Export of FT protein from phloem companion cells is sufficient for floral induction in Arabidopsis

The mobile RNAs, StBEL11 and StBEL29, suppress growth of tubers in potato

We demonstrate that RNAs of StBEL11 and StBEL29 are phloem-mobile and function antagonistically to the growth-promoting characteristics of StBEL5 in potato. Both these RNAs appear to inhibit tuber growth by repressing the activity of target genes of StBEL5 in potato. Moreover, upstream sequence driving GUS expression in transgenic potato lines demonstrated that both StBEL11 and -29 promoter activity is robust in leaf veins, petioles, stems, and vascular tissues and induced by short days in leaves and stolons. Steady-state levels of their mRNAs were also enhanced by short-day conditions in selective organs. There are thirteen functional BEL1-like genes in potato that encode for a family of transcription factors (TF) ubiquitous in the plant kingdom. These BEL1 TFs work in tandem with KNOTTED1-types to regulate the expression of numerous target genes involved in hormone metabolism and growth processes. One of the StBELs, StBEL5, functions as a long-distance mRNA signal that is transcribed in leaves and moves into roots and stolons to stimulate growth. The two most closely related StBELs to StBEL5 are StBEL11 and -29. Together these three genes make up more than 70% of all StBEL transcripts present throughout the potato plant. They share a number of common features, suggesting they may be co-functional in tuber development. Upstream sequence driving GUS expression in transgenic potato lines demonstrated that both StBEL11 and -29 promoter activity is robust in leaf veins, petioles, stems, and vascular tissues and induced by short-days in leaves and stolons. Steady-state levels of their mRNAs were also enhanced by short-day conditions in specific organs. Using a transgenic approach and heterografting experiments, we show that both these StBELs inhibit growth in correlation with the long distance transport of their mRNAs from leaves to roots and stolons, whereas suppression lines of these two RNAs exhibited enhanced tuber yields. In summary, our results indicate that the RNAs of StBEL11 and StBEL29 are phloem-mobile and function antagonistically to the growth-promoting characteristics of StBEL5. Both these RNAs appear to inhibit growth in tubers by repressing the activity of target genes of StBEL5.

Hormonal control of the floral transition: Can one catch them all?

The transition to flowering marks a key adaptive developmental switch in plants which impacts on their survival and fitness. Different signaling pathways control the floral transition, conveying both endogenous and environmental cues. These cues are often relayed and/or modulated by different hormones, which might confer additional developmental flexibility to the floral process in the face of varying conditions. Among the different hormonal pathways, the phytohormone gibberellic acid (GA) plays a dominant role. GA is connected with the other floral pathways through the GA-regulated DELLA proteins, acting as versatile interacting modules for different signaling proteins. In this review, I will highlight the role of DELLAs as spatial and temporal modulators of different consolidated floral pathways. Next, building on recent data, I will provide an update on some emerging themes connecting other hormone signaling cascades to flowering time control. I will finally provide examples for some established as well as potential cross-regulatory mechanisms between hormonal pathways mediated by the DELLA proteins.

Beyond flexibility: controlling stem cells in an ever changing environment

Developmental plasticity is a defining feature of plants allowing them to colonize a wide range of different ecosystems by promoting environmental adaptation. Their postembryonic development requires life-long maintenance of stem cells, which are embedded into specialized tissues, called meristems. The shoot apical meristem gives rise to all above ground tissues and is a complex and dynamic three-dimensional structure harboring cells of different clonal origins and fates. Functionally divergent subdomains are stably maintained despite permanent cell division, however their relative sizes are modified in response to developmental and environmental signals. In this review, we briefly describe the core regulatory program of the shoot apical meristem and discuss progress in the fields of mechanical and environmental control of its activity.

Potato Tuber Induction is Regulated by Interactions Between Components of a Tuberigen Complex

Photoperiod-regulated flowering and potato tuber formation involve leaf-produced mobile signals, florigen and tuberigen, respectively. The major protein component of florigen has been identified as the FLOWERING LOCUS T (FT) protein. In rice, an FT-like protein, Heading date 3a (Hd3a), induces flowering by making the florigen activation complex (FAC) through interactions with 14-3-3 and OsFD1, a rice FD-like protein. In potato, StSP6A, an FT-like protein, was identified as a major component of tuberigen. However, the molecular mechanism of how StSP6A triggers tuber formation remains elusive. Here we analyzed the significance of the formation of a complex including StSP6A, 14-3-3 and FD-like proteins in tuberization. Yeast two-hybrid, bimolecular fluorescence complementation and in vitro pull-down assays showed that StSP6A and StFDL1, a potato FD-like protein, interact with St14-3-3s. StSP6A overexpression induced early tuberization in a 14-3-3-dependent manner, and suppression of StFDL1 delayed tuberization. These results strongly suggest that an FAC-like complex, the tuberigen activation complex (TAC), comprised of StSP6A, St14-3-3s and StFDL1, regulates potato tuber formation.

Inflorescence Development and the Role of LsFT in Regulating Bolting in Lettuce (Lactuca sativa L.)

Lettuce (Lactuca sativa L.) is one of the most important leafy vegetable that is consumed during its vegetative growth. The transition from vegetative to reproductive growth is induced by high temperature, which has significant economic effect on lettuce production. However, the progression of floral transition and the molecular regulation of bolting are largely unknown. Here we morphologically characterized the inflorescence development and functionally analyzed the FLOWERING LOCUS T (LsFT) gene during bolting regulation in lettuce. We described the eight developmental stages during floral transition process. The expression of LsFT was negatively correlated with bolting in different lettuce varieties, and was promoted by heat treatment. Overexpression of LsFT could recover the late-flowering phenotype of ft-2 mutant. Knockdown of LsFT by RNA interference dramatically delayed bolting in lettuce, and failed to respond to high temperature. Therefore, this study dissects the process of inflorescence development and characterizes the role of LsFT in bolting regulation in lettuce.

Companion cells: a diamond in the rough

Vascular plants have developed highly specialized cells to transport nutrients and developmental signals. The differentiation process includes the degradation of multiple organelles of the sieve element cells (SEs) to facilitate transport and, as a consequence, SEs become dependent on neighboring companion cells (CCs). Despite its importance for phloem function and flowering time control, CCs are still a mysterious cell type. In this review, we gather all the genes known to be expressed in CCs, in different organs and organisms, with the objective of better understanding CC identity and function.

The Importance of Being on Time: Regulatory Networks Controlling Photoperiodic Flowering in Cereals

Flowering is the result of the coordination between genetic information and environmental cues. Gene regulatory networks have evolved in plants in order to measure diurnal and seasonal variation of day length (or photoperiod), thus aligning the reproductive phase with the most favorable season of the year. The capacity of plants to discriminate distinct photoperiods classifies them into long and short day species, depending on the conditions that induce flowering. Plants of tropical origin and adapted to short day lengths include rice, maize, and sorghum, whereas wheat and barley were originally domesticated in the Fertile Crescent and are considered long day species. In these and other crops, day length measurement mechanisms have been artificially modified during domestication and breeding to adapt plants to novel areas, to the extent that a wide diversity of responses exists within any given species. Notwithstanding the ample natural and artificial variation of day length responses, some of the basic molecular elements governing photoperiodic flowering are widely conserved. However, as our understanding of the underlying mechanisms improves, it becomes evident that specific regulators exist in many lineages that are not shared by others, while apparently conserved components can be recruited to novel functions during evolution.

NUCLEAR FACTOR Y, Subunit A (NF-YA) Proteins Positively Regulate Flowering and Act Through FLOWERING LOCUS T

Photoperiod dependent flowering is one of several mechanisms used by plants to initiate the developmental transition from vegetative growth to reproductive growth. The NUCLEAR FACTOR Y (NF-Y) transcription factors are heterotrimeric complexes composed of NF-YA and histone-fold domain (HFD) containing NF-YB/NF-YC, that initiate photoperiod-dependent flowering by cooperatively interacting with CONSTANS (CO) to drive the expression of FLOWERING LOCUS T (FT). This involves NF-Y and CO binding at distal CCAAT and proximal “CORE” elements, respectively, in the FT promoter. While this is well established for the HFD subunits, there remains some question over the potential role of NF-YA as either positive or negative regulators of this process. Here we provide strong support, in the form of genetic and biochemical analyses, that NF-YA, in complex with NF-YB/NF-YC proteins, can directly bind the distal CCAAT box in the FT promoter and are positive regulators of flowering in an FT-dependent manner.

For plants to have reproductive success, they must time their flowering with the most beneficial biotic and abiotic environmental conditions—after all, reproductive success would likely be low if flowers developed when pollinators were not present or freezing temperatures were on the horizon. Proper timing mechanisms for flowering vary significantly between different species, but can be connected to a variety of environmental cues, including water availability, temperature, and day length. Numerous labs have studied the molecular aspects of these timing mechanisms and discovered that many of these pathways converge on the gene FLOWERING LOCUS T (FT). This means that understanding precisely how this gene is regulated can teach us a lot about many plant species in both natural and agricultural settings. In the current study, we focus on day length as an essential cue for flowering in the plant species Arabidopsis thaliana. We further unravel the complexity of FT regulation by clarifying the roles of NUCLEAR FACTOR Y genes in day length perception.

SPL3/4/5 Integrate Developmental Aging and Photoperiodic Signals into the FT-FD Module in Arabidopsis Flowering

Environmental sensitivity varies across developmental phases in flowering plants. In the juvenile phase, microRNA156 (miR156)-mediated repression of SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) transcription factors renders Arabidopsis plants incompetent to floral inductive signals, including long-day (LD) photoperiod. During the vegetative phase transition, which accompanies a reduction of miR156 and a concomitant elevation of its targets, plants acquire reproductive competence such that LD signals promote flowering. However, it remains largely unknown how developmental signals are associated with photoperiodic flowering. Here, we show that SPL3, SPL4, and SPL5 (SPL3/4/5) potentiate the FLOWERING LOCUS T (FT)-FD module in photoperiodic flowering. SPL3/4/5 function as transcriptional activators through the interaction with FD, a basic leucine zipper transcription factor which plays a critical role in photoperiodic flowering. SPL3/4/5 can directly bind to the promoters of APETALA1, LEAFY, and FRUITFULL, thus mediating their activation by the FT-FD complex. Our findings demonstrate that SPL3/4/5 act synergistically with the FT-FD module to induce flowering under LDs, providing a long-sought molecular knob that links developmental aging and photoperiodic flowering.

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.

ABA-dependent control of GIGANTEA signalling enables drought escape via up-regulation of FLOWERING LOCUS T in Arabidopsis thaliana

One strategy deployed by plants to endure water scarcity is to accelerate the transition to flowering adaptively via the drought escape (DE) response. In Arabidopsis thaliana, activation of the DE response requires the photoperiodic response gene GIGANTEA (GI) and the florigen genes FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF). The phytohormone abscisic acid (ABA) is also required for the DE response, by promoting the transcriptional up-regulation of the florigen genes. The mode of interaction between ABA and the photoperiodic genes remains obscure. In this work we use a genetic approach to demonstrate that ABA modulates GI signalling and consequently its ability to activate the florigen genes. We also reveal that the ABA-dependent activation of FT, but not TSF, requires CONSTANS (CO) and that impairing ABA signalling dramatically reduces the expression of florigen genes with little effect on the CO transcript profile. ABA signalling thus has an impact on the core genes of photoperiodic signalling GI and CO by modulating their downstream function and/or activities rather than their transcript accumulation. In addition, we show that as well as promoting flowering, ABA simultaneously represses flowering, independent of the florigen genes. Genetic analysis indicates that the target of the repressive function of ABA is the flowering-promoting gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1), a transcription factor integrating floral cues in the shoot meristem. Our study suggests that variations in ABA signalling provide different developmental information that allows plants to co-ordinate the onset of the reproductive phase according to the available water resources.

FT and florigen long-distance flowering control in plants

The great hunt for florigen, the universal, long distance flowering regulator proposed by Chailakhan in the 1930s, resulted in the discovery a decade ago that FT-like proteins fulfilled the predictions for florigen. They are small (∼175 amino acids), globular, phosphatidylethanolamine-binding (PEBP) proteins, phloem-expressed, graft-transmissible and able to move to the shoot apex to act as potent stimulators of flowering in many plants. Genes that regulate Arabidopsis FT protein movement and some features of Arabidopsis FT protein that make it an effective florigen have recently been identified. Although floral promotion via graft transmission of FT has not been demonstrated in trees, FT-like genes have been successfully applied to reducing the long juvenile (pre-flowering) phase of many trees enabling fast track breeding.

Control of floral transition in the bioenergy crop switchgrass

Switchgrass (Panicum virgatum L.), a perennial warm season bunchgrass native to North America, has been a target in the U.S. as a renewable bioenergy crop because of its ability to produce moderate to high biomass yield on marginal soils. Delaying flowering can increase vegetative biomass production by allowing prolonged growth before switching to the reproductive phase. Despite the identification of flowering time as a biomass trait in switchgrass, the molecular regulatory factors involved in controlling floral transition are poorly understood. Here we identified PvFT1, PvAPL1-3 and PvSL1, 2 as key flowering regulators required from floral transition initiation to development of floral organs. PvFT1 expression in leaves is developmentally regulated peaking at the time of floral transition, and diurnally regulated with peak at approximately 2 h into the dark period. Ectopic expression of PvFT1 in Arabidopsis, Brachypodium and switchgrass led to extremely early flowering, and activation of FT downstream target genes, confirming that it is a strong activator of flowering in switchgrass. Ectopic expression of PvAPL1-3 and PvSL1, 2 in Arabidopsis also activated early flowering with distinct floral organ phenotypes. Our results suggest that switchgrass has conserved flowering pathway regulators similar to Arabidopsis and rice.

Integration of light and metabolic signals for stem cell activation at the shoot apical meristem

A major feature of embryogenesis is the specification of stem cell systems, but in contrast to the situation in most animals, plant stem cells remain quiescent until the postembryonic phase of development. Here, we dissect how light and metabolic signals are integrated to overcome stem cell dormancy at the shoot apical meristem. We show on the one hand that light is able to activate expression of the stem cell inducer WUSCHEL independently of photosynthesis and that this likely involves inter-regional cytokinin signaling. Metabolic signals, on the other hand, are transduced to the meristem through activation of the TARGET OF RAPAMYCIN (TOR) kinase. Surprisingly, TOR is also required for light signal dependent stem cell activation. Thus, the TOR kinase acts as a central integrator of light and metabolic signals and a key regulator of stem cell activation at the shoot apex.

DOI:http://dx.doi.org/10.7554/eLife.17023.001

Plants are able to grow and develop throughout their lives thanks to groups of stem cells at the tips of their shoots and roots, which can constantly divide to produce new cells. Energy captured from sunlight during a process called photosynthesis is the main source of energy for most plants. Therefore, the amount and quality of light in the environment has a big influence on how plants grow and develop. An enzyme called TOR kinase can sense energy levels in animal cells and regulate many processes including growth and cell division. Plants also have a TOR kinase, but it is less clear if it plays the same role in plants, and whether it can respond to light.

Plant stem cells only start to divide after the seed germinates. In shoots, a protein called WUSCHEL is required to maintain stem cells in an active state. Here, Pfeiffer et al. studied how shoot stem cells are activated in response to environmental signals in a plant known as Arabidopsis. The experiments show that light is able to activate the production of WUSCHEL independently of photosynthesis via a signal pathway that depends on TOR kinase. The stem cells do not directly sense light; instead other cells detect the light and relay the information to the stem cells with the help of a hormone called cytokinin.

Further experiments show that information about energy levels in cells is relayed via another signal pathway that also involves the TOR kinase. Therefore, Pfeiffer et al.’s findings suggest that the activation of TOR by light allows plant cells to anticipate how much energy will be available and efficiently tune their growth and development to cope with the environmental conditions. Future challenges are to understand how TOR kinase is regulated by light signals and how this enzyme is able to act on WUSCHEL to trigger stem cell division.

DOI:http://dx.doi.org/10.7554/eLife.17023.002

Apple latent spherical virus vector-induced flowering for shortening the juvenile phase in Japanese gentian and lisianthus plants

Infection by apple latent spherical virus (ALSV) vectors that promote the expression of Arabidopsis thaliana FLOWERING LOCUS T ( AtFT ) or Gentiana triflora GtFT s accelerates flowering in gentian and lisianthus plants. Apple latent spherical virus (ALSV) has isometric virus particles (25 nm in diameter) that contain two ssRNA species (RNA1 and RNA2) and three capsid proteins (Vp25, Vp20, and Vp24). ALSV vectors are used for foreign gene expression and virus-induced gene silencing in a broad range of plant species. Here, we report the infection by ALSV vectors that express FLOWERING LOCUS T (AtFT) from Arabidopsis thaliana or its homolog GtFT1 from Gentiana triflora in three gentian cultivars ('Iwate Yume Aoi' [early flowering], 'Iwate' [medium flowering], and 'Alta' [late flowering]), and two lisianthus cultivars ('Newlination Pink ver. 2' and 'Torukogikyou daburu mikkusu') promotes flowering within 90 days post-inoculation using particle bombardment. Additionally, seedlings from the progeny of the early-flowering plants were tested by tissue blot hybridization, and the results showed that ALSV was not transmitted to the next generation. The promotion of flowering in the family Gentianaceae by ALSV vectors shortened the juvenile phase from 1-3 years to 3-5 months, and thus, it could be considered as a new plant breeding technique in ornamental gentian and lisianthus plants.

Post-translational control of genetic circuits using Potyvirus proteases

Genetic engineering projects often require control over when a protein is degraded. To this end, we use a fusion between a degron and an inactivating peptide that can be added to the N-terminus of a protein. When the corresponding protease is expressed, it cleaves the peptide and the protein is degraded. Three protease:cleavage site pairs from Potyvirus are shown to be orthogonal and active in exposing degrons, releasing inhibitory domains and cleaving polyproteins. This toolbox is applied to the design of genetic circuits as a means to control regulator activity and degradation. First, we demonstrate that a gate can be constructed by constitutively expressing an inactivated repressor and having an input promoter drive the expression of the protease. It is also shown that the proteolytic release of an inhibitory domain can improve the dynamic range of a transcriptional gate (200-fold repression). Next, we design polyproteins containing multiple repressors and show that their cleavage can be used to control multiple outputs. Finally, we demonstrate that the dynamic range of an output can be improved (8-fold to 190-fold) with the addition of a protease-cleaved degron. Thus, controllable proteolysis offers a powerful tool for modulating and expanding the function of synthetic gene circuits.

Photoperiodic and thermosensory pathways interact through CONSTANS to promote flowering at high temperature under short days

Plants detect changes in day length to induce seasonal patterns of flowering. The photoperiodic pathway accelerates the flowering of Arabidopsis thaliana under long days (LDs) whereas it is inactive under short days (SDs), resulting in delayed flowering. This delay is overcome by exposure of plants to high temperature (27°C) under SDs (27°C-SD). Previously, the high-temperature flowering response was proposed to involve either the impaired activity of MADS-box transcription factor (TF) floral repressors or PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) TF-mediated activation of FLOWERING LOCUS T (FT), which encodes the output signal of the photoperiodic pathway. We integrate these observations by studying several PIFs, the MADS-box SHORT VEGETATIVE PHASE (SVP) and the photoperiodic pathway under 27°C-SD. We find that the mRNAs of FT and its paralogue TWIN SISTER OF FT (TSF) are increased at dusk under 27°C-SD compared with 21°C-SD, and that this requires PIF4 and PIF5 as well as CONSTANS (CO), a TF that promotes flowering under LDs. The CO and PIF4 proteins are present at dusk under 27°C-SD, and they physically interact. Although Col-0 plants flower at similar times under 27°C-SD and 21°C-LD the expression level of FT is approximately 10-fold higher under 21°C-LD, suggesting that responsiveness to FT is also increased under 27°C-SD, perhaps as a result of the reduced activity of SVP in the meristem. Accordingly, only svp-41 ft-10 tsf-1 plants flowered at the same time under 21°C-SD and 27°C-SD. Thus, we propose that under non-inductive SDs, elevated temperatures increase the activity and sensitize the response to the photoperiod pathway.

NaKR1 regulates long-distance movement of FLOWERING LOCUS T in Arabidopsis

Flowering plants perceive photoperiodic signals in leaves to generate mobile stimuli required for the induction of flower formation at shoot apices. Although FLOWERING LOCUS T (FT) has been identified as part of the mobile floral stimuli in Arabidopsis thaliana, the mechanisms underlying long-distance movement of FT from leaves to shoot apices remain largely unclear. Here we show that a heavy-metal-associated (HMA) domain-containing protein, SODIUM POTASSIUM ROOT DEFECTIVE 1 (NaKR1), is activated by CONSTANS (CO) under long-day conditions and regulates long-distance movement of FT in Arabidopsis. Loss of function of NaKR1 compromises FT transport to shoot apices through sieve elements, causing late flowering under long-day conditions. NaKR1 and FT share similar expression patterns and subcellular localization, and interact with each other in vivo. Grafting experiments demonstrate that NaKR1 promotes flowering through mediating FT translocation from leaves to shoot apices. Thus, photoperiodic control of floral induction requires NaKR1-mediated long-distance delivery of florigenic signals.

Transcriptomic analysis of Arabidopsis overexpressing flowering locus T driven by a meristem-specific promoter that induces early flowering

Here we analyzed in leaves the effect of FT overexpression driven by meristem-specific KNAT1 gene homolog of Arabidopsis thaliana (Lincoln et al., 1994; Long et al., 1996) on the transcriptomic response during plant development. Our results demonstrated that meristematic FT overexpression generates a phenotype with an early flowering independent of photoperiod when compared with wild type (WT) plants. Arabidopsis FT-overexpressor lines (AtFTOE) did not show significant differences compared with WT lines neither in leaf number nor in rosette diameter up to day 21, when AtFTOE flowered. After this period AtFTOE plants started flower production and no new rosette leaves were produced. Additionally, WT plants continued on vegetative stage up to day 40, producing 12-14 rosette leaves before flowering. Transcriptomic analysis of rosette leaves studied by sequencing Illumina RNA-seq allowed us to determine the differential expression in mature leaf rosette of 3652 genes, being 626 of them up-regulated and 3026 down-regulated. Overexpressed genes related with flowering showed up-regulated transcription factors such as MADS-box that are known as flowering markers in meristem and which overexpression has been related with meristem identity preservation and the transition from vegetative to floral stage. Genes related with sugar transport have shown a higher demand of monosaccharides derived from the hydrolysis of sucrose to glucose and probably fructose, which can also be influenced by reproductive stage of AtFTOE plants.

Natural Variation in Tomato Reveals Differences in the Recognition of AvrPto and AvrPtoB Effectors from Pseudomonas syringae

The Pto protein kinase from Solanum pimpinellifolium interacts with Pseudomonas syringae effectors AvrPto or AvrPtoB to activate effector-triggered immunity. The previously solved crystal structures of the AvrPto-Pto and AvrPtoB-Pto complexes revealed that Pto binds each effector through both a shared and a unique interface. Here we use natural variation in wild species of tomato to further investigate Pto recognition of these two effectors. One species, Solanum chmielewskii, was found to have many accessions that recognize only AvrPtoB. The Pto ortholog from one of these accessions was responsible for recognition of AvrPtoB and it differed from Solanum pimpinellifolium Pto by only 14 amino acids, including two in the AvrPto-specific interface, glutamate-49/glycine-51. Converting these two residues to those in Pto (histidine-49/valine-51) did not restore recognition of AvrPto. Subsequent experiments revealed that a single substitution of a histidine-to-aspartate at position 193 in Pto, which is not near the AvrPto-specific interface, was sufficient for conferring recognition of AvrPto in plant cells. The reciprocal substitution of aspartate-to-histidine-193 in Pto abolished AvrPto recognition, confirming the importance of this residue. Our results reveal new aspects about effector recognition by Pto and demonstrate the value of using natural variation to understand the interaction between resistance proteins and pathogen effectors.

Three FLOWERING LOCUS T-like genes function as potential florigens and mediate photoperiod response in sorghum

Sorghum is a typical short-day (SD) plant and its use in grain or biomass production in temperate regions depends on its flowering time control, but the underlying molecular mechanism of floral transition in sorghum is poorly understood. Here we characterized sorghum FLOWERING LOCUS T (SbFT) genes to establish a molecular road map for mechanistic understanding. Out of 19 PEBP genes, SbFT1, SbFT8 and SbFT10 were identified as potential candidates for encoding florigens using multiple approaches. Phylogenetic analysis revealed that SbFT1 clusters with the rice Hd3a subclade, while SbFT8 and SbFT10 cluster with the maize ZCN8 subclade. These three genes are expressed in the leaf at the floral transition initiation stage, expressed early in grain sorghum genotypes but late in sweet and forage sorghum genotypes, induced by SD treatment in photoperiod-sensitive genotypes, cooperatively repressed by the classical sorghum maturity loci, interact with sorghum 14-3-3 proteins and activate flowering in transgenic Arabidopsis plants, suggesting florigenic potential in sorghum. SD induction of these three genes in sensitive genotypes is fully reversed by 1 wk of long-day treatment, and yet, some aspects of the SD treatment may still make a small contribution to flowering in long days, indicating a complex photoperiod response mediated by SbFT genes.

Light-Mediated Hormonal Regulation of Plant Growth and Development

Light is crucial for plant life, and perception of the light environment dictates plant growth, morphology, and developmental changes. Such adjustments in growth and development in response to light conditions are often established through changes in hormone levels and signaling. This review discusses examples of light-regulated processes throughout a plant's life cycle for which it is known how light signals lead to hormonal regulation. Light acts as an important developmental switch in germination, photomorphogenesis, and transition to flowering, and light cues are essential to ensure light capture through architectural changes during phototropism and the shade avoidance response. In describing well-established links between light perception and hormonal changes, we aim to give insight into the mechanisms that enable plants to thrive in variable light environments.

The mitogen-activated protein kinase phosphatase PHS1 regulates flowering in Arabidopsis thaliana

Arabidopsis PHS1, initially known as an actor of cytoskeleton organization, is a positive regulator of flowering in the photoperiodic and autonomous pathways by modulating both CO and FLC mRNA levels. Protein phosphorylation and dephosphorylation is a major type of post-translational modification, controlling many biological processes. In Arabidopsis thaliana, five genes encoding MAPK phosphatases (MKP)-like proteins have been identified. Among them, PROPYZAMIDE HYPERSENSITIVE 1 (PHS1) encoding a dual-specificity protein tyrosine phosphatase (DsPTP) has been shown to be involved in microtubule organization, germination and ABA-regulated stomatal opening. Here, we demonstrate that PHS1 also regulates flowering under long-day and short-day conditions. Using physiological, genetic and molecular approaches, we have shown that the late flowering phenotype of the knock-out phs1-5 mutant is linked to a higher expression of FLOWERING LOCUS C (FLC). In contrast, a decline of both CONSTANS (CO) and FLOWERING LOCUS T (FT) expression is observed in the knock-out phs1-5 mutant, especially at the end of the light period under long-day conditions when the induction of flowering occurs. We show that this partial loss of sensitivity to photoperiodic induction is independent of FLC. Our results thus indicate that PHS1 plays a dual role in flowering, in the photoperiodic and autonomous pathways, by modulating both CO and FLC mRNA levels. Our work reveals a novel actor in the complex network of the flowering regulation.

Identification, Functional Study, and Promoter Analysis of HbMFT1, a Homolog of MFT from Rubber Tree (Hevea brasiliensis)

A homolog of MOTHER OF FT AND TFL1 (MFT) was isolated from Heveabrasiliensis and its biological function was investigated. Protein multiple sequence alignment and phylogenetic analysis revealed that HbMFT1 conserved critical amino acid residues to distinguish MFT, FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1)-like proteins and showed a closer genetic relationship to the MFT-like group. The accumulation of HbMFT1 was generally detected in various tissues except pericarps, with the highest expression in embryos and relatively higher expression in roots and stems of seedlings, flowering inflorescences, and male and female flowers. HbMFT1 putative promoter analysis showed that tissue-specific, environmental change responsive and hormone-signaling responsive elements were generally present. HbMFT1 was strongly induced under a short-day condition at 28 °C, with the highest expression after the onset of a day. Overexpression of HbMFT1 inhibited seed germination, seedling growth, and flowering in transgenic Arabidopsis. The qRT-PCR further confirmed that APETALA1 (AP1) and FRUITFULL (FUL) were drastically down-regulated in 35S::HbMFT1 plants. A histochemical β-glucuronidase (GUS) assay showed that HbMFT1::GUS activity was mainly detected in stamens and mature seeds coinciding with its original expression and notably induced in rosette leaves and seedlings of transgenic Arabidopsis by exogenous abscisic acid (ABA) due to the presence of ABA cis-elements in HbMFT1 promoter. These results suggested that HbMFT1 was mainly involved in maintenance of seed maturation and stamen development, but negatively controlled germination, growth and development of seedlings and flowering. In addition, the HbMFT1 promoter can be utilized in controlling transgene expression in stamens and seeds of rubber tree or other plant species.

Flowering and trichome development share hormonal and transcription factor regulation

Gibberellins (GAs) and cytokinins (CKs) are plant hormones that act either synergistically or antagonistically during the regulation of different developmental processes. In Arabidopsis thaliana, GAs and CKs overlap in the positive regulation of processes such as the transition from the vegetative to the reproductive phase and the development of epidermal adaxial trichomes. Despite the fact that both developmental processes originate in the rosette leaves, they occur separately in time and space. Here we review how, as genetic and molecular mechanisms are being unraveled, both processes might be closely related. Additionally, this shared genetic network is not only dependent on GA and CK hormone signaling but is also strictly controlled by specific clades of transcription factor families. Some key flowering genes also control other rosette leaf developmental processes such as adaxial trichome formation. Conversely, most of the trichome activator genes, which belong to the MYB, bHLH and C2H2 families, were found to positively control the floral transition. Furthermore, three MADS floral organ identity genes, which are able to convert leaves into floral structures, are also able to induce trichome proliferation in the flower. These data lead us to propose that the spatio-temporal regulation and integration of diverse signals control different developmental processes, such as floral induction and trichome formation, which are intimately connected through similar genetic pathways.

Shoot-to-Root Mobile Transcription Factor HY5 Coordinates Plant Carbon and Nitrogen Acquisition

Coordination of shoot photosynthetic carbon fixation with root inorganic nitrogen uptake optimizes plant performance in a fluctuating environment [1]. However, the molecular basis of this long-distance shoot-root coordination is little understood. Here we show that Arabidopsis ELONGATED HYPOCOTYL5 (HY5), a bZIP transcription factor that regulates growth in response to light [2, 3], is a shoot-to-root mobile signal that mediates light promotion of root growth and nitrate uptake. Shoot-derived HY5 auto-activates root HY5 and also promotes root nitrate uptake by activating NRT2.1, a gene encoding a high-affinity nitrate transporter [4]. In the shoot, HY5 promotes carbon assimilation and translocation, whereas in the root, HY5 activation of NRT2.1 expression and nitrate uptake is potentiated by increased carbon photoassimilate (sucrose) levels. We further show that HY5 function is fluence-rate modulated and enables homeostatic maintenance of carbon-nitrogen balance in different light environments. Thus, mobile HY5 coordinates light-responsive carbon and nitrogen metabolism, and hence shoot and root growth, in a whole-organismal response to ambient light fluctuations.

Detecting N-myristoylation and S-acylation of host and pathogen proteins in plants using click chemistry

BACKGROUND

The plant plasma membrane is a key battleground in the war between plants and their pathogens. Plants detect the presence of pathogens at the plasma membrane using sensor proteins, many of which are targeted to this lipophilic locale by way of fatty acid modifications. Pathogens secrete effector proteins into the plant cell to suppress the plant's defense mechanisms. These effectors are able to access and interfere with the surveillance machinery at the plant plasma membrane by hijacking the host's fatty acylation apparatus. Despite the important involvement of protein fatty acylation in both plant immunity and pathogen virulence mechanisms, relatively little is known about the role of this modification during plant-pathogen interactions. This dearth in our understanding is due largely to the lack of methods to monitor protein fatty acid modifications in the plant cell.

RESULTS

We describe a rapid method to detect two major forms of fatty acylation, N-myristoylation and S-acylation, of candidate proteins using alkyne fatty acid analogs coupled with click chemistry. We applied our approach to confirm and decisively demonstrate that the archetypal pattern recognition receptor FLS2, the well-characterized pathogen effector AvrPto, and one of the best-studied intracellular resistance proteins, Pto, all undergo plant-mediated fatty acylation. In addition to providing a means to readily determine fatty acylation, particularly myristoylation, of candidate proteins, this method is amenable to a variety of expression systems. We demonstrate this using both Arabidopsis protoplasts and stable transgenic Arabidopsis plants and we leverage Agrobacterium-mediated transient expression in Nicotiana benthamiana leaves as a means for high-throughput evaluation of candidate proteins.

CONCLUSIONS

Protein fatty acylation is a targeting tactic employed by both plants and their pathogens. The metabolic labeling approach leveraging alkyne fatty acid analogs and click chemistry described here has the potential to provide mechanistic details of the molecular tactics used at the host plasma membrane in the battle between plants and pathogens.

The Arabidopsis RING Domain Protein BOI Inhibits Flowering via CO-dependent and CO-independent Mechanisms

BOTRYTIS SUSCEPTIBLE1 INTERACTOR (BOI) and its three homologs (BOIs) are RING domain-containing proteins that repress flowering. Here, we investigated how BOIs repress flowering. Genetic analysis of the boiQ quadruple mutant indicates that BOIs repress flowering mainly through FLOWERING LOCUS T (FT). BOIs repress the expression of FT by CONSTANS (CO)-dependent and -independent mechanisms: in the CO-dependent mechanism, BOIs bind to CO, inhibit the targeting of CO to the FT locus, and thus repress the expression of FT; in the CO-independent mechanism, BOIs target the FT locus via a mechanism that requires DELLAs but not CO. This dual repression of FT makes BOIs strong repressors of flowering in both CO-dependent and CO-independent pathways in Arabidopsis thaliana. Our finding that BOIs inhibit CO targeting further suggests that, in addition to modulating CO mRNA expression and CO protein stability, flowering regulation can also modulate the targeting of CO to FT.

Gibberellic acid signaling is required for ambient temperature-mediated induction of flowering in Arabidopsis thaliana

Distinct molecular mechanisms integrate changes in ambient temperature into the genetic pathways that govern flowering time in Arabidopsis thaliana. Temperature-dependent eviction of the histone variant H2A.Z from nucleosomes has been suggested to facilitate the expression of FT by PIF4 at elevated ambient temperatures. Here we show that, in addition to PIF4, PIF3 and PIF5, but not PIF1 and PIF6, can promote flowering when expressed specifically in phloem companion cells (PCC), where they can induce FT and its close paralog, TSF. However, despite their strong potential to promote flowering, genetic analyses suggest that the PIF genes seem to have only a minor role in adjusting flowering in response to photoperiod or high ambient temperature. In addition, loss of PIF function only partially suppressed the early flowering phenotype and FT expression of the arp6 mutant, which is defective in H2A.Z deposition. In contrast, the chemical inhibition of gibberellic acid (GA) biosynthesis resulted in a strong attenuation of early flowering and FT expression in arp6. Furthermore, GA was able to induce flowering at low temperature (15°C) independently of FT, TSF, and the PIF genes, probably directly at the shoot apical meristem. Together, our results suggest that the timing of the floral transition in response to ambient temperature is more complex than previously thought and that GA signaling might play a crucial role in this process.

Expression of flowering locus T2 transgene from Pyrus communis L. delays dormancy and leaf senescence in Malus×domestica Borkh, and causes early flowering in tobacco

Annual and perennial plants represent two different evolutionary strategies based on differential synchronization of their reproductive development. The mobile signal protein FLOWERING LOCUS T (FT) plays a central role in mediating the onset of reproduction in both plant types. Two novel FT-like genes from pear (Pyrus communis)-PcFT1 and PcFT2-were isolated, and their expression profiles were determined for one annual cycle. The effects of PcFT2 on flowering were investigated in annual (tobacco) and perennial (apple) plants by means of grafting and generating transgenic plants. Long-distance graft transmission of PcFT2 in both annual and perennial plants was confirmed using a 35S::PcFT2-YFP construct. Ectopic overexpression of PcFT2 caused early flowering in tobacco but not in apple. Transgenic apples were less sensitive to short-day-induced dormancy, and this phenotype was also observed in wild-type apples grafted onto the transgenic plants. Comparison of PcFT2 protein structure to the paralogous FT proteins from apple and pear showed alterations that could influence protein structure and thus the florigen-activation complex. PcFT2 protein seems to function by promoting flowering as all other FT proteins in the annual plant tobacco while in the perennial plant apple PcFT2 does not promote flowering but delays senescence. This observation may hint to a modified function of FT2 in perennial plants.

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

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

Comprehensive analysis of the flowering genes in Chinese cabbage and examination of evolutionary pattern of CO-like genes in plant kingdom

In plants, flowering is the most important transition from vegetative to reproductive growth. The flowering patterns of monocots and eudicots are distinctly different, but few studies have described the evolutionary patterns of the flowering genes in them. In this study, we analysed the evolutionary pattern, duplication and expression level of these genes. The main results were as follows: (i) characterization of flowering genes in monocots and eudicots, including the identification of family-specific, orthologous and collinear genes; (ii) full characterization of CONSTANS-like genes in Brassica rapa (BraCOL genes), the key flowering genes; (iii) exploration of the evolution of COL genes in plant kingdom and construction of the evolutionary pattern of COL genes; (iv) comparative analysis of CO and FT genes between Brassicaceae and Grass, which identified several family-specific amino acids, and revealed that CO and FT protein structures were similar in B. rapa and Arabidopsis but different in rice; and (v) expression analysis of photoperiod pathway-related genes in B. rapa under different photoperiod treatments by RT-qPCR. This analysis will provide resources for understanding the flowering mechanisms and evolutionary pattern of COL genes. In addition, this genome-wide comparative study of COL genes may also provide clues for evolution of other flowering genes.

Florigens and antiflorigens: a molecular genetic understanding

Florigens, the leaf-derived signals that initiate flowering, have been described as ‘mysterious’, ‘elusive’ and the ‘Holy Grail’ of plant biology. They are synthesized in response to appropriate photoperiods and move through the phloem tissue. It has been proposed that their composition is complex. The evidence that FLOWERING LOCUS T (FT) protein and its paralogue TWIN SISTER OF FT (TSF) act as florigen, or represent at least part of it, in diverse plant species has attracted considerable attention. In Arabidopsis thaliana, inductive photoperiodic conditions perceived in the leaf lead to stabilization of CONSTANS protein, which induces FT and TSF transcription. When they have been translated in the phloem companion cells, FT and TSF enter the phloem stream and are conveyed to the shoot apical meristem, where they act together with FLOWERING LOCUS D to activate transcription of floral meristem identity genes, resulting in floral initiation. At least part of this model is conserved, with some variations in several species. In addition to florigen(s), a systemic floral inhibitor or antiflorigen contributes to floral initiation. This chapter provides an overview of the different molecules that have been demonstrated to have florigenic or antiflorigenic functions in plants, and suggests possible directions for future research.

The dynamics of FLOWERING LOCUS T expression encodes long-day information

Long days repeatedly enhance the expression of the FLOWERING LOCUS T (FT) gene during the evening and early night. This signal induces flowering despite low FT expression the rest of the day. To investigate whether this temporal behaviour transmits information, plants of Arabidopsis thaliana were exposed to different day-night cycles, including combinations that induced FT expression out of normal hours. Flowering time best correlated with the integral of FT expression over several days, corrected for a higher evening and early night sensitivity to FT. We generated a system to induce FT expression in a leaf removed 8-12 h later. The expression of flowering genes in the apex and flowering required cycles of induction repeated over several days. Evening and early night FT induction was the most effective. The temporal pattern of FT expression encodes information that discriminates long days from other inputs.

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.

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

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

Genome-wide expression analysis of soybean NF-Y genes reveals potential function in development and drought response

Nuclear factor-Y (NF-Y), a heterotrimeric transcription factor, is composed of NF-YA, NF-YB and NF-YC proteins. In plants, there are usually more than 10 genes for each family and their members have been identified to be key regulators in many developmental and physiological processes controlling gametogenesis, embryogenesis, nodule development, seed development, abscisic acid (ABA) signaling, flowering time, primary root elongation, blue light responses, endoplasmic reticulum (ER) stress response and drought tolerance. Taking the advantages of the recent soybean genome draft and information on functional characterizations of nuclear factor Y (NF-Y) transcription factor family in plants, we identified 21 GmNF-YA, 32 GmNF-YB, and 15 GmNF-YC genes in the soybean (Glycine max) genome. Phylogenetic analyses show that soybean's proteins share strong homology to Arabidopsis and many of them are closely related to functionally characterized NF-Y in plants. Expression analysis in various tissues of flower, leaf, root, seeds of different developmental stages, root hairs under rhizobium inoculation, and drought-treated roots and leaves revealed that certain groups of soybean NF-Y are likely involved in specific developmental and stress responses. This study provides extensive evaluation of the soybean NF-Y family and is particularly useful for further functional characterization of GmNF-Y proteins in seed development, nodulation and drought adaptation of soybean.

THO2, a core member of the THO/TREX complex, is required for microRNA production in Arabidopsis

The THO/TREX complex mediates transport of nascent mRNAs from the nucleus towards the cytoplasm in animals, and has a role in small interfering RNA-dependent processes in plants. Here we describe five mutant alleles of Arabidopsis thaliana THO2, which encodes a core subunit of the plant THO/TREX complex. tho2 mutants present strong developmental defects resembling those in plants compromised in microRNA (miRNA) activity. In agreement, not only were the levels of siRNAs reduced in tho2 mutants, but also those of mature miRNAs. As a consequence, a feedback mechanism is triggered, increasing the amount of miRNA precursors, and finally causing accumulation of miRNA-targeted mRNAs. Yeast two-hybrid experiments and confocal microscopy showed that THO2 does not appear to interact with any of the known miRNA biogenesis components, but rather with the splicing machinery, implying an indirect role of THO2 in small RNA biogenesis. Using an RNA immunoprecipitation approach, we found that THO2 interacts with miRNA precursors, and that tho2 mutants fail to recruit such precursors into the miRNA-processing complex, explaining the reduction in miRNA production in this mutant background. We also detected alterations in the splicing pattern of genes encoding serine/arginine-rich proteins in tho2 mutants, supporting a previously unappreciated role of the THO/TREX complex in alternative splicing.

Molecular cloning and functional analysis of the FLOWERING LOCUS T (FT) homolog GhFT1 from Gossypium hirsutum

FLOWERING LOCUS T (FT) encodes a member of the phosphatidylethanolamine-binding protein (PEBP) family that functions as the mobile floral signal, playing an important role in regulating the floral transition in angiosperms. We isolated an FT-homolog (GhFT1) from Gossypium hirsutum L. cultivar, Xinluzao 33 GhFT1 was predominantly expressed in stamens and sepals, and had a relatively higher expression level during the initiation stage of fiber development. GhFT1 mRNA displayed diurnal oscillations in both long-day and short-day condition, suggesting that the expression of this gene may be under the control of the circadian clock. Subcellular analysis revealed that GhFT1 protein located in the cytoplasm and nucleus. Ectopic expression of GhFT1 in transgenic arabidopsis plants resulted in early flowering compared with wild-type plants. In addition, ectopic expression of GhFT1 in arabidopsis ft-10 mutants partially rescued the extremely late flowering phenotype. Finally, several flowering related genes functioning downstream of AtFT were highly upregulated in the 35S::GhFT1 transgenic arabidopsis plants. In summary, GhFT1 is an FT-homologous gene in cotton that regulates flower transition similar to its orthologs in other plant species and thus it may be a candidate target for promoting early maturation in cotton breeding.

Evolution of CONSTANS Regulation and Function after Gene Duplication Produced a Photoperiodic Flowering Switch in the Brassicaceae

Environmental control of flowering allows plant reproduction to occur under optimal conditions and facilitates adaptation to different locations. At high latitude, flowering of many plants is controlled by seasonal changes in day length. The photoperiodic flowering pathway confers this response in the Brassicaceae, which colonized temperate latitudes after divergence from the Cleomaceae, their subtropical sister family. The CONSTANS (CO) transcription factor of Arabidopsis thaliana, a member of the Brassicaceae, is central to the photoperiodic flowering response and shows characteristic patterns of transcription required for day-length sensing. CO is believed to be widely conserved among flowering plants; however, we show that it arose after gene duplication at the root of the Brassicaceae followed by divergence of transcriptional regulation and protein function. CO has two close homologs, CONSTANS-LIKE1 (COL1) and COL2, which are related to CO by tandem duplication and whole-genome duplication, respectively. The single CO homolog present in the Cleomaceae shows transcriptional and functional features similar to those of COL1 and COL2, suggesting that these were ancestral. We detect cis-regulatory and codon changes characteristic of CO and use transgenic assays to demonstrate their significance in the day-length-dependent activation of the CO target gene FLOWERING LOCUS T. Thus, the function of CO as a potent photoperiodic flowering switch evolved in the Brassicaceae after gene duplication. The origin of CO may have contributed to the range expansion of the Brassicaceae and suggests that in other families CO genes involved in photoperiodic flowering arose by convergent evolution.

Coordinated Expression of FLOWERING LOCUS T and DORMANCY ASSOCIATED MADS-BOX-Like Genes in Leafy Spurge

Leafy spurge (Euphorbia esula L.) is a noxious perennial weed that produces underground adventitious buds, which are crucial for generating new vegetative shoots following periods of freezing temperatures or exposure to various control measures. It is also capable of flowering and producing seeds, but requires vernalization in some cases. DORMANCY ASSOCIATED MADS-BOX (DAM) genes have been proposed to play a direct role in the transition to winter-induced dormancy and maintenance through regulation of the FLOWERING LOCUS T (FT) gene, which also is likely involved in the vernalization process. To explore the regulation of FT and DAM during dormancy transitions in leafy spurge, the transcript accumulation of two previously cloned DAM splice variants and two different previously cloned FT genes was characterized. Under long-photoperiods (16 h light), both DAM and FT transcripts accumulate in a diurnal manner. Tissue specific expression patterns indicated the tissues with high DAM expression had low FT expression and vice versa. DAM expression is detected in leaves, stems, shoot tips, and crown buds. FT transcripts were detected mainly in leaves and flowers. Under dormancy inducing conditions, DAM and FT genes had an inverse expression pattern. Additionally, chromatin immunoprecipitation assays were performed using DAM-like protein specific antibodies to demonstrate that DAM or related proteins likely bind to cryptic and/or conserved CArG boxes in the promoter regions of FT genes isolated from endodormant crown buds. These results are consistent with the hypothesis that DAM proteins play a crucial role in leafy spurge dormancy transition and maintenance, potentially by negatively regulating the expression of FT.

Enabling photoperiodic control of flowering by timely chromatin silencing of the florigen gene

Many plants synchronize their flowering times with changing seasons to maximize reproductive success. A key seasonal cue is the change in day length (photoperiod), that induces the production of a systemic flowering signaling molecule called florigen. A major florigen component is FLOWERING LOCUS T (FT) or its orthologs. In the long-day plant Arabidopsis thaliana, FT expression is well known to be activated by the photoperiod pathway output specifically near dusk in long days; however, underappreciated is the importance of FT silencing at other times of the day, in enabling Arabidopsis to respond only to long days in flowering. We have recently reported that a plant-specific chromatin-silencing complex called EMF1c represses FT expression at times other than around dusk in long days to prevent its temporal ectopic expression from "spoiling" the long-day floral induction in Arabidopsis. Here I further discuss in other day-length sensitive plants the potential involvement of a chromatin mechanism similar to the Arabidopsis EMF1c-mediated silencing, in repressing the expression of FT orthologs to enable diverse photoperiodic control of flowering.

The Cloning and Functional Characterization of Peach CONSTANS and FLOWERING LOCUS T Homologous Genes PpCO and PpFT

Flowering is an essential stage of plant growth and development. The successful transition to flowering not only ensures the completion of plant life cycles, it also serves as the basis for the production of economically important seeds and fruits. CONSTANS (CO) and FLOWERING LOCUS T (FT) are two genes playing critical roles in flowering time control in Arabidopsis. Through homology-based cloning and rapid-amplifications of cDNA ends (RACE), we obtained full-lengths cDNA sequences of Prunus persica CO (PpCO) and Prunus persica FT (PpFT) from peach (Prunus persica (L.) Batsch) and investigated their functions in flowering time regulation. PpCO and PpFT showed high homologies to Arabidopsis CO and FT at DNA, mRNA and protein levels. We showed that PpCO and PpFT were nucleus-localized and both showed transcriptional activation activities in yeast cells, consistent with their potential roles as transcription activators. Moreover, we established that the over-expression of PpCO could restore the late flowering phenotype of the Arabidopsis co-2 mutant, and the late flowering defect of the Arabidopsis ft-1 mutant can be rescued by the over-expression of PpFT, suggesting functional conservations of CO and FT genes in peach and Arabidopsis. Our results suggest that PpCO and PpFT are homologous genes of CO and FT in peach and they may function in regulating plant flowering time.

Floral induction and flower formation--the role and potential applications of miRNAs

The multiple regulatory pathways controlling flowering and flower development are varied and complex, and they require tight control of gene expression and protein levels. MicroRNAs (miRNAs) act at both the transcriptional and post-transcriptional level to regulate key genes involved in flowering-related processes such as the juvenile-adult transition, the induction of floral competence and flower development. Many different miRNA families are involved in these processes and their roles are summarized in this review, along with potential biotechnological applications for miRNAs in controlling processes related to flowering and flower development.

Germline-transmitted genome editing in Arabidopsis thaliana Using TAL-effector-nucleases

Transcription activator-like effector nucleases (TALENs) are custom-made bi-partite endonucleases that have recently been developed and applied for genome engineering in a wide variety of organisms. However, they have been only scarcely used in plants, especially for germline-modification. Here we report the efficient creation of small, germline-transmitted deletions in Arabidopsis thaliana via TALENs that were delivered by stably integrated transgenes. Using meristem specific promoters to drive expression of two TALEN arms directed at the CLV3 coding sequence, we observed very high phenotype frequencies in the T2 generation. In some instances, full CLV3 loss-of-function was already observed in the T1 generation, suggesting that transgenic delivery of TALENs can cause highly efficient genome modification. In contrast, constitutive TALEN expression in the shoot apical meristem (SAM) did not cause additional phenotypes and genome re-sequencing confirmed little off-target effects, demonstrating exquisite target specificity.

A quantitative and dynamic model of the Arabidopsis flowering time gene regulatory network

Various environmental signals integrate into a network of floral regulatory genes leading to the final decision on when to flower. Although a wealth of qualitative knowledge is available on how flowering time genes regulate each other, only a few studies incorporated this knowledge into predictive models. Such models are invaluable as they enable to investigate how various types of inputs are combined to give a quantitative readout. To investigate the effect of gene expression disturbances on flowering time, we developed a dynamic model for the regulation of flowering time in Arabidopsis thaliana. Model parameters were estimated based on expression time-courses for relevant genes, and a consistent set of flowering times for plants of various genetic backgrounds. Validation was performed by predicting changes in expression level in mutant backgrounds and comparing these predictions with independent expression data, and by comparison of predicted and experimental flowering times for several double mutants. Remarkably, the model predicts that a disturbance in a particular gene has not necessarily the largest impact on directly connected genes. For example, the model predicts that SUPPRESSOR OF OVEREXPRESSION OF CONSTANS (SOC1) mutation has a larger impact on APETALA1 (AP1), which is not directly regulated by SOC1, compared to its effect on LEAFY (LFY) which is under direct control of SOC1. This was confirmed by expression data. Another model prediction involves the importance of cooperativity in the regulation of APETALA1 (AP1) by LFY, a prediction supported by experimental evidence. Concluding, our model for flowering time gene regulation enables to address how different quantitative inputs are combined into one quantitative output, flowering time.

The arabidopsis DNA polymerase δ has a role in the deposition of transcriptionally active epigenetic marks, development and flowering

DNA replication is a key process in living organisms. DNA polymerase α (Polα) initiates strand synthesis, which is performed by Polε and Polδ in leading and lagging strands, respectively. Whereas loss of DNA polymerase activity is incompatible with life, viable mutants of Polα and Polε were isolated, allowing the identification of their functions beyond DNA replication. In contrast, no viable mutants in the Polδ polymerase-domain were reported in multicellular organisms. Here we identify such a mutant which is also thermosensitive. Mutant plants were unable to complete development at 28°C, looked normal at 18°C, but displayed increased expression of DNA replication-stress marker genes, homologous recombination and lysine 4 histone 3 trimethylation at the SEPALLATA3 (SEP3) locus at 24°C, which correlated with ectopic expression of SEP3. Surprisingly, high expression of SEP3 in vascular tissue promoted FLOWERING LOCUS T (FT) expression, forming a positive feedback loop with SEP3 and leading to early flowering and curly leaves phenotypes. These results strongly suggest that the DNA polymerase δ is required for the proper establishment of transcriptionally active epigenetic marks and that its failure might affect development by affecting the epigenetic control of master genes.

Three DNA polymerases replicate DNA in Eukaryotes. DNA polymerase α (Polα) initiates strand synthesis, which is performed by Polε and Polδ in leading and lagging strands, respectively. Not only the information encoded in the DNA, but also the inheritance of chromatin states is essential during development. Loss of function mutants in DNA polymerases lead to lethal phenotypes. Hence, hypomorphic alleles are necessary to study their roles beyond DNA replication. Here we identify a thermosensitive mutant of the Polδ in the model plant Arabidopsis thaliana, which bears an aminoacid substitution in the polymerase-domain. The mutants were essentially normal at 18°C but arrested development at 28°C. Interestingly, at 24°C we were able to study the roles of Polδ in epigenetic inheritance and plant development. We observed a tight connection between DNA replication stress and an increase the deposition of transcriptionally active chromatin marks in the SEPALLATA3 (SEP3) locus. Finally, we tested by genetic means that the ectopic expression of SEP3 was indeed the cause of early flowering and the leaf phenotypes by promoting the expression of FLOWERING LOCUS T (FT). These results link Polδ activity to the proper establishment of transcriptionally active epigenetic marks, which then impact the development of multicellular organisms.

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.

Calcium-dependent protein kinases responsible for the phosphorylation of a bZIP transcription factor FD crucial for the florigen complex formation

Appropriate timing of flowering is critical for reproductive success and necessarily involves complex genetic regulatory networks. A mobile floral signal, called florigen, is a key molecule in this process, and flowering locus T (FT) protein is its major component in Arabidopsis. FT is produced in leaves, but promotes the floral transition in the shoot apex, where it forms a complex with a basic region/leucine-zipper (bZIP) transcription factor, FD. Formation of the florigen complex depends on the supposed phosphorylation of FD; hitherto, however, the responsible protein kinase(s) have not been identified. In this study, we prepared protein extracts from shoot apices of plants around the floral transition, and detected a protein kinase activity that phosphorylates a threonine residue at position 282 of FD (FD T282), which is a crucial residue for the complex formation with FT via 14-3-3. The kinase activity was calcium-dependent. Subsequent biochemical, cellular, and genetic analyses showed that three calcium-dependent protein kinases (CDPKs) efficiently phosphorylate FD T282. Two of them (CPK6 and CPK33) are expressed in shoot apical meristem and directly interact with FD, suggesting they have redundant functions. The loss of function of one CDPK (CPK33) resulted in a weak but significant late-flowering phenotype.