fve mutants. It is therefore clear that HOS1 regulates FLC transcription via chromatin remodeling, providing new insights into the signaling crosstalks between cold response and flowering time control.