Non-cell-autonomous regulation of petal initiation in Arabidopsis thaliana.

In many flowering plants, petals initiate in alternate positions from first whorl sepals, suggesting possible signaling between sepal boundaries and petal initiation sites. PETAL LOSS (PTL) and RABBIT EARS (RBE) regulate petal initiation in Arabidopsis thaliana and their transcripts are expressed in sepal boundary and petal initiation sites, respectively, suggesting that PTL acts in a non-cell-autonomous manner. Here, we determined that cells expressing PTL and RBE fusion proteins did not overlap but were adjacent, confirming the non-cell-autonomous function of PTL. Genetic ablation of intersepal cells by expressing the diphtheria toxin-A chain gene driven by the PTL promoter resulted in flowers lacking petals, suggesting these cells are required for petal initiation. Transcriptome analysis combined with a PTL induction system revealed 42 genes that were upregulated under PTL activation, including UNUSUAL FLORAL ORGANS (UFO), which likely plays an important role in petal initiation. These findings suggest a molecular mechanism in which PTL indirectly regulates petal initiation and UFO mediates positional signaling between the sepal boundary and petal initiation sites.

Inhibition of cysteine protease disturbs the topological relationship between bone resorption and formation in vitro.

INTRODUCTION: Osteoporosis is a global health issue. Bisphosphonates that are commonly used to treat osteoporosis suppress both bone resorption and subsequent bone formation. Inhibition of cathepsin K, a cysteine proteinase secreted by osteoclasts, was reported to suppress bone resorption while preserving or increasing bone formation. Analyses of the different effects of antiresorptive reagents such as bisphosphonates and cysteine proteinase inhibitors will contribute to the understanding of the mechanisms underlying bone remodeling. MATERIALS AND METHODS: Our team has developed an in vitro system in which bone remodeling can be temporally observed at the cellular level by 2-photon microscopy. We used this system in the present study to examine the effects of the cysteine proteinase inhibitor E-64 and those of zoledronic acid on bone remodeling. RESULTS: In the control group, the amount of the reduction and the increase in the matrix were correlated in each region of interest, indicating the topological and quantitative coordination of bone resorption and formation. Parameters for osteoblasts, osteoclasts, and matrix resorption/formation were also correlated. E-64 disrupted the correlation between resorption and formation by potentially inhibiting the emergence of spherical osteoblasts, which are speculated to be reversal cells in the resorption sites. CONCLUSION: These new findings help clarify coupling mechanisms and will contribute to the development of new drugs for osteoporosis.

Transcriptomic heterochrony and completely cleistogamous flower development in the mycoheterotrophic orchid Gastrodia.

Cleistogamy, in which plants can reproduce via self-fertilization within permanently closed flowers, has evolved in > 30 angiosperm lineages; however, consistent with Darwin's doubts about its existence, complete cleistogamy - the production of only cleistogamous flowers - has rarely been recognized. Thus far, the achlorophyllous orchid genus, Gastrodia, is the only known genus with several plausible completely cleistogamous species. Here, we analyzed the floral developmental transcriptomes of two recently evolved, completely cleistogamous Gastrodia species and their chasmogamous sister species to elucidate the possible changes involved in producing common cleistogamous traits. The ABBA-BABA test did not support introgression and protein sequence convergence as evolutionary mechanisms leading to cleistogamy, leaving convergence in gene expression as a plausible mechanism. Regarding transcriptomic differentiation, the two cleistogamous species had common modifications in the expression of developmental regulators, exhibiting a gene family-wide signature of convergent expression changes in MADS-box genes. Our transcriptomic pseudotime analysis revealed a prolonged juvenile state and eventual maturation, a heterochronic pattern consistent with partial neoteny, in cleistogamous flower development. These findings indicate that transcriptomic partial neoteny, arising from changes in the expression of conserved developmental regulators, might have contributed to the rapid and repeated evolution of cleistogamous flowers in Gastrodia.

Quantitative analyses of matrices, osteoblasts, and osteoclasts during bone remodeling using an in vitro system.

INTRODUCTION: Bone remodeling plays a central role in the maintenance of bone homeostasis. Our group has established an in vitro system by which the cellular events during bone remodeling can be observed longitudinally. This study used this system to quantitatively analyze osteoblasts, osteoclasts, and matrices to elucidate their temporal changes and correlations. MATERIALS AND METHODS: Osteoblasts from EGFP mice were cultured to form calcified nodules, followed by co-culture with bone marrow macrophages from Tnfrsf11aCre/+ x Ai14 mice for 3 weeks (resorption phase). Then cells were cultured with osteoblast differentiation medium for 3 weeks (formation phase). The same sites were observed weekly using 2-photon microscopy. Matrices were detected using second harmonic generation. Parameters related to matrices, osteoblasts, and osteoclasts were quantified and statistically analyzed. RESULTS: Resorption and replenishment of the matrix were observed at the same sites by 2 photon microscopy. Gross quantification revealed that matrix and osteoblast parameters decreased in the resorption phase and increased in the formation phase, while osteoclast parameters showed the opposite pattern. When one field of view was divided into 16 regions of interest (ROIs) and correlations between parameters were analyzed in each ROI, decreased and increased matrix volumes were moderately correlated. Parameters of matrices and osteoblasts, and those of matrices and osteoclasts exhibited moderate correlations, while those of osteoblasts and osteoclasts were only weakly correlated. CONCLUSION: Several correlations between cells and matrix during remodeling were demonstrated quantitatively. This system may be a powerful tool for the research of bone remodeling.

Leaf Cell Morphology Alternation in Response to Environmental Signals in Rorippa aquatica.

Heterophylly, the phenomenon by which plants alter leaf forms to adapt to surrounding conditions, is apparent in amphibious plant species. In response to submergence, they emerge leaves with narrower blade areas. The pathway that receives the submergence signals and the mechanism regulating leaf form via cell proliferation and/or expansion systems have not yet been fully identified yet. Our anatomical study of Rorippa aquatica, an amphibious plant that exhibits heterophylly in response to various signals, showed that leaf thickness increased upon submergence; this was caused by the expansion of mesophyll cell size. Additionally, these submergence effects were inhibited under blue-light conditions. The ANGUSTIFOLIA3 (AN3)/GROWTH-REGULATING FACTOR (GRF) pathway regulating cell proliferation and cell expansion was downregulated in response to submergence; and the response was blocked under the blue-light conditions. These results suggest that submergence and light quality determine leaf cell morphology via the AN3/GRF pathway.

ERdj3B-Mediated Quality Control Maintains Anther Development at High Temperatures.

Pollen development is highly sensitive to heat stress, which impairs cellular proteostasis by causing misfolded proteins to accumulate. Therefore, each cellular compartment possesses a dedicated protein quality control system. An elaborate quality control system involving molecular chaperones, including immunoglobulin-binding protein (BiP), heat shock protein70, and regulatory J domain-containing cochaperones (J proteins), allows the endoplasmic reticulum (ER) to withstand a large influx of proteins. Here, we found that Arabidopsis (Arabidopsis thaliana) mutants of ER-localized DnaJ family 3B (ERdj3B), one of three ER-resident J proteins involved in ER quality control, produced few seeds at high temperatures (29°C) due to defects in anther development. This temperature-sensitive fertility defect is specific to the defective interactions of BiP with ERdj3B but not with the other two J proteins, indicating functional differences between ERdj3B and the other J proteins. RNA sequencing analysis revealed that heat stress affects pollen development in both wild-type and mutant buds, but the erdj3b mutant is more susceptible, possibly due to defects in ER quality control. Our results highlight the importance of a specific ER quality control factor, ERdj3B, for plant reproduction, particularly anther development, at high temperatures.

BELL1-like homeobox genes regulate inflorescence architecture and meristem maintenance in rice.

Inflorescence architecture is diverse in angiosperms, and is mainly determined by the arrangement of the branches and flowers, known as phyllotaxy. In rice (Oryza sativa), the main inflorescence axis, called the rachis, generates primary branches in a spiral phyllotaxy, and flowers (spikelets) are formed on these branches. Here, we have studied a classical mutant, named verticillate rachis (ri), which produces branches in a partially whorled phyllotaxy. Gene isolation revealed that RI encodes a BELL1-type homeodomain transcription factor, similar to Arabidopsis PENNYWISE/BELLRINGER/REPLUMLESS, and is expressed in the specific regions within the inflorescence and branch meristems where their descendant meristems would soon initiate. Genetic combination of an ri homozygote and a mutant allele of RI-LIKE1 (RIL1) (designated ri ril1/+ plant), a close paralog of RI, enhanced the ri inflorescence phenotype, including the abnormalities in branch phyllotaxy and rachis internode patterning. During early inflorescence development, the timing and arrangement of primary branch meristem (pBM) initiation were disturbed in both ri and ri ril1/+ plants. These findings suggest that RI and RIL1 were involved in regulating the phyllotactic pattern of the pBMs to form normal inflorescences. In addition, both RI and RIL1 seem to be involved in meristem maintenance, because the ri ril1 double-mutant failed to establish or maintain the shoot apical meristem during embryogenesis.

SUPPRESSOR OF GAMMA RESPONSE 1 acts as a regulator coordinating crosstalk between DNA damage response and immune response in Arabidopsis thaliana.

Plants live in constantly changing and often unfavorable or stressful environments. Environmental changes induce biotic and abiotic stress, which, in turn, may cause genomic DNA damage. Hence, plants simultaneously suffer abiotic/biotic stress and DNA damage. However, little information is available on the signaling crosstalk that occurs between DNA damage and abiotic/biotic stresses. Arabidopsis thaliana SUPPRESSOR OF GAMMA RESPONSE1 (SOG1) is a pivotal transcription factor that regulates thousands of genes in response to DNA double-strand break (DSB), and we recently reported that SOG1 has a role in immune responses. In the present study, the effects of SOG1 overexpression on the DNA damage and immune responses were examined. Results found that SOG1 overexpression enhances the regulation of numerous downstream genes. Relative to the wild type plants, then, DNA damage responses were observed to be strongly induced. SOG1 overexpression also upregulates chitin (a major components of fungal cell walls) responsive genes in the presence of DSBs, implying that pathogen defense response is activated by DNA damage via SOG1. Further, SOG1 overexpression enhances fungal resistance. These results suggest that SOG1 regulates crosstalk between DNA damage response and the immune response and that plants have evolved a sophisticated defense network to contend with environmental stress.

Molecular Basis for Natural Vegetative Propagation via Regeneration in North American Lake Cress, Rorippa aquatica (Brassicaceae).

Some plant species have a striking capacity for regeneration in nature, including regeneration of the entire individual from explants. However, due to the lack of suitable experimental models, the regulatory mechanisms of spontaneous whole plant regeneration are mostly unknown. In this study, we established a novel model system to study these mechanisms using an amphibious plant within Brassicaceae, Rorippa aquatica, which naturally undergoes vegetative propagation via regeneration from leaf fragments. Morphological and anatomical observation showed that both de novo root and shoot organogenesis occurred from the proximal side of the cut edge transversely with leaf vascular tissue. Time-series RNA-seq analysis revealed that auxin and cytokinin responses were activated after leaf amputation and that regeneration-related genes were upregulated mainly on the proximal side of the leaf explants. Accordingly, we found that both auxin and cytokinin accumulated on the proximal side. Application of a polar auxin transport inhibitor retarded root and shoot regeneration, suggesting that the enhancement of auxin responses caused by polar auxin transport enhanced de novo organogenesis at the proximal wound site. Exogenous phytohormone and inhibitor applications further demonstrated that, in R. aquatica, both auxin and gibberellin are required for root regeneration, whereas cytokinin is important for shoot regeneration. Our results provide a molecular basis for vegetative propagation via de novo organogenesis.

Increased Phosphorylation of Ser-Gln Sites on SUPPRESSOR OF GAMMA RESPONSE1 Strengthens the DNA Damage Response in Arabidopsis thaliana.

The Arabidopsis thaliana transcription factor SUPPRESSOR OF GAMMA RESPONSE1 (SOG1) regulates hundreds of genes in response to DNA damage, and this results in the activation of cell cycle arrest, DNA repair, endoreduplication, and programmed cell death. However, it is not clear how this single transcription factor regulates each of these pathways. We previously reported that phosphorylation of five Ser-Gln (SQ) motifs in the C-terminal region of SOG1 are required to activate downstream pathways. In this study, we introduced Ser-to-Ala (AQ) substitutions in these five SQ motifs to progressively eliminate them and then we examined the effects on DNA damage responses. We found that all SQs are required for the full activation of SOG1 and that the expression level of most downstream genes changed incrementally depending on the number of phosphorylated SQ sites. Genes involved in DNA repair and cell cycle progression underwent stepwise activation and inhibition respectively as the number of phosphorylated SQ sites increased. Also, inhibition of DNA synthesis, programmed cell death, and cell differentiation were incrementally induced as the number of phosphorylated SQ sites increased. These results show that the extent of SQ phosphorylation in SOG1 regulates gene expression levels and determines the strength of DNA damage responses.

Identifying the target genes of SUPPRESSOR OF GAMMA RESPONSE 1, a master transcription factor controlling DNA damage response in Arabidopsis.

In mammalian cells, the transcription factor p53 plays a crucial role in transmitting DNA damage signals to maintain genome integrity. However, in plants, orthologous genes for p53 and checkpoint proteins are absent. Instead, the plant-specific transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1) controls most of the genes induced by gamma irradiation and promotes DNA repair, cell cycle arrest, and stem cell death. To date, the genes directly controlled by SOG1 remain largely unknown, limiting the understanding of DNA damage signaling in plants. Here, we conducted a microarray analysis and chromatin immunoprecipitation (ChIP)-sequencing, and identified 146 Arabidopsis genes as direct targets of SOG1. By using ChIP-sequencing data, we extracted the palindromic motif [CTT(N)7 AAG] as a consensus SOG1-binding sequence, which mediates target gene induction in response to DNA damage. Furthermore, DNA damage-triggered phosphorylation of SOG1 is required for efficient binding to the SOG1-binding sequence. Comparison between SOG1 and p53 target genes showed that both transcription factors control genes responsible for cell cycle regulation, such as CDK inhibitors, and DNA repair, whereas SOG1 preferentially targets genes involved in homologous recombination. We also found that defense-related genes were enriched in the SOG1 target genes. Consistent with this finding, SOG1 is required for resistance against the hemi-biotrophic fungus Colletotrichum higginsianum, suggesting that SOG1 has a unique function in controlling the immune response.

A Missense Mutation in the NSF Gene Causes Abnormal Golgi Morphology in Arabidopsis thaliana.

The Golgi apparatus is a key station of glycosylation and membrane traffic. It consists of stacked cisternae in most eukaryotes. However, the mechanisms how the Golgi stacks are formed and maintained are still obscure. The model plant Arabidopsis thaliana provides a nice system to observe Golgi structures by light microscopy, because the Golgi in A. thaliana is in the form of mini-stacks that are distributed throughout the cytoplasm. To obtain a clue to understand the molecular basis of Golgi morphology, we took a forward-genetic approach to isolate A. thaliana mutants that show abnormal structures of the Golgi under a confocal microscope. In the present report, we describe characterization of one of such mutants, named #46-3. The #46-3 mutant showed pleiotropic Golgi phenotypes. The Golgi size was in majority smaller than the wild type, but varied from very small ones, sometimes without clear association of cis and trans cisternae, to abnormally large ones under a confocal microscope. At the ultrastructual level by electron microscopy, queer-shaped large Golgi stacks were occasionally observed. By positional mapping, genome sequencing, and complementation and allelism tests, we linked the mutant phenotype to the missense mutation D374N in the NSF gene, encoding the N-ethylmaleimide-sensitive factor (NSF), a key component of membrane fusion. This residue is near the ATP-binding site of NSF, which is very well conserved in eukaryotes, suggesting that the biochemical function of NSF is important for maintaining the normal morphology of the Golgi.Key words: Golgi morphology, N-ethylmaleimide-sensitive factor (NSF), Arabidopsis thaliana.

Identification of Arabidopsis genic and non-genic promoters by paired-end sequencing of TSS tags.

Information about transcription start sites (TSSs) provides baseline data for the analysis of promoter architecture. In this paper we used paired- and single-end deep sequencing to analyze Arabidopsis TSS tags from several libraries prepared from roots, shoots, flowers and etiolated seedlings. The clustering of approximately 33 million mapped TSS tags led to the identification of 324 461 promoters that covered 79.7% (21 672/27 206) of protein-coding genes in the Arabidopsis genome. In addition we identified intragenic, antisense and orphan promoters that were not associated with any gene models. Of these, intragenic promoters exhibited unique characteristics regarding dinucleotide sequences at TSSs and core promoter element composition, suggesting that these promoters use different mechanisms of transcriptional initiation. An analysis of base composition with regard to promoter position revealed a low GC content throughout the promoter region and several local strand biases that were evident for TATA-type promoters, but not for Coreless-type promoters. Most observed strand biases coincided with strand biases of single nucleotide polymorphism rate. Our analysis also revealed that transcription of a gene is supported by an average of 2.7 genic promoters, among which one specific promoter, designated as a top promoter, substantially determines the expression level of the gene.

Protein S-Nitrosylation Regulates Xylem Vessel Cell Differentiation in Arabidopsis.

Post-translational modifications of proteins have important roles in the regulation of protein activity. One such modification, S-nitrosylation, involves the covalent binding of nitric oxide (NO)-related species to a cysteine residue. Recent work showed that protein S-nitrosylation has crucial functions in plant development and environmental responses. In the present study, we investigated the importance of protein S-nitrosylation for xylem vessel cell differentiation using a forward genetics approach. We performed ethyl methanesulfonate mutagenesis of a transgenic Arabidopsis 35S::VND7-VP16-GR line in which the activity of VASCULAR-RELATED NAC-DOMAIN7 (VND7), a key transcription factor involved in xylem vessel cell differentiation, can be induced post-translationally by glucocorticoid treatment, with the goal of obtaining suppressor mutants that failed to differentiate ectopic xylem vessel cells; we named these mutants suppressor of ectopic vessel cell differentiation induced by VND7 (seiv) mutants. We found the seiv1 mutant to be a recessive mutant in which ectopic xylem cell differentiation was inhibited, especially in aboveground organs. In seiv1 mutants, a single nucleic acid substitution (G to A) leading to an amino acid substitution (E36K) was present in the gene encoding S-NITROSOGLUTATHIONE REDUCTASE 1 (GSNOR1), which regulates the turnover of the natural NO donor, S-nitrosoglutathione. An in vitro S-nitrosylation assay revealed that VND7 can be S-nitrosylated at Cys264 and Cys320 located near the transactivation activity-related domains, which were shown to be important for transactivation activity of VND7 by transient reporter assay. Our results suggest crucial roles for GSNOR1-regulated protein S-nitrosylation in xylem vessel cell differentiation, partly through the post-translational modification of VND7.

Detection of Impurities in Organic Crystals by High-Accuracy Terahertz Absorption Spectroscopy.

Quantitative detection of impurities in organic crystals was demonstrated by accurately measuring absorption frequencies using a continuous wave gallium phosphide terahertz spectrometer. THz spectra of l-asparagine monohydrate doped with l-aspartic acid at 0.05 to 12.5 wt % were obtained at 10 K. The three lowest frequency absorption peaks were baseline-resolved, allowing them to be examined independently. Using a least-squares curve fitting technique, impurities were detected at levels as low as 500 ppm. The sensitivity and detection limits of the technique depended strongly on the nature of both the host and the impurities. The projected limit of detection using the current system, given optimal materials, was estimated to be 51.7 ppm. In addition to quantitative assessments, impurities may also be identified by comparing frequency shifts of multiple absorptions.

FT-like proteins induce transposon silencing in the shoot apex during floral induction in rice.

Floral induction is a crucial developmental step in higher plants. Florigen, a mobile floral activator that is synthesized in the leaf and transported to the shoot apex, was recently identified as a protein encoded by FLOWERING LOCUS T (FT) and its orthologs; the rice florigen is Heading date 3a (Hd3a) protein. The 14-3-3 proteins mediate the interaction of Hd3a with the transcription factor OsFD1 to form a ternary structure called the florigen activation complex on the promoter of OsMADS15, a rice APETALA1 ortholog. However, crucial information, including the spatiotemporal overlap among FT-like proteins and the components of florigen activation complex and downstream genes, remains unclear. Here, we confirm that Hd3a coexists, in the same regions of the rice shoot apex, with the other components of the florigen activation complex and its transcriptional targets. Unexpectedly, however, RNA-sequencing analysis of shoot apex from wild-type and RNA-interference plants depleted of florigen activity revealed that 4,379 transposable elements (TEs; 58% of all classifiable rice TEs) were expressed collectively in the vegetative and reproductive shoot apex. Furthermore, in the reproductive shoot apex, 214 TEs were silenced by florigen. Our results suggest a link between floral induction and regulation of TEs.

Plant Temperature Sensors.

Temperature is one of the most important environmental signals for plants. High and low temperatures have a variety of effects that affect plant growth and development profoundly. Further, temperature is an indication of seasonal change. Plants must survive under severe conditions in winter and prepare to resume growth and reach their reproductive stage in the following spring. Recent studies have focused on plant mechanisms responsible for sensing temperature and the molecular systems underlying plant reactions in response to this signal. In this review, we describe how plants sense ambient temperature to adapt to ambient-temperature changes.

Genetic Enhancer Analysis Reveals that FLORAL ORGAN NUMBER2 and OsMADS3 Co-operatively Regulate Maintenance and Determinacy of the Flower Meristem in Rice.

Meristems such as the shoot apical meristem and flower meristem (FM) act as a reservoir of stem cells, which reproduce themselves and supply daughter cells for the differentiation of lateral organs. In Oryza sativa (rice), the FLORAL ORGAN NUMBER2 (FON2) gene, which is similar to Arabidopsis CLAVATA3, is involved in meristem maintenance. In fon2 mutants, the numbers of floral organs are increased due to an enlargement of the FM. To identify new factors regulating meristem maintenance in rice, we performed a genetic screening of mutants that enhanced the fon2 mutation, and found a mutant line (2B-424) in which pistil number was dramatically increased. By using a map-based approach and next-generation sequencing, we found that the line 2B-424 had a complete loss-of-function mutation (a large deletion) in OsMADS3, a class C MADS-box gene that is known to be involved in stamen specification. Disruption of OsMADS3 in the fon2 mutant by CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats-CRISPR-associated protein 9) technology caused a flower phenotype similar to that of 2B-424, confirming that the gene responsible for enhancement of fon2 was OsMADS3. Morphological analysis showed that the fon2 and osmads3 mutations synergistically affected pistil development and FM determinacy. We also found that whorl 3 was duplicated in mature flowers and the FM was enlarged at an early developmental stage in severe osmads3 single mutants. These findings suggest that OsMADS3 is involved not only in FM determinacy in late flower development but also in FM activity in early flower development.

The Plasma Membrane H+-ATPase AHA1 Plays a Major Role in Stomatal Opening in Response to Blue Light.

Stomata open in response to a beam of weak blue light under strong red light illumination. A blue light signal is perceived by phototropins and transmitted to the plasma membrane H(+)-ATPase that drives stomatal opening. To identify the components in this pathway, we screened for mutants impaired in blue light-dependent stomatal opening. We analyzed one such mutant, provisionally named blus2 (blue light signaling2), and found that stomatal opening in leaves was impaired by 65%, although the magnitude of red light-induced opening was not affected. Blue light-dependent stomatal opening in the epidermis and H(+) pumping in guard cell protoplasts were inhibited by 70% in blus2 Whole-genome resequencing identified a mutation in the AHA1 gene of the mutant at Gly-602. T-DNA insertion mutants of AHA1 exhibited a similar phenotype to blus2; this phenotype was complemented by the AHA1 gene. We renamed blus2 as aha1-10 T-DNA insertion mutants of AHA2 and AHA5 did not show any impairment in stomatal response, although the transcript levels of AHA2 and AHA5 were higher than those of AHA1 in wild-type guard cells. Stomata in ost2, a constitutively active AHA1 mutant, did not respond to blue light. A decreased amount of H(+)-ATPase in aha1-10 accounted for the reduced stomatal blue light responses and the decrease was likely caused by proteolysis of misfolded AHA1. From these results, we conclude that AHA1 plays a major role in blue light-dependent stomatal opening in Arabidopsis and that the mutation made the AHA1 protein unstable in guard cells.

FE, a phloem-specific Myb-related protein, promotes flowering through transcriptional activation of FLOWERING LOCUS T and FLOWERING LOCUS T INTERACTING PROTEIN 1.

In many flowering plants, the transition to flowering is primarily affected by seasonal changes in day length (photoperiod). An inductive photoperiod promotes flowering via synthesis of a floral stimulus, called florigen. In Arabidopsis thaliana, the FLOWERING LOCUS T (FT) protein is an essential component of florigen, which is synthesized in leaf phloem companion cells and is transported through phloem tissue to the shoot apical meristem where floral morphogenesis is initiated. However, the molecular mechanism involved in the long-distance transport of FT protein remains elusive. In this study, we characterized the classic Arabidopsis mutant fe, which is involved in the photoperiodic induction of flowering, and showed that FE encodes a phloem-specific Myb-related protein that was previously reported as ALTERED PHLOEM DEVELOPMENT. Phenotypic analyses of the fe mutant showed that FT expression is reduced in leaf phloem companion cells. In addition, the transport of FT protein from leaves to the shoot apex is impaired in the fe mutant. Expression analyses further demonstrated that FE is also required for transcriptional activation of FLOWERING LOCUS T INTERACTING PROTEIN 1 (FTIP1), an essential regulator for selective trafficking of the FT protein from companion cells to sieve elements. These findings indicate that FE plays a dual role in the photoperiodic induction of flowering: as a transcriptional activator of FT on the one hand, and its transport machinery component, FTIP1, on the other hand. Thus, FE is likely to play a role in regulating FT by coordinating FT synthesis and FT transport in phloem companion cells.