Analysis of flowering pathway integrators in Arabidopsis

Phytotoxic effects of silver nanoparticles and silver ions to Arabidopsis thaliana as revealed by analysis of molecular responses and of metabolic pathways

The acute (3 days) and chronic (whole life history) responses of Arabidopsis thaliana following exposure to silver nanoparticles (AgNPs) and Ag⁺ ions (AgNO₃) in respectively a hydroponic medium and in soil were studied. After 3 days of hydroponic exposure, AgNPs (1.0 and 2.5 mg/L) exerted more severe inhibitory effects on plant (shoot and root) growth and photosynthesis than the same concentrations of Ag⁺ ions. In soil cultivation, the photoperiod, the autonomous, and the vernalization pathways were down-regulated to 0.15- to 0.5-fold of the control after 12.5 mg/kg AgNPs treatment. This exposure caused a decrease of approximately 25%-40% as compared to the control of the transcription of flowering key genes including AP1, LFY, FT and SOC1, and finally resulted in a delayed flowering time of 5 days. Only autonomous and vernalization pathways were inhibited by Ag⁺ ion treatment and ultimately the time of flowering in treated plants was delayed by 3 days. The energy production related metabolic pathways in the tricarboxylic acid cycle and in sugar metabolism were stimulated stronger by AgNPs than by Ag⁺ ion treatment, thus releasing more energy and accelerating the physiological metabolic responses against stress in the AgNPs treatment while subsequently reducing the plant growth and yield at the maturation stage. Importantly, shikimate-phenylpropanoid biosynthesis, and tryptophan and galactose metabolisms were regulated only by the AgNPs treatment, which was a specific effect of nanoparticles. This work provides a systematic understanding at the molecular, physiological as well as metabolic level of the effects of AgNPs and Ag⁺ ions in A. thaliana.

Genetic dissection of the mechanism of flowering time based on an environmentally stable and specific QTL in Brassica napus

Flowering time is an important agronomic trait that is highly influenced by the environment. To elucidate the genetic mechanism of flowering time in rapeseed (Brassica napus L.), a genome-wide QTL analysis was performed in a doubled haploid population grown in winter, semi-winter and spring ecological conditions. Fifty-five consensus QTLs were identified after combining phenotype and genomic data, including 12 environment-stable QTLs and 43 environment-specific QTLs. Importantly, six major QTLs for flowering time were identified, of which two were considered environment-specific QTLs in spring ecological condition and four were considered environment-stable QTLs in winter and semi-winter ecological conditions. Through QTL comparison, 18 QTLs were colocalized with QTLs from six other published studies. Combining the candidate genes with their functional annotation, in 49 of 55 consensus QTLs, 151 candidate genes in B. napus corresponding to 95 homologous genes in Arabidopsis thaliana related to flowering were identified, including BnaC03g32910D (CO), BnaA02g12130D (FT) and BnaA03g13630D (FLC). Most of the candidate genes were involved in different flowering regulatory pathways. Based on re-sequencing and differences in sequence annotation between the two parents, we found that regions containing some candidate genes have numerous non-frameshift InDels and many non- synonymous mutations, which might directly lead to gene functional variation. Flowering time was negativly correlated with seed yield and thousand seed weight based on a QTL comparison of flowering time and seed yield traits, which has implications in breeding new early-maturing varieties of B. napus. Moreover, a putative flowering regulatory network was constructed, including the photoperiod, circadian clock, vernalization, autonomous and gibberellin pathways. Multiple copies of genes led to functional difference among the different copies of homologous genes, which also increased the complexity of the flowering regulatory networks. Taken together, the present results not only provide new insights into the genetic regulatory network underlying the control of flowering time but also improve our understanding of flowering time regulatory pathways in rapeseed.

Genome-wide identification and characterization of CONSTANS-like gene family in radish (Raphanus sativus)

Floral induction that initiates bolting and flowering is crucial for reproductive fitness in radishes. CONSTANS-like (CO-like, COL) genes play an important role in the circadian clock, which ensures regular development through complicated time-keeping mechanisms. However, the specific biological and functional roles of each COL transcription factor gene in the radish remain unknown. In this study, we performed a genome-wide identification of COL genes in the radish genome of three cultivars including ‘Aokubi’, ‘kazusa’ and ‘WK10039’, and we analyzed their exon-intron structure, gene phylogeny and synteny, and expression levels in different tissues. The bioinformatics analysis identified 20 COL transcription factors in the radish genome, which were divided into three subgroups (Group I to Group III). RsaCOL-09 and RsaCOL-12 might be tandem duplicated genes, whereas the others may have resulted from segmental duplication. The Ka/Ks ratio indicated that all the COL genes in radish, Arabidopsis, Brassica rapa, Brassica oleracea, Capsella rubella and rice were under purifying selection. We identified 6 orthologous and 19 co-orthologous COL gene pairs between the radish and Arabidopsis, and we constructed an interaction network among these gene pairs. The expression values for each COL gene during vegetable and flower development showed that the majority of Group I members had similar expression patterns. In general, the expression of radish COL genes in Groups I and III decreased during development, whereas the expression of radish COL genes in Group II first increased and then decreased. Substantial numbers of radish COL genes were differentially expressed after vernalization treatment. The expression levels of RsaCOL-02 and RsaCOL-04 were significantly increased during vernalization treatment, while the expression of RsaCOL-10 was significantly decreased. These outcomes provide insights for improving the genetic control of bolting and flowering in radish and other root vegetable crops, and they facilitate genetic improvements to radish yields and quality.

Identification and Characterization of miRNAs in Self-Rooted and Grafted Malus Reveals Critical Networks Associated with Flowering

Grafting can improve the agricultural traits of crop plants, especially fruit trees. However, the regulatory networks and differentially expressed microRNAs (miRNAs) related to grafting in apple remain unclear. Herein, we conducted high-throughput sequencing and identified differentially expressed miRNAs among self-rooted Fuji, self-rooted M9, and grafted Fuji/M9. We analyzed the flowering rate, leaf morphology, and nutrient and carbohydrate contents in the three materials. The flowering rate, element and carbohydrate contents, and expression levels of flowering genes were higher in Fuji/M9 than in Fuji. We detected 206 known miRNAs and 976 novel miRNAs in the three materials, and identified those that were up- or downregulated in response to grafting. miR156 was most abundant in Fuji, followed by Fuji/M9, and then self-rooted M9, while miR172 was most abundant in M9, followed by Fuji/M9, and then Fuji. These expression patterns suggest that that these miRNAs were related to grafting. A Gene Ontology (GO) analysis showed that the differentially expressed miRNAs controlled genes involved in various biological processes, including cellular biosynthesis and metabolism. The expression of differentially expressed miRNAs and flowering-related genes was verified by qRT-PCR. Altogether, this comprehensive analysis of miRNAs related to grafting provides valuable information for breeding and grafting of apple and other fruit trees.

LCM-seq reveals the crucial role of LsSOC1 in heat-promoted bolting of lettuce (Lactuca sativa L.)

Lettuce (Lactuca sativa L.) is one of the most economically important vegetables. The floral transition in lettuce is accelerated under high temperatures, which can significantly decrease yields. However, the molecular mechanism underlying the floral transition in lettuce is poorly known. Using laser capture microdissection coupled with RNA sequencing, we isolated shoot apical meristem cells from the bolting-sensitive lettuce line S39 at four critical stages of development. Subsequently, we screened specifically for the flowering-related gene LsSOC1 during the floral transition through comparative transcriptomic analysis. Molecular biology, developmental biology, and biochemical tools were combined to investigate the biological function of LsSOC1 in lettuce. LsSOC1 knockdown by RNA interference resulted in a significant delay in the timing of bolting and insensitivity to high temperature, which indicated that LsSOC1 functions as an activator during heat-promoted bolting in lettuce. We determined that two heat shock transcription factors, HsfA1e and HsfA4c, bound to the promoter of LsSOC1 to confirm that LsSOC1 played an important role in heat-promoted bolting. This study indicates that LsSOC1 plays a crucial role in the heat-promoted bolting process in lettuce. Further investigation of LsSOC1 may be useful for clarification of the bolting mechanism in lettuce.

UV-B radiation delays flowering time through changes in the PRC2 complex activity and miR156 levels in Arabidopsis thaliana

UV-B is a high-energy component of the solar radiation perceived by the plant and induces a number of modifications in plant growth and development, including changes in flowering time. However, the molecular mechanisms underlying these changes are largely unknown. In the present work, we demonstrate that Arabidopsis plants grown under white light supplemented with UV-B show a delay in flowering time, and this developmental reprogramming is mediated by the UVR8 photoreceptor. Using a combination of gene expression analyses and UV-B irradiation of different flowering mutants, we gained insight into the pathways involved in the observed flowering time delay in UV-B-exposed Arabidopsis plants. We provide evidence that UV-B light downregulates the expression of MSI1 and CLF, two of the components of the polycomb repressive complex 2, which in consequence drives a decrease in H3K27me3 histone methylation of MIR156 and FLC genes. Modification in the expression of several flowering time genes as a consequence of the decrease in the polycomb repressive complex 2 activity was also determined. UV-B exposure of flowering mutants supports the involvement of this complex in the observed delay in flowering time, mostly through the age pathway.

Sequence variation and functional analysis of a FRIGIDA orthologue (BnaA3.FRI) in Brassica napus

BACKGROUND

Allelic variation at the FRIGIDA (FRI) locus is a major contributor to natural variation of flowering time and vernalization requirement in Arabidopsis thaliana. Dominant FRI inhibits flowering by activating the expression of the MADS box transcriptional repressor FLOWERING LOCUS C (FLC), which represses flowering prior to vernalization. Four FRI orthologues had been identified in the domesticated amphidiploid Brassica napus. Linkage and association studies had revealed that one of the FRI orthologues, BnaA3.FRI, contributes to flowering time variation and crop type differentiation.

RESULTS

Sequence analyses indicated that three out of the four BnaFRI paralogues, BnaA3.FRI, BnaA10.FRI and BnaC3.FRI, contained a large number of polymorphic sites. Haplotype analysis in a panel of 174 B. napus accessions using PCR markers showed that all the three paralogues had a biased distribution of haplotypes in winter type oilseed rape (P < 0.01). Association analysis indicated that only BnaA3.FRI contributes to flowering time variation in B. napus. In addition, transgenic functional complementation demonstrated that mutations in the coding sequence of BnaA3.FRI lead to weak alleles, and subsequently to flowering time variation.

CONCLUSION

This study for the first time provides a molecular basis for flowering time control by BnaA3.FRI in B. napus, and will facilitate predictive oilseed rape breeding to select varieties with favorable flowering time and better adaption to latitude and seasonal shifts due to changing climate.

Overexpression of PvGF14c from Phyllostachys violascens Delays Flowering Time in Transgenic Arabidopsis

14-3-3 Proteins are a family of highly conserved regulatory molecules expressed in all eukaryotic cells and regulate a diverse set of biological responses in plants. However, their functions in flowering of Phyllostachys violascens are poorly understood. In this study, four non-𝜀 Pv14-3-3 genes from P. violascens were identified and named PvGF14b, PvGF14c, PvGF14e, and PvGF14f. qRT-PCR analyses revealed that PvGF14b and PvGF14e exhibited widely expressed in all tested bamboo tissues. PvGF14b was highest expression in root and lowest in immature leaf. Whereas PvGF14c and PvGF14f showed tissue-specific expression. PvGF14c was mainly expressed in immature and mature leaves. PvGF14f was highest expression in mature leaves. These four genes were not significantly differentially expressed in mature leaf before bamboo flowering and during flower development. PvGF14b and PvGF14c were not induced by circadian rhythm. PvGF14c displayed subcellular localization in the cytoplasm and PvFT in nucleus and cytoplasm. Yeast two-hybrid screening and bimolecular fluorescence complementation confirmed the interaction between PvGF14c and PvFT. The overexpression of PvGF14b, PvGF14c, and PvGF14e significantly delayed flowering time in transgenic Arabidopsis under long-day condition. These findings suggested that at least three PvGF14 genes are involved in flowering and may act as a negative regulator of flowering by interacting with PvFT in bamboo.

A Predictive Model for Time-to-Flowering in the Common Bean Based on QTL and Environmental Variables

The common bean is a tropical facultative short-day legume that is now grown in tropical and temperate zones. This observation underscores how domestication and modern breeding can change the adaptive phenology of a species. A key adaptive trait is the optimal timing of the transition from the vegetative to the reproductive stage. This trait is responsive to genetically controlled signal transduction pathways and local climatic cues. A comprehensive characterization of this trait can be started by assessing the quantitative contribution of the genetic and environmental factors, and their interactions. This study aimed to locate significant QTL (G) and environmental (E) factors controlling time-to-flower in the common bean, and to identify and measure G × E interactions. Phenotypic data were collected from a biparental [Andean × Mesoamerican] recombinant inbred population (F_11:14, 188 genotypes) grown at five environmentally distinct sites. QTL analysis using a dense linkage map revealed 12 QTL, five of which showed significant interactions with the environment. Dissection of G × E interactions using a linear mixed-effect model revealed that temperature, solar radiation, and photoperiod play major roles in controlling common bean flowering time directly, and indirectly by modifying the effect of certain QTL. The model predicts flowering time across five sites with an adjusted r-square of 0.89 and root-mean square error of 2.52 d. The model provides the means to disentangle the environmental dependencies of complex traits, and presents an opportunity to identify in silico QTL allele combinations that could yield desired phenotypes under different climatic conditions.

Age-Related Resistance in Arabidopsis thaliana Involves the MADS-Domain Transcription Factor SHORT VEGETATIVE PHASE and Direct Action of Salicylic Acid on Pseudomonas syringae

Arabidopsis thaliana exhibits a developmentally regulated disease-resistance response known as age-related resistance (ARR), a process that requires intercellular accumulation of salicylic acid (SA), which is thought to act as an antimicrobial agent. ARR is characterized by enhanced resistance to some pathogens at the late adult-vegetative and reproductive stages. While the transition to flowering does not cause the onset of ARR, both processes involve the MADS-domain transcription factor SHORT VEGETATIVE PHASE (SVP). In this study, ARR-defective svp mutants were found to accumulate reduced levels of intercellular SA compared with wild type in response to Pseudomonas syringae pv. tomato. Double mutant and overexpression analyses suggest that SVP and SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CO 1) act antagonistically, such that SVP is required for ARR to alleviate the negative effects of SOC1 on SA accumulation. In vitro, SA exhibited antibacterial and antibiofilm activity at concentrations similar to those measured in the intercellular space during ARR. In vivo, P. syringae pv. tomato formed biofilm-like aggregates in young susceptible plants, while this was drastically reduced in mature ARR-competent plants, which accumulate intercellular SA. Collectively, these results reveal a novel role for the floral regulators SVP and SOC1 in disease resistance and provide evidence that SA acts directly on pathogens as an antimicrobial agent. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Functional characterization of Brassica napus DNA topoisomerase Iα-1 and its effect on flowering time when expressed in Arabidopsis thaliana

Previous studies have shown that DNA topoisomerase Iα (AtTOP1α) has specific developmental functions during growth and development in Arabidopsis thaliana. However, little is known about the roles of DNA topoisomerases in the closely related and commercially important plant, rapeseed (Brassica napus). Here, the full-length BnTOP1α-1 coding sequence was cloned from the A2 subgenome of the Brassica napus inbred line L111. We determine that all BnTOP1α paralogs showed differing patterns of expression in different organs of L111, and that when expressed in tobacco leaves as a fusion protein with green fluorescent protein, BnTOP1α-1 localized to the nucleus. We further showed that ectopic expression of BnTOP1α-1 in the A. thaliana top1α-7 mutant fully complemented the early flowering phenotype of the mutant. Moreover, altered expression levels in top1α-7 seedlings of several key genes controlling flowering time were restored to wild type levels by ectopic expression of BnTOP1α-1. These results provide valuable insights into the roles of rapeseed DNA topoisomerases in flowering time, and provide a promising target for genetic manipulation of this commercially significant process in rapeseed.

Thermoperiodic Control of Floral Induction Involves Modulation of the Diurnal FLOWERING LOCUS T Expression Pattern

Thermoperiodism is defined as the ability to discriminate between day temperature (DT) and night temperature (NT). Our aim was to shed light on the mechanistic basis of thermoperiodic floral induction with acceleration under lower DT than NT compared with other DT-NT combinations at the same average daily temperature (ADT), a response exploited in temperate area greenhouses. Arabidopsis thaliana floral pathway mutants and a lhy circadian clock mutant as well as the expression of floral integrators and LHY (LATE ELONGATED HYPOCOTYL) were studied under different DT-NT combinations, all at the same ADT. We show that acceleration of floral induction under lower DT than NT is linked to increased FT expression early during the day and generally increased LFY expression preceding visible flower buds, compared with higher DT than NT or equal DT and NT. Consistent with FLOWERING LOCUS T (FT) action through LEAFY (LFY), time to floral transition in ft-1 and lfy-1 was similar under all treatments, in contrast to the situation for soc1-1, which behaved like the wild type (WT). The lhy-21 mutants did not discriminate between opposite DT-NT combinations, whereas LHY expression in the WT differed in these temperature regimes. This might suggest that LHY plays a role in thermoperiodic control of floral induction. We conclude that thermoperiodic control of floral transition is associated with modulation of the diurnal expression patterns of FT, with timing of temperature alteration being important rather than ADT.

Small RNA-Sequencing Links Physiological Changes and RdDM Process to Vegetative-to-Floral Transition in Apple

Transition from vegetative to floral buds is a critical physiological change during flower induction that determines fruit productivity. Small non-coding RNAs (sRNAs) including microRNAs (miRNAs) and small interfering RNAs (siRNAs) are pivotal regulators of plant growth and development. Although the key role of sRNAs in flowering regulation has been well-described in Arabidopsis and some other annual plants, their relevance to vegetative-to-floral transition (hereafter, referred to floral transition) in perennial woody trees remains under defined. Here, we performed Illumina sequencing of sRNA libraries prepared from vegetative and floral bud during flower induction of the apple trees. A large number of sRNAs exemplified by 33 previously annotated miRNAs and six novel members display significant differential expression (DE) patterns. Notably, most of these DE-miRNAs in floral transition displayed opposite expression changes in reported phase transition in apple trees. Bioinformatics analysis suggests most of the DE-miRNAs targeted transcripts involved in SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) gene regulation, stress responses, and auxin and gibberellin (GA) pathways, with further suggestion that there is an inherent link between physiological stress response and metabolism reprogramming during floral transition. We also observed significant changes in 24 nucleotide (nt) sRNAs that are hallmarks for RNA-dependent DNA methylation (RdDM) pathway, suggestive of the correlation between epigenetic modifications and the floral transition. The study not only provides new insight into our understanding of fundamental mechanism of poorly studied floral transition in apple and other woody plants, but also presents important sRNA resource for future in-depth research in the apple flowering physiology.

The sweet cherry (Prunus avium) FLOWERING LOCUS T gene is expressed during floral bud determination and can promote flowering in a winter-annual Arabidopsis accession

FT gene is expressed in leaves and buds and is involved in floral meristem determination and bud development in sweet cherry. In woody fruit perennial trees, floral determination, dormancy and bloom, depends on perception of different environmental and endogenous cues which converge to a systemic signaling gene known as FLOWERING LOCUS T (FT). In long-day flowering plants, FT is expressed in the leaves on long days. The protein travels through the phloem to the shoot apical meristem, where it induces flower determination. In perennial plants, meristem determination and flowering are separated by a dormancy period. Meristem determination takes place in summer, but flowering occurs only after a dormancy period and cold accumulation during winter. The roles of FT are not completely clear in meristem determination, dormancy release, and flowering in perennial plants. We cloned FT from sweet cherry (Prunus avium) and analyzed its expression pattern in leaves and floral buds during spring and summer. Phylogenetic analysis shows high identity of the FT cloned sequence with orthologous genes from other Rosaceae species. Our results show that FT is expressed in both leaves and floral buds and increases when the daylight reached 12 h. The peak in FT expression was coincident with floral meristem identity genes expression and morphological changes typical of floral meristem determination. The Edi-0 Arabidopsis ecotype, which requires vernalization to flower, was transformed with a construct for overexpression of PavFT. These transgenic plants showed an early-flowering phenotype without cold treatment. Our results suggest that FT is involved in floral meristem determination and bud development in sweet cherry. Moreover, we show that FT is expressed in both leaves and floral buds in this species, in contrast to annual plants.

DELLA proteins interact with FLC to repress flowering transition

Flowering is a highly orchestrated and extremely critical process in a plant's life cycle. Previous study has demonstrated that SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FLOWERING LOCUS T (FT) integrate the gibberellic acid (GA) signaling pathway and vernalization pathway in regulating flowering time, but detailed molecular mechanisms remain largely unclear. In GA signaling pathway, DELLA proteins are a group of master transcriptional regulators, while in vernalization pathway FLOWERING LOCUS C (FLC) is a core transcriptional repressor that down-regulates the expression of SOC1 and FT. Here, we report that DELLA proteins interact with FLC in vitro and in vivo, and the LHRI domains of DELLAs and the C-terminus of MADS domain of FLC are required for these interactions. Phenotypic and gene expression analysis showed that mutation of FLC reduces while over-expression of FLC enhances the GA response in the flowering process. Further, DELLA-FLC interactions promote the repression ability of FLC on its target genes. In summary, these findings report that the interaction between MADS box transcription factor FLC and GRAS domain regulator DELLAs may integrate various signaling inputs in flowering time control, and shed new light on the regulatory mechanism both for FLC and DELLAs in regulating gene expression.

De novo transcriptome analysis in radish (Raphanus sativus L.) and identification of critical genes involved in bolting and flowering

Background

The appropriate timing of bolting and flowering is pivotal for reproductive success in Brassicaceae crops including radish (Raphanus sativus L.). Although several flowering regulatory pathways had been described in some plant species, no study on genetic networks of bolting and flowering regulation was performed in radish. In this study, to generate dataset of radish unigene sequences for large-scale gene discovery and functional pathway identification, a cDNA library from mixed radish leaves at different developmental stages was subjected to high-throughput RNA sequencing (RNA-seq).

Results

A total of 54.64 million clean reads and 111,167 contigs representing 53,642 unigenes were obtained from the radish leaf transcriptome. Among these, 50,385 unigenes were successfully annotated by BLAST searching against the public protein databases. Functional classification and annotation indicated that 42,903 and 15,382 unique sequences were assigned to 55 GO terms and 25 COG categories, respectively. KEGG pathway analysis revealed that 25,973 unigenes were classified into 128 functional pathways, among which 24 candidate genes related to plant circadian rhythm were identified. Moreover, 142 potential bolting and flowering-related genes involved in various flowering pathways were identified. In addition, seven critical bolting and flowering-related genes were isolated and profiled by T-A cloning and RT-qPCR analysis. Finally, a schematic network model of bolting and flowering regulation and pathways was put forward in radish.

Conclusions

This study is the first report on systematic identification of bolting and flowering-related genes based on transcriptome sequencing and assembly in radish. These results could provide a foundation for further investigating bolting and flowering regulatory networks in radish, and facilitate dissecting molecular genetic mechanisms underlying bolting and flowering in Brassicaceae vegetable crops.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2633-2) contains supplementary material, which is available to authorized users.

LATE ELONGATED HYPOCOTYL regulates photoperiodic flowering via the circadian clock in Arabidopsis

BACKGROUND

Plants constantly monitor changes in photoperiod or day length to trigger the flowering cycle at the most appropriate time of the year. It is well established that photoperiodic flowering is intimately associated with the circadian clock in Arabidopsis. In support of this notion, many clock-defective mutants exhibit altered photoperiodic sensitivity in inducing flowering. LATE ELONGATED HYPOCOTYL (LHY) and its functional paralogue CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) constitute the core of the circadian clock together with TIMING OF CAB EXPRSSION 1 (TOC1). While it is known that TOC1 contributes to the timing of flowering entirely by modulating the clock function, molecular mechanisms by which LHY and CCA1 regulate flowering time have not been explored.

RESULTS

We investigated how LHY and CCA1 regulate photoperiodic flowering through molecular genetic and biochemical studies. It was found that LHY-defective mutants (lhy-7 and lhy-20) exhibit accelerated flowering under both long days (LDs) and short days (SDs). Consistent with the accelerated flowering phenotypes, gene expression analysis revealed that expression of the floral integrator FLOWERING LOCUS T (FT) is up-regulated in the lhy mutants. In addition, the expression peaks of GIGANTEA (GI) and FLAVIN-BINDING, KELCH REPEAT, F-BOX PROTEIN 1 (FKF1) genes, which constitute the clock output pathway that is linked with photoperiodic flowering, were advanced by approximately 4 h in the mutants. Furthermore, the up-regulation of FT disappeared when the endogenous circadian period is matched to the external light/dark cycles in the lhy-7 mutant. Notably, whereas CCA1 binds strongly to FT gene promoter, LHY does not show such DNA-binding activity.

CONCLUSIONS

Our data indicate that the advanced expression phases of photoperiodic flowering genes are associated with the clock defects in the lhy mutants and responsible for the reduced photoperiodic sensitivity of the mutant flowering, demonstrating that LHY regulates photoperiodic flowering via the circadian clock, similar to what has been shown with TOC1. It is notable that while LHY regulates photoperiodic flowering in a similar manner as with TOC1, the underlying molecular mechanism would be somewhat distinct from that exerted by CCA1 in Arabidopsis.

DNA-binding protein phosphatase AtDBP1 acts as a promoter of flowering in Arabidopsis

We provide evidence that AtDBP1 promotes flowering by regulating the transcript levels of several important integrators and floral meristem identity genes, including FLC, CO, SOC1, LFY, FT and FD. DNA-binding protein phosphatases (DBP) which exhibit both sequence specific DNA-binding and protein phosphatase 2C activities are important regulators that are involved in both the transcriptional and post-translational regulations. DBP factors are known to mediate susceptibility to potyviruses; however, whether they are involved in other processes is still unclear. In this study, under both long day (LD) and short day conditions, AtDBP1 overexpressing plants displayed early flowering, while the knock out mutants, atdbp1, exhibited a delay in flowering relative to the wild-type plants; both the overexpressing lines and atdbp1 mutants remained photoperiodic sensitive, indicating that AtDBP1 was involved in the autonomous pathway. AtDBP1 does not respond to vernalization at transcript level, and both AtDBP1 overexpressing plants and atdbp1 mutants remain responsive to vernalization, indicating that AtDBP1 may not be directly involved in vernalization. Real-time PCR analysis showed that AtDBP1 can suppress FLOWERING LOCUC C (FLC) expression, a key integrator of the autonomous and vernalization pathways, and enhance the expression levels of CONSTANS and FLOWERING LOCUC T, key regulators of the LD pathway. Furthermore, expression of floral meristem identity genes including SUPPRESSOR OF OVEREXPRESSION OF CO 1, LEAFY and FD was also promoted in AtDBP1 overexpressing plants. AtDBP1 transcription can be detected in root, leaf, stem, flower and silique. AtDBP1-GFP and YFP-AtDBP1 fusion protein were localized in the cytosol and nucleus. Our results provide the evidence demonstrating the effective role of AtDBP1 for flowering time regulation and report a novel function of DBP factors in planta besides in plant defense.

OsHAL3, a Blue Light-Responsive Protein, Interacts with the Floral Regulator Hd1 to Activate Flowering in Rice

In flowering plants, photoperiodic flowering is controlled by a complicated network. Light is one of the most important environmental stimuli that control the timing of the transition from vegetative growth to reproductive development. Several photoreceptors, including PHYA, PHYB, CRY2, and FKF1 in Arabidopsis and their homologs (OsPHYA, OsPHYB, OsPHYC, and OsCRY2) in rice, have been identified to be related to flowering. Our previous study suggests that OsHAL3, a flavin mononucleotide-binding protein, may function as a blue-light sensor. Here, we report the identification of OsHAL3 as a positive regulator of flowering in rice. OsHAL3 overexpression lines exhibited an early flowering phenotype, whereas downregulation of OsHAL3 expression by RNA interference delayed flowering under an inductive photoperiod (short-day conditions). The change in flowering time was not accompanied by altered Hd1 expression but rather by reduced accumulation of Hd3a and MADS14 transcripts. OsHAL3 and Hd1 colocalized in the nucleus and physically interacted in vivo under the dark, whereas their interaction was inhibited by white or blue light. Moreover, OsHAL3 directly bound to the promoter of Hd3a, especially before dawn. We conclude that OsHAL3, a novel light-responsive protein, plays an essential role in photoperiodic control of flowering time in rice, which is probably mediated by forming a complex with Hd1. Our findings open up new perspectives on the photoperiodic flowering pathway.

Functional characterization of GhSOC1 and GhMADS42 homologs from upland cotton (Gossypium hirsutum L.)

In Arabidopsis flowering pathway, MADS-box genes encode transcription factors, with their structures and functions highly conserved in many species. In our study, two MADS-box genes GhSOC1 and GhMADS42 (Gossypium hirsutum L.) were cloned from upland cotton CCRI36 and transformed into Arabidopsis. GhSOC1 was additionally transformed into upland cotton. Comparative analysis demonstrated sequence conservation between GhSOC1 and GhMADS42 and genes of other plant species. Tissue-specific expression analysis of GhSOC1 and GhMADS42 revealed spatiotemporal expression patterns involving high transcript levels in leaves, shoot apical buds, and flowers. In addition, overexpression of both GhSOC1 and GhMADS42 in Arabidopsis accelerated flowering, with GhMADS42 transgenic plants showing abnormal floral organ phenotypes. Overexpression of GhSOC1 in upland cotton also produced variations in floral organs. Furthermore, chromatin immunoprecipitation assay demonstrated that GhSOC1 could regulate GhMADS41 and GhMADS42, but not FLOWERING LOCUS T, by directly binding to the genes promoter. Finally, yeast two-hybrid and bimolecular fluorescence complementation approaches were undertaken to better understand the interaction of GhSOC1 and other MADS-box factors. These experiments showed that GhSOC1 can interact with APETALA1/FRUITFULL-like proteins in cotton.

Transcriptomic Analysis Identifies Differentially Expressed Genes (DEGs) Associated with Bolting and Flowering in Radish (Raphanus sativus L.)

The transition of vegetative growth to bolting and flowering is an important process in the life cycle of plants, which is determined by numerous genes forming an intricate network of bolting and flowering. However, no comprehensive identification and profiling of bolting and flowering-related genes have been carried out in radish. In this study, RNA-Seq technology was applied to analyze the differential gene expressions during the transition from vegetative stage to reproductive stage in radish. A total of 5922 differentially expressed genes (DEGs) including 779 up-regulated and 5143 down-regulated genes were isolated. Functional enrichment analysis suggested that some DEGs were involved in hormone signaling pathways and the transcriptional regulation of bolting and flowering. KEGG-based analysis identified 37 DEGs being involved in phytohormone signaling pathways. Moreover, 95 DEGs related to bolting and flowering were identified and integrated into various flowering pathways. Several critical genes including FT, CO, SOC1, FLC, and LFY were characterized and profiled by RT-qPCR analysis. Correlation analysis indicated that 24 miRNA-DEG pairs were involved in radish bolting and flowering. Finally, a miRNA-DEG-based schematic model of bolting and flowering regulatory network was proposed in radish. These outcomes provided significant insights into genetic control of radish bolting and flowering, and would facilitate unraveling molecular regulatory mechanism underlying bolting and flowering in root vegetable crops.

CAPRICE family genes control flowering time through both promoting and repressing CONSTANS and FLOWERING LOCUS T expression

CAPRICE (CPC) and six additional CPC family genes encode R3-type MYB transcription factors involved in epidermal cell fate determination, including Arabidopsis root hair and trichome differentiation. Previously, we reported that the CPC and CPC family genes TRIPTYCHON (TRY) and CAPRICE LIKE MYB3 (CPL3) also affect flowering time. The cpl3 mutant plants flower earlier, with fewer but larger leaves, than do wild type plants, and mutations in CPC or TRY delay flowering in the cpl3 mutant. In this study, we examined flowering time, leaf number, and fresh weight for CPC family gene double and triple mutants. Mutation in ENHANCER OF TRY AND CPC1 (ETC1) shortened the flowering time of the cpl3 single mutant. Mutation in ETC2 significantly reduced fresh weight in the cpl3 mutant. Expression levels of the flowering-related genes CONSTANS (CO) and FLOWERING LOCUS T (FT) were higher in the cpl3 mutant than in wild type plants. The high expression levels of CO and FT in cpl3 were significantly reduced by mutations in CPC, TRY, ETC1, or ETC2. Our results suggest that CPC family genes antagonistically regulate flowering time through CO and FT expression.

Functional Characterization of PhapLEAFY, a FLORICAULA/LEAFY Ortholog in Phalaenopsis aphrodite

The plant-specific transcription factor LEAFY (LFY) is considered to be a master regulator of flower development in the model plant, Arabidopsis. This protein plays a dual role in plant growth, integrating signals from the floral inductive pathways and acting as a floral meristem identity gene by activating genes for floral organ development. Although LFY occupies an important position in flower development, the functional divergence of LFY homologs has been demonstrated in several plants including monocots and gymnosperms. In particular, the functional roles of LFY genes from orchid species such as Phalaenopsis that contain unique floral morphologies with distinct expression patterns of floral organ identity genes remain elusive. Here, PhapLFY, an ortholog of Arabidopsis LFY from Phalaenopsis aphrodite subsp. formosana, a Taiwanese native monopodial orchid, was isolated and characterized through analyses of expression and protein activity. PhapLFY transcripts accumulated in the floral primordia of developing inflorescences, and the PhapLFY protein had transcriptional autoactivation activity forming as a homodimer. Furthermore, PhapLFY rescues the aberrant floral phenotypes of Arabidopsis lfy mutants. Overexpression of PhapLFY alone or together with PhapFT1, a P. aphrodite subsp. formosana homolog of Arabidopsis FLOWERING LOCUS T (FT) in rice, caused precocious heading. Consistently, a higher Chl content in the sepals and morphological changes in epidermal cells were observed in the floral organs of PhapLFY knock-down orchids generated by virus-induced gene silencing. Taken together, these results suggest that PhapLFY is functionally distinct from RICE FLORICAULA/LEAFY (RFL) but similar to Arabidopsis LFY based on phenotypes of our transgenic Arabidopsis and rice plants.

Functional and expression analyses of kiwifruit SOC1-like genes suggest that they may not have a role in the transition to flowering but may affect the duration of dormancy

The MADS-domain transcription factor SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) is one of the key integrators of endogenous and environmental signals that promote flowering in the annual species Arabidopsis thaliana. In the deciduous woody perennial vine kiwifruit (Actinidia spp.), environmental signals are integrated to regulate annual cycles of growth and dormancy. Accumulation of chilling during winter is required for dormancy break and flowering in spring. In order to understand the regulation of dormancy and flowering in kiwifruit, nine kiwifruit SOC1-like genes were identified and characterized. All genes affected flowering time of A. thaliana Col-0 and were able to rescue the late flowering phenotype of the soc1-2 mutant when ectopically expressed. A differential capacity for homodimerization was observed, but all proteins were capable of strong interactions with SHORT VEGETATIVE PHASE (SVP) MADS-domain proteins. Largely overlapping spatial domains but distinct expression profiles in buds were identified between the SOC1-like gene family members. Ectopic expression of AcSOC1e, AcSOC1i, and AcSOC1f in Actinidia chinensis had no impact on establishment of winter dormancy and failed to induce precocious flowering, but AcSOC1i reduced the duration of dormancy in the absence of winter chilling. These findings add to our understanding of the SOC1-like gene family and the potential diversification of SOC1 function in woody perennials.

CYCLIN-DEPENDENT KINASE G2 regulates salinity stress response and salt mediated flowering in Arabidopsis thaliana

Cyclin-dependent protein kinases are involved in many crucial cellular processes and aspects of plant growth and development, but their precise roles in abiotic stress responses are largely unknown. Here, Arabidopsis thaliana CYCLIN-DEPENDENT KINASE G2 (CDKG2) was shown to act as a negative regulator of the salinity stress response, as well as being involved in the control of flowering time. GUS expression experiments based on a pCDKG2::GUS transgene suggested that CDKG2 was expressed throughout plant development, with especially high expression levels recorded in the seed and in the flower. The loss-of-function of CDKG2 led to an increased tolerance of salinity stress and the up-regulation of the known stress-responsive genes SOS1, SOS2, SOS3, NHX3, RD29B, ABI2, ABI3, MYB15 and P5CS1. Flowering was accelerated in the cdkg2 mutants via the repression of FLC and the consequent up-regulation of FT, SOC1, AP1 and LFY. Transgenic lines constitutively expressing CDKG2 showed greater sensitivity to salinity stress and were delayed in flowering. Furthermore, the CDKG2 genotype affected the response of flowering time to salinity stress. Our data connect CDKG2 to undescribed functions related to salt stress tolerance and flowering time through the regulation of specific target genes.

Negative regulatory roles of DE-ETIOLATED1 in flowering time in Arabidopsis

Arabidopsis flowers early under long days (LD) and late under short days (SD). The repressor of photomorphogenesis DE-ETIOLATED1 (DET1) delays flowering; det1-1 mutants flower early, especially under SD, but the molecular mechanism of DET1 regulation remains unknown. Here we examine the regulatory function of DET1 in repression of flowering. Under SD, the det1-1 mutation causes daytime expression of FKF1 and CO; however, their altered expression has only a small effect on early flowering in det1-1 mutants. Notably, DET1 interacts with GI and binding of GI to the FT promoter increases in det1-1 mutants, suggesting that DET1 mainly restricts GI function, directly promoting FT expression independent of CO expression. Moreover, DET1 interacts with MSI4/FVE, which epigenetically inhibits FLC expression, indicating that the lack of FLC expression in det1-1 mutants likely involves altered histone modifications at the FLC locus. These data demonstrate that DET1 acts in both photoperiod and autonomous pathways to inhibit expression of FT and SOC1. Consistent with this, the early flowering of det1-1 mutants disappears completely in the ft-1 soc1-2 double mutant background. Thus, we propose that DET1 is a strong repressor of flowering and has a pivotal role in maintaining photoperiod sensitivity in the regulation of flowering time.

Comparative transcriptome analysis of the petal degeneration mutant pdm in Chinese cabbage (Brassica campestris ssp. pekinensis) using RNA-Seq

Flowering, which plays a crucial role in the growth and development of flowering plants, is a crucial point from vegetative growth to reproductive growth. The goal of this study was to examine the differences between the transcriptomes of the Chinese cabbage mutant pdm and the corresponding wild-type line 'FT'. We performed transcriptome analysis on mRNA isolated from flower buds of pdm and 'FT' using Illumina RNA sequencing (RNA-Seq) data. A total of 117 differentially expressed genes (DEGs) were detected. Among the DEGs, we identified a number of genes involved in floral development and flowering, including an F-box protein gene, EARLY FLOWERING 4 (ELF4), and transcription factors BIGPETAL (BPE) and MYB21 (v-myb avian myeloblastosis viral oncogene homolog); differential expression of these genes could potentially explain the difference in the flowers between pdm and 'FT'. In addition, the expression patterns of 20 DEGs, including 12 floral development and flowering-related genes and eight randomly selected genes, were validated by qRT-PCR, and the results were highly concordant with the RNA-Seq results. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed to better understand the functions of these DEGs. We also identified a large number of single nucleotide polymorphism and insertion/deletion markers, which will be a rich resource for future marker development and breeding research in Chinese cabbage. Also, our analysis revealed numerous novel transcripts and alternative splicing events. The transcriptome analysis provides valuable information for furthering our understanding of the molecular mechanisms that regulate the flowering process, and establishes a solid foundation for future genetic and functional genomic studies in Chinese cabbage.

Photoperiodic Regulation of Florigen Function in Arabidopsis thaliana

One mechanism through which flowering in response to seasonal change is brought about is by sensing the fluctuation in day-length; the photoperiod. Flowering induction occurs through the production of the florigenic protein FLOWERING LOCUS T (FT) and its movement from the phloem companion cells in the leaf vasculature into the shoot apex, where meristematic reprogramming occurs. FT activation in response to photoperiod condition is accomplished largely through the activity of the transcription factor CONSTANS (CO). Regulation of CO expression and protein stability, as well as the timing of other components via the circadian clock, is a critical mechanism by which plants are able to respond to photoperiod to initiate the floral transition. Modulation of FT expression in response to external and internal stimuli via components of the flowering network is crucial to mediate a fluid flowering response to a variety of environmental parameters. In addition, the regulated movement of FT protein from the phloem to the shoot apex, and interactions that determine floral meristem cell fate, constitute novel mechanisms through which photoperiodic information is translated into flowering time.

The trxG family histone methyltransferase SET DOMAIN GROUP 26 promotes flowering via a distinctive genetic pathway

Histone methylation is a major component in numerous processes such as determination of flowering time, which is fine-tuned by multiple genetic pathways that integrate both endogenous and environmental signals. Previous studies identified SET DOMAIN GROUP 26 (SDG26) as a histone methyltransferase involved in the activation of flowering, as loss of function of SDG26 caused a late-flowering phenotype in Arabidopsis thaliana. However, the SDG26 function and the underlying molecular mechanism remain largely unknown. In this study, we undertook a genetic analysis by combining the sdg26 mutant with mutants of other histone methylation enzymes, including the methyltransferase mutants Arabidopsis trithorax1 (atx1), sdg25 and curly leaf (clf), as well as the demethylase double mutant lsd1-like1 lsd1-like2 (ldl1 ldl2). We found that the early-flowering mutants sdg25, atx1 and clf interact antagonistically with the late-flowering mutant sdg26, whereas the late-flowering mutant ldl1 ldl2 interacts synergistically with sdg26. Based on microarray analysis, we observed weak overlaps in the genes that were differentially expressed between sdg26 and the other mutants. Our analyses of the chromatin of flowering genes revealed that the SDG26 protein binds at the key flowering integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1/AGAMOUS-LIKE 20 (SOC1/AGL20), and is required for histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 36 trimethylation (H3K36me3) at this locus. Together, our results indicate that SDG26 promotes flowering time through a distinctive genetic pathway, and that loss of function of SDG26 causes a decrease in H3K4me3 and H3K36me3 at its target gene SOC1, leading to repression of this gene and the late-flowering phenotype.

Transcriptomic analysis of floral initiation in litchi (Litchi chinensis Sonn.) based on de novo RNA sequencing

Comparative transcriptome analysis of litchi ( Litchi chinensis Sonn.) buds at two developmental stages revealed multiple processes involving various phytohormones regulating floral initiation, and expression of numerous flowering-related genes. Floral initiation is a critical and complicated plant developmental process involving interactions of numerous endogenous and environmental factors, but little is known about the complex network regulating floral initiation in litchi (Litchi chinensis Sonn.). Illumina second-generation sequencing is an efficient method for obtaining massive transcriptional information resulting from phase changes in plant development. In this study, comparative transcriptomic analysis was performed with resting and emerging panicle stage buds, to gain further understanding of the molecular mechanisms involved in floral initiation in litchi. Abundance analysis identified 5,928 unigenes exhibiting at least twofold differences in expression between the two bud stages. Of these, 4,622 unigenes were up-regulated and 1,306 were down-regulated in panicle-emerging buds compared with resting buds. KEGG pathway enrichment analysis revealed that unigenes exhibiting differential expression were involved in the metabolism and signal transduction of various phytohormones. The expression levels of unigenes annotated as auxin, cytokinin, jasmonic acid, and salicylic acid biosynthesis were up-regulated, whereas those unigenes annotated as abscisic acid biosynthesis were down-regulated during floral initiation. In addition, 188 unigenes exhibiting sequence similarities to known flowering-related genes from other plants were differentially expressed during floral initiation. Thirteen genes were selected for confirmation of expression levels using quantitative-PCR. Our results provide abundant sequence resources for studying mechanisms underlying floral initiation in litchi and establish a platform for further studies of litchi and other evergreen fruit trees.

BBX19 interacts with CONSTANS to repress FLOWERING LOCUS T transcription, defining a flowering time checkpoint in Arabidopsis

The timely transition of vegetative to reproductive development is coordinated through quantitative regulation of floral pathway genes in response to physiological and environmental cues. Here, we show that the circadian-controlled expression of the Arabidopsis thaliana floral transition regulators FLOWERING LOCUS T (FT) and CONSTANS (CO) is antiphasic to that of BBX19, a transcription factor with two B-Box motifs. Diminished expression of BBX19 by RNA interference accelerates flowering, and constitutive expression of BBX19 delays flowering under inductive photoperiods. This delay is not accompanied by the alteration of CO expression levels but rather by a reduction of transcript levels of FT and the FT-regulated genes SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1, LEAFY, and FRUITFUL. Similar to CO, BBX19 is expressed in vasculature. BBX19 and CO colocalize in the nucleus and interact physically in vivo. In transient assays, coinfiltration of 10-fold more CO overcomes the BBX19-mediated repression of FT activation. Substitution of the conserved Cys-25 to Ser in the BBX19 Box1 motif abolishes the BBX19-CO interaction and eliminates the negative regulation of flowering time, while the analogous C76S substitution in the Box2 motif is ineffective. Together, these results implicate BBX19 as a circadian clock output that depletes the active CO pool to accurately monitor daylength and precisely time FT expression.

GmFT2a and GmFT5a redundantly and differentially regulate flowering through interaction with and upregulation of the bZIP transcription factor GmFDL19 in soybean

FLOWERING LOCUS T (FT) is the key flowering integrator in Arabidopsis (Arabidopsis thaliana), and its homologs encode florigens in many plant species regardless of their photoperiodic response. Two FT homologs, GmFT2a and GmFT5a, are involved in photoperiod-regulated flowering and coordinately control flowering in soybean. However, the molecular and genetic understanding of the roles played by GmFT2a and GmFT5a in photoperiod-regulated flowering in soybean is very limited. In this study, we demonstrated that GmFT2a and GmFT5a were able to promote early flowering in soybean by overexpressing these two genes in the soybean cultivar Williams 82 under noninductive long-day (LD) conditions. The soybean homologs of several floral identity genes, such as GmAP1, GmSOC1 and GmLFY, were significantly upregulated by GmFT2a and GmFT5a in a redundant and differential pattern. A bZIP transcription factor, GmFDL19, was identified as interacting with both GmFT2a and GmFT5a, and this interaction was confirmed by yeast two-hybridization and bimolecular fluorescence complementation (BiFC). The overexpression of GmFDL19 in soybean caused early flowering, and the transcription levels of the flowering identity genes were also upregulated by GmFDL19, as was consistent with the upregulation of GmFT2a and GmFT5a. The transcription of GmFDL19 was also induced by GmFT2a. The results of the electrophoretic mobility shift assay (EMSA) indicated that GmFDL19 was able to bind with the cis-elements in the promoter of GmAP1a. Taken together, our results suggest that GmFT2a and GmFT5a redundantly and differentially control photoperiod-regulated flowering in soybean through both physical interaction with and transcriptional upregulation of the bZIP transcription factor GmFDL19, thereby inducing the expression of floral identity genes.

Sequential action of FRUITFULL as a modulator of the activity of the floral regulators SVP and SOC1

The role in flowering time of the MADS-box transcription factor fruitfulL (FUL) has been proposed in many works. FUL has been connected to several flowering pathways as a target of the photoperiod, ambient temperature, and age pathways and it is has been shown to promote flowering in a partially redundant manner with suppressor of overexpression of constans 1 (SOC1). However, the position of FUL in these genetic networks, as well as the functional output of FUL activity during floral transition, remains unclear. In this work, a genetic approach has been undertaken to understand better the functional hierarchies involving FUL and other MADS-box factors with well established roles as floral integrators such as SOC1, short vegetative phase (svp) or flowering locus C (FLC). Our results suggest a prominent role of FUL in promoting reproductive transition when photoinductive signalling is suppressed by short-day conditions or by high levels of FLC expression, as in non-vernalized winter ecotypes. A model is proposed where the sequential formation of FUL-SVP and FUL-SOC1 heterodimers may mediate the vegetative and meristem identity transitions, counteracting the repressive effect of FLC and SVP on flowering.

The circadian clock gene regulatory module enantioselectively mediates imazethapyr-induced early flowering in Arabidopsis thaliana

Plant growth and development are strongly affected by environmental pollutants, such as herbicides. Widely used herbicides can remain in soil or aquatic systems for long periods of time. Herbicide pollutants have been reported to heavily affect global plant growth and pose a significant challenge to agriculture. However, it is unclear whether herbicides affect plant flowering. Here, we demonstrated that imazethapyr (IM), a chiral herbicide, can enantioselectively promote flowering in Arabidopsis thaliana. We clarified the possible mechanism by which IM promotes flowering and found that the photoperiod pathway may play an important role in propagating the IM stress signal. IM enantiomers decreased the amplitude of core oscillators (CIRCADIAN CLOCK ASSOCIATED 1 and LATE ELONGATED HYPOCOTYL) and utilized the up-regulation of the GIGANTEA-(CONSTANS)-FLOWERING LOCUS T pathway to induce floral gene, APETALA1 over-expression enantioselectively; this treatment ultimately caused early flowering. Our findings provide new insight into the method by which plants control reproductive timing in response to herbicide stress. Flowering time is an important trait in crops and affects the life cycles of pollinator species. The persistence of herbicides in the biosphere will alter plant life cycles and diversity.

Light quality regulates flowering in FvFT1/FvTFL1 dependent manner in the woodland strawberry Fragaria vesca

Control of flowering in the perennial model, the woodland strawberry (Fragaria vesca L.), involves distinct molecular mechanisms that result in contrasting photoperiodic flowering responses and growth cycles in different accessions. The F. vesca homolog of TERMINAL FLOWER1 (FvTFL1) functions as a key floral repressor that causes short-day (SD) requirement of flowering and seasonal flowering habit in the SD strawberry. In contrast, perpetual flowering F. vesca accessions lacking functional FvTFL1 show FLOWERING LOCUS T (FvFT1)-dependent early flowering specifically under long-days (LD). We show here that the end-of-day far-red (FR) and blue (B) light activate the expression of FvFT1 and the F. vesca homolog of SUPPRESSOR OF THE OVEREXPRESSION OF CONSTANS (FvSOC1) in both SD and LD strawberries, whereas low expression levels are detected in red (R) and SD treatments. By using transgenic lines, we demonstrate that FvFT1 advances flowering under FR and B treatments compared to R and SD treatments in the LD strawberry, and that FvSOC1 is specifically needed for the B light response. In the SD strawberry, flowering responses to these light quality treatments are reversed due to up-regulation of the floral repressor FvTFL1 in parallel with FvFT1 and FvSOC1. Our data highlights the central role of FvFT1 in the light quality dependent flower induction in the LD strawberry and demonstrates that FvTFL1 reverses not only photoperiodic requirements but also light quality effects on flower induction in the SD strawberry.

Florigen is involved in axillary bud development at multiple stages in Arabidopsis

The wide variety of plant architectures is largely based on diverse and flexible modes of axillary shoot development. In Arabidopsis, floral transition (flowering) stimulates axillary bud development. The mechanism that links flowering and axillary bud development is, however, largely unknown. We recently showed that FLOWERING LOCUS T (FT) protein, which acts as florigen, promotes the phase transition of axillary meristems, whereas BRANCHED1 (BRC1) antagonizes the florigen action in axillary buds. Here, we present evidences for another possible role of florigen in axillary bud development. Ectopic overexpression of FT or another florigen gene TWIN SISTER OF FT (TSF) with LEAFY (LFY) induces ectopic buds at cotyledonary axils, confirming the previous proposal that these genes are involved in formation of axillary buds. Taken together with our previous report that florigen promotes axillary shoot elongation, we propose that florigen regulates axillary bud development at multiple stages to coordinate it with flowering in Arabidopsis.

MlWRKY12, a novel Miscanthus transcription factor, participates in pith secondary cell wall formation and promotes flowering

WRKY proteins play crucial roles in various plant processes. An AtWRKY12 homologous gene, named MlWRKY12, was isolated from Miscanthus lutarioriparius. The MlWRKY12 gene encodes a WRKY transcription factor belonging to the group IIc subfamily. MlWRKY12 is a nuclear protein. Gene expression pattern analysis revealed a relatively high MlWRKY12 expression level in rhizomes, stems and leaf sheaths. In situ hybridization analysis further demonstrated that MlWRKY12 was expressed in vascular bundle sheath, sclerenchyma and parenchyma tissues. The heterologous expression of MlWRKY12 in an atwrky12 background mutant successfully rescued the phenotype of pith cell walls caused by the defect of AtWRKY12. Most strikingly, the transgenic Arabidopsis plants overexpressing MlWRKY12 exhibited early flowering. The transcript abundance of flowering related genes was measured by quantitative RT-PCR analysis, suggesting that overexpression of MlWRKY12 in Arabidopsis had a significant impact on the expression level of CONSTANS (CO). Moreover, the expression levels of FLOWERING LOCUS T (FT), LFY (LEAFY), APETALA1 (AP1), CAULIFLOWER (CAL) and FRUITFULL (FUL) were upregulated in transgenic plants. These results demonstrated the conserved function of MlWRKY12 existing in secondary cell wall formation of monocotyledonous species and implied a possible impact of MlWRKY12 on flowering control.

The Fragaria vesca homolog of suppressor of overexpression of constans1 represses flowering and promotes vegetative growth

In the annual long-day plant Arabidopsis thaliana, suppressor of overexpression of constans1 (SOC1) integrates endogenous and environmental signals to promote flowering. We analyzed the function and regulation of the SOC1 homolog (Fragaria vesca [Fv] SOC1) in the perennial short-day plant woodland strawberry (Fragaria vesca). We found that Fv SOC1 overexpression represses flower initiation under inductive short days, whereas its silencing causes continuous flowering in both short days and noninductive long days, similar to mutants in the floral repressor Fv terminal flower1 (Fv TFL1). Molecular analysis of these transgenic lines revealed that Fv SOC1 activates Fv TFL1 in the shoot apex, leading to the repression of flowering in strawberry. In parallel, Fv SOC1 regulates the differentiation of axillary buds to runners or axillary leaf rosettes, probably through the activation of gibberellin biosynthetic genes. We also demonstrated that Fv SOC1 is regulated by photoperiod and Fv flowering locus T1, suggesting that it plays a central role in the photoperiodic control of both generative and vegetative growth in strawberry. In conclusion, we propose that Fv SOC1 is a signaling hub that regulates yearly cycles of vegetative and generative development through separate genetic pathways.

Genetic identification of a novel locus, ACCELERATED FLOWERING 1 that controls chromatin modification associated with histone H3 lysine 27 trimethylation in Arabidopsis thaliana

Flowering on time is a critically important for successful reproduction of plants. Here we report an early-flowering mutant in Arabidopsis thaliana, accelerated flowering 1-1D (afl1-1D) that exhibited pleiotropic developmental defects including semi-dwarfism, curly leaf, and increased branching. Genetic analysis showed that afl1-1D mutant is a single, dominant mutant. Chromosomal mapping indicates that AFL1 resides at the middle of chromosome 4, around which no known flowering-related genes have been characterized. Expression analysis and double mutant studies with late flowering mutants in various floral pathways indicated that elevated FT is responsible for the early-flowering of afl1-1D mutant. Interestingly, not only flowering-related genes, but also several floral homeotic genes were ectopically overexpressed in the afl1-1D mutants in both FT-dependent and -independent manner. The degree of histone H3 Lys27-trimethylation (H3K27me3) was reduced in several chromatin including FT, FLC, AG and SEP3 in the afl1-1D, suggesting that afl1-1D might be involved in chromatin modification. In support, double mutant analysis of afl1-1D and lhp1-4 revealed epistatic interaction between afl1-1D and lhp1-4 in regard to flowering control. Taken together, we propose that AFL1 regulate various aspect of development through chromatin modification, particularly associated with H3K27me3 in A. thaliana.

The inhibitory effect of ABA on floral transition is mediated by ABI5 in Arabidopsis

Seed germination and flowering initiation are both transitions responding to similar seasonal cues. This study shows that ABSCISIC ACID-INSENSITIVE MUTANT 5 (ABI5), a bZIP transcription factor, which plays an important role in the abscisic acid (ABA)-arrested seed germination, is robustly associated with the floral transition in Arabidopsis. Under long-day conditions, overexpression of ABI5 could delay floral transition through upregulating FLOWERING LOCUS C (FLC) expression. In contrast, ectopically overexpressing FLC in an abi5 mutant reversed the earlier flowering phenotype. Further analysis indicated that transactivation of FLC could be promoted by ABI5 and/or other abscisic acid-responsive element (ABRE)-binding factors (ABFs). The expression of FLC that was promoted by ABI5 and/or other ABFs could be blocked in a triple SNF1-related protein kinase (SnRK) mutant, snrk2.2/2.3/2.6, despite the presence of ABA. In sharp contrast, when SnRK2.6 was coexpressed, the reduction of transactivity of FLC was reverted in mesophyll protoplasts of snrk2.2/2.3/2.6. Additional results from analysing transgenic plants carrying mutations of phosphoamino acids (ABI5 ( S42AS145AT201A )), which are conserved in ABI5, suggested that SnRK2-mediated ABI5 and/or ABF phosphorylation may be crucial for promoting FLC expression. The transgenic plants ABI5 ( S42AS145AT201A ) were insensitive to ABA in seed germination, in addition to having an earlier flowering phenotype. Direct binding of ABI5 to the ABRE/G-box promoter elements existing in FLC was demonstrated by chromatin immunoprecipitation. Mutations at the ABRE/G-box regions in FLC promoter sequences abolished the ABI5-promoted transactivation of FLC. In summary, these results may decipher the inhibitory effect of ABA on floral transition in Arabidopsis.

Promoter variation and transcript divergence in Brassicaceae lineages of FLOWERING LOCUS T

Brassica napus (AACC, 2n = 38), an oil crop of world-wide importance, originated from interspecific hybridization of B. rapa (AA, 2n = 20) and B. oleracea (CC, 2n = 18), and has six FLOWERING LOCUS T (FT) paralogues. Two located on the homeologous chromosomes A2 and C2 arose from a lineage distinct from four located on A7 and C6. A set of three conserved blocks A, B and C, which were found to be essential for FT activation by CONSTANS (CO) in Arabidopsis, was identified within the FT upstream region in B. napus and its progenitor diploids. However, on chromosome C2, insertion of a DNA transposable element (TE) and a retro-element in FT upstream blocks A and B contributed to significant structural divergence between the A and C genome orthologues. Phylogenetic analysis of upstream block A indicated the conserved evolutionary relationships of distinct FT genes within Brassicaceae. We conclude that the ancient At-α whole genome duplication contributed to distinct ancestral lineages for this key adaptive gene, which co-exist within the same genus. FT-A2 was found to be transcribed in all leaf samples from different developmental stages in both B. rapa and B. napus, whereas FT-C2 was not transcribed in either B. napus or B. oleracea. Silencing of FT-C2 appeared to result from TE insertion and consequent high levels of cytosine methylation in TE sequences within upstream block A. Interestingly, FT-A7/C6 paralogues were specifically silenced in winter type B. napus but abundantly expressed in spring type cultivars under vernalization-free conditions. Motif prediction indicated the presence of two CO protein binding sites within all Brassica block A and additional sites for FT activation in block C. We propose that the ancestral whole genome duplications have contributed to more complex mechanisms of floral regulation and niche adaptation in Brassica compared to Arabidopsis.

Vernalization-mediated chromatin changes

Proper flowering time is vital for reproductive fitness in flowering plants. In Arabidopsis, vernalization is mediated primarily through the repression of a MADS box transcription factor, FLOWERING LOCUS C (FLC). The induction of a plant homeodomain-containing protein, VERNALIZATION INSENSITIVE 3 (VIN3), by vernalizing cold is required for proper repression of FLC. One of a myriad of changes that occurs after VIN3 is induced is the establishment of FLC chromatin at a mitotically repressed state due to the enrichment of repressive histone modifications. VIN3 induction by cold is the earliest known event during the vernalization response and includes changes in histone modifications at its chromatin. Here, the current understanding of the vernalization-mediated chromatin changes in Arabidopsis is discussed, with a focus on the roles of shared chromatin-modifying machineries in regulating VIN3 and FLC gene family expression during the course of vernalization.

UGT87A2, an Arabidopsis glycosyltransferase, regulates flowering time via FLOWERING LOCUS C

• Family 1 glycosyltransferases comprise the greatest number of glycosyltransferases found in plants. The widespread occurrence and diversity of glycosides throughout the plant kingdom underscore the importance of these glycosyltransferases. • Here, we describe the identification and characterization of a late-flowering Arabidopsis (Arabidopsis thaliana) mutant, in which a putative family 1 glycosyltransferase gene, UGT87A2, was disrupted. The role and possible mechanism of UGT87A2 in the regulation of flowering were analyzed by molecular, genetic and cellular approaches. • The ugt87a2 mutant exhibited late flowering in both long and short days, and its flowering was promoted by vernalization and gibberellin. Furthermore, the mutant flowering phenotype was rescued by the wild-type UGT87A2 gene in complementation lines. Interestingly, the expression of the flowering repressor FLOWERING LOCUS C was increased substantially in the mutant, but decreased to the wild-type level in complementation lines, with corresponding changes in the expression levels of the floral integrators and floral meristem identity genes. The expression of UGT87A2 was developmentally regulated and its protein products were distributed in both cytoplasm and nucleus. • Our findings imply that UGT87A2 regulates flowering time via the flowering repressor FLOWERING LOCUS C. These data highlight an important role for the family 1 glycosyltransferases in the regulation of plant flower development.

Isolation of a CENTRORADIALIS/TERMINAL FLOWER1 homolog in saffron (Crocus sativus L.): characterization and expression analysis

Genes in the phosphatidyl-ethanolamine-binding protein (PEBP) family are instrumental in regulating the fate of meristems and flowering time. To investigate the role of these genes in the monocotyledonous plant Crocus (Crocus sativus L), an industrially important crop cultivated for its nutritional and medicinal properties, we have cloned and characterized a CENTRORADIALIS/TERMINAL FLOWER1 (CEN/TFL1) like gene, named CsatCEN/TFL1-like, the first reported CEN/TFL1 gene characterized from such a perennial geophyte. Sequence analysis revealed that CsatCEN/TFL1 shows high similarity to its homologous PEBP family genes CEN/TFL1, FT and MFT from a variety of plant species and maintains the same exon/intron organization. Phylogenetic analysis of the CsatCEN/TFL1 amino acid sequence confirmed that the isolated sequences belong to the CEN/TFL1 clade of the PEBP family. CsatCEN/TFL1 transcripts could be detected in corms, flower and flower organs but not in leaves. An alternative spliced transcript was also detected in the flower. Comparison of expression levels of CsatCEN/TFL1 and its alternative spliced transcript in wild type flower and a double flower mutant showed no significant differences. Overexpression of CsatCEN/TFL1 transcript in Arabidopsis tfl1 plants reversed the phenotype of early flowering and terminal flowering of the tfl1 plants to a normal one. Computational analysis of the obtained promoter sequences revealed, next to common binding motifs in CEN/TFL1-like genes as well as other flowering gene promoters, the presence of two CArG binding sites indicative of control of CEN/TFL1 by MADS-box transcription factors involved in crocus flowering and flower organ formation.

Reduced transcription of a LEAFY-like gene in Alstroemeria sp. cultivar Green Coral that cannot develop floral meristems

Alstroemeria sp. cv. Green Coral has numerous bracts instead of flowers, and its cyme structures are repeated eternally. Observations of the development and morphology of inflorescence in cv. Green Coral revealed that transition from inflorescence to floral meristem was restricted. We isolated and characterized floral meristem identity genes LEAFY-like (AlsLFY) and SQUAMOSA-like (AlsSQa and AlsSQb) genes from Alstroemeria ligtu. In situ hybridization results indicated that AlsSQa and AlsSQb were expressed in the dome-shaped floral meristems and all floral organ primordia in A. ligtu. Transcripts of AlsLFY accumulated early in the dome-shaped floral meristems; the signals were restricted later to the outer region of the floral meristem. These results indicate that AlsLFY, AlsSQa, and AlsSQb function as floral meristem identity genes. Expression profiles of AlsLFY, AlsSQa, AlsSQb, and other MADS-box genes were compared between A. ligtu and cv. Green Coral. AlsLFY, AlsDEFa, and AlsAGL6 transcripts were not detected at the shoot apices of cv. Green Coral but were detected in A. ligtu. The early induction and accumulation of AlsLFY transcripts in the inflorescence meristem of A. ligtu prior to development of the floral meristem suggest that downregulation of AlsLFY is likely to restrict the inflorescence-to-floral meristem transition in cv. Green Coral.

CRYPTIC PRECOCIOUS/MED12 is a novel flowering regulator with multiple target steps in Arabidopsis

The proper timing of flowering is of crucial importance for reproductive success of plants. Regulation of flowering is orchestrated by inputs from both environmental and endogenous signals such as daylength, light quality, temperature and hormones, and key flowering regulators construct several parallel and interactive genetic pathways. This integrative regulatory network has been proposed to create robustness as well as plasticity of the regulation. Although knowledge of key genes and their regulation has been accumulated, there still remains much to learn about how they are organized into an integrative regulatory network. Here, we have analyzed the CRYPTIC PRECOCIOUS (CRP) gene for the Arabidopsis counterpart of the MED12 subunit of the Mediator. A novel dominant mutant, crp-1D, which causes up-regulation of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), FRUITFULL (FUL) and APETALA1 (AP1) expression in a FLOWERING LOCUS T (FT)-dependent manner, was identified in an enhancer screen of the early-flowering phenotype of 35S::FT. Genetic and molecular analysis of both crp-1D and crp loss-of-function alleles showed that MED12/CRP is required not only for proper regulation of SOC1, FUL and AP1, but also for up-regulation of FT, TWIN SISTER OF FT (TSF) and FD, and down-regulation of FLOWERING LOCUS C (FLC). These observations suggest that MED12/CRP is a novel flowering regulator with multiple regulatory target steps both upstream and downstream of the key flowering regulators including FT florigen. Our work, taken together with recent studies of other Mediator subunit genes, supports an emerging view that the Mediator plays multiple roles in the regulation of flowering.

The SOC1-SPL module integrates photoperiod and gibberellic acid signals to control flowering time in Arabidopsis

miR156 and its target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes constitute an endogenous flowering pathway in Arabidopsis. The SPL genes are regulated post-transcriptionally by miR156, and incorporate endogenous aging signals into floral gene networks. Intriguingly, the SPL genes are also regulated transcriptionally by FLOWERING LOCUS T (FT)-mediated photoperiod signals. However, it is unknown how photoperiod regulates the SPL genes. Here, we show that SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FT regulate the SPL3, SPL4 and SPL5 genes by directly binding to the gene promoters in response to photoperiod signals. Notably, the SOC1 regulation of the SPL genes, termed the SOC1-SPL module, also mediates gibberellic acid (GA) signals to promote flowering under non-inductive short days (SDs). Under SDs, the inductive effects of GA on the SPL genes disappeared in the soc1-2 mutant, and the flowering of SPL3-overexpressing transgenic plants (35S:SPL3) was less sensitive to GA. In addition, the 35S:SPL3 × soc1-2 plants flowered much earlier than the soc1-2 mutant, supporting SOC1 regulation of the SPL genes. Our observations indicate that the SOC1-SPL module serves as a molecular link that integrates photoperiod and GA signals to promote flowering in Arabidopsis.

Ca2+-dependent GTPase, extra-large G protein 2 (XLG2), promotes activation of DNA-binding protein related to vernalization 1 (RTV1), leading to activation of floral integrator genes and early flowering in Arabidopsis

Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, play important roles in plant development and cell signaling. In Arabidopsis, in addition to one prototypical G protein α subunit, GPA1, there are three extra-large G proteins, XLG1, XLG2, and XLG3, of largely unknown function. Each extra-large G (XLG) protein has a C-terminal Gα-like region and a ∼400 amino acid N-terminal extension. Here we show that the three XLG proteins specifically bind and hydrolyze GTP, despite the fact that these plant-specific proteins lack key conserved amino acid residues important for GTP binding and hydrolysis of GTP in mammalian Gα proteins. Moreover, unlike other known Gα proteins, these activities require Ca(2+) instead of Mg(2+) as a cofactor. Yeast two-hybrid library screening and in vitro protein pull-down assays revealed that XLG2 interacts with the nuclear protein RTV1 (related to vernalization 1). Electrophoretic mobility shift assays show that RTV1 binds to DNA in vitro in a non-sequence-specific manner and that GTP-bound XLG2 promotes the DNA binding activity of RTV1. Overexpression of RTV1 results in early flowering. Combined overexpression of XLG2 and RTV1 enhances this early flowering phenotype and elevates expression of the floral pathway integrator genes, FT and SOC1, but does not repress expression of the floral repressor, FLC. Chromatin immunoprecipitation assays show that XLG2 increases RTV1 binding to FT and SOC1 promoters. Thus, a Ca(2+)-dependent G protein, XLG2, promotes RTV1 DNA binding activity for a subset of floral integrator genes and contributes to floral transition.