Florigen Research Articles

Shoot-Silicon-Signal protein to regulate root silicon uptake in rice

Plants accumulate silicon to protect them from biotic and abiotic stresses. Especially in rice (Oryza sativa), a typical Si-accumulator, tremendous Si accumulation is indispensable for healthy growth and productivity. Here, we report a shoot-expressed signaling protein, Shoot-Silicon-Signal (SSS), an exceptional homolog of the flowering hormone “florigen” differentiated in Poaceae. SSS transcript is only detected in the shoot, whereas the SSS protein is also detected in the root and phloem sap. When Si is supplied from the root, the SSS transcript rapidly decreases, and then the SSS protein disappears. In sss mutants, root Si uptake and expression of Si transporters are decreased to a basal level regardless of the Si supply. The grain yield of the mutants is decreased to 1/3 due to insufficient Si accumulation. Thus, SSS is a key phloem-mobile protein for integrating root Si uptake and shoot Si accumulation underlying the terrestrial adaptation strategy of grasses.

Analysis of the 2OG‐Fe (II) oxygenase family reveals new insights associated with flowering regulation in Saccharum spontaneum

AbstractPlant flowering is a crucial phenomenon affecting crop yield and crossbreeding. Saccharum spontaneum is the closest wild relative sugarcane species widely used in sugarcane genetic improvement. However, the molecular interactions among the components of the flowering regulatory network of S. spontaneum are still unclear. In this study, we conducted a transcriptome sequencing approach to analyze the expressed genes on four developmental stages of panicle samples in S. spontaneum. The weighted gene co‐expression network analysis (WGCNA) results suggested that genes from the MEred (p = 3 × 10−4) module were significantly enriched in pathways of ABC transporters, photosynthesis, and circadian rhythm‐plant. Thus, 23 genes associated with flower development‐related genes were screened, including 7 AGL24, 1 FLC, 3 VIN3, 3 GA20ox, and 9 ELF3. In the gibberellic acid (GA) synthesis pathway, GA20ox, GA3ox, and GA2ox were the three enzymes that catalyzed later reactions of GA biosynthesis and belong to the 2‐oxoglutarate Fe (II) oxygenase (2OG) superfamily. Flavonoid synthesis was promoted by 2‐oxoglutarate Fe (II) oxygenase genes, a flowering hormone, which catalyzed and oxidized naringenin to dihydrokaempferol, an important precursor of florigen. Genome‐wide analysis revealed that 122 Ss2OG genes were identified in S. spontaneum, of which 99 genes were enriched in the panicle transcriptome. Furthermore, a quantitative reverse transcription polymerase chain reaction analysis showed that Ss2OG‐FeII_Oxy90, Ss2OG‐FeII_Oxy47, and Ss2OG‐FeII_Oxy89 could introduce the flower development, while Ss2OG‐FeII_Oxy34, Ss2OG‐FeII_Oxy37, and Ss2OG‐FeII_Oxy50 inhibited it. These findings provide new clues and resources for exploring the molecular mechanism of flowering regulation in S. spontaneum and promoting sugarcane breeding.

The effect of phytohormones on the flowering of plants

During the development of angiosperms, one of the most critical stages of plants is the transition from vegetative to reproductive stage, and successfully producing seeds is necessary. Plants have developed a complex signaling pathway to recognize and combine endogenous and environmental signals. Plant growth regulators (PGRs) play a role in regulating flower growth on shoots. Physiological and biochemical processes work together to differentiate and produce flower buds. The impact of PGRs on floral bud differentiation has been the subject of several publications in recent years. In addition, the dynamic variations in gibberellin (GA), auxin, and cytokinin levels in buds and the hormonal-related signatures in gene regulatory networks indicate a crucial function for these hormones during floral bud development in plants. Especially the flowering hormone GA has a key role in regulating the activities related to flowering genes as well as controlling the activity of the DELLA protein. Abscisic acid (ABA) and ethylene (ET) have an inhibitory role in flowering but in some cases stimulate flowering depending on environmental conditions. This study aims to understand the regulation of phytohormones on flowering of plants and its effects on plant development during the flowering stage.

Transposon insertions within alleles of BnaFT.A2 are associated with seasonal crop type in rapeseed.

We identified two new transposon insertions within the promoter of BnaFT.A2 in addition to an existing 288bp MITE within the second intron. Each insertion event corresponds to a distinct BnaFT.A2 haplotype and is closely associated with established crop seasonal ecotypes. Florigen, encoded by FLOWERING LOCUS T (FT), plays key roles not only as a flowering hormone, but also a universal growth factor affecting several aspects of plant architecture. In rapeseed, BnaFT.A2 has been revealed as one of the major loci associated with flowering time and different ecotypes. However, it is unclear how allelic variations of BnaFT.A2 affect its function in flowering time regulation and beyond. In this study, we confirmed an existing 288bp miniature inverted-repeat transposable element (MITE) insertion within the second intron and identified two new insertions within the promoter of BnaFT.A2-a 3971bp CACTA and a 1079bp Helitron. Each insertion event corresponds to a distinct BnaFT.A2 haplotype and is closely associated with established crop seasonal ecotypes. These alleles have similar tissue-specific expression patterns but discrete transcriptional patterns tightly associated with rapeseed flowering time and ecotype. RNAi lines and mutants of BnaFT.A2 flowered significantly later than controls. Differentially expressed genes (DEGs), identified in transcriptomic profiling of seedling leaves from two loss-of-function mutants (Bnaft.a2-L1 and Bnaft.a2-L2) compared with controls, indicated significant enrichment for hormone metabolic genes and roles related to plant cell wall synthesis and photosynthesis. Plants with loss-of-function BnaFT.A2 had smaller leaves and lower net photosynthetic rate compared to controls. These findings not only further clarify the genetic basis of flowering time variation and ecotype formation in B. napus, but also provide an additional toolbox for genetic improvement of seasonal adaptation and production.

Comprehensive Effects of Flowering Locus T-Mediated Stem Growth in Tobacco.

In flowering plants, Flowering locus T (FT) encodes a major florigen. It is a key flowering hormone in controlling flowering time and has a wide range of effects on plant development. Although the mechanism by which FT promotes flowering is currently clearly understood, comprehensive effects of the FT gene on plant growth have not been evaluated. Therefore, the effects of FT on vegetative growth need to be explored for a complete understanding of the molecular functions of the FT gene. In this study, the Jatropha curcas L. FT gene was overexpressed in tobacco (JcFTOE) in order to discover multiple aspects and related mechanisms of how the FT gene affects plant development. In JcFTOE plants, root, stem, and leaf development was strongly affected. Stem tissues were selected for further transcriptome analysis. In JcFTOE plants, stem growth was affected because of changes in the nucleus, cytoplasm, and cell wall. In the nucleus of JcFTOE plants, the primary effect was to weaken all aspects of DNA replication, which ultimately affected the cell cycle and cell division. The number of stem cells decreased significantly in JcFTOE plants, which decreased the thickness and height of tobacco stems. In the cell wall of JcFTOE plants, hemicellulose and cellulose contents increased, with the increase in hemicellulose associated with up-regulation of xylan synthase-related genes expression. In the cytoplasm of JcFTOE plants, the primary effects were on biogenesis of ribonucleoprotein complexes, photosynthesis, carbohydrate biosynthesis, and the cytoskeleton. In addition, in the cytoplasm of JcFTOE plants, there were changes in certain factors of the core oscillator, expression of many light-harvesting chlorophyll a/b binding proteins was down-regulated, and expression of fructose 1,6-bisphosphatase genes was up-regulated to increase starch content in tobacco stems. Changes in the xylem and phloem of JcFTOE plants were also identified, and in particular, xylem development was affected by significant increases in expression of irregular xylem genes.

Microstructural observation on pistil abortion of ‘Li Guang’ apricot and transcriptome reveal the mechanism of endogenous hormones involved in pistil abortion

‘Li Guang’ apricot, a famous local variety, originated in Dunhuang city, Gansu Province, China. The fruit quality of 'Li Guang' apricot is excellent and It will generate greater financial returns. However, due to the change of natural environment and planting structure, pistil abortion has become an important factor affecting fruit setting rate, yield and quality. It seriously affects the economic benefits of this geographical indication product.The distribution and regulation of hormones play an important role in signal molecules of flower abortion. In order to elucidate the key molecular mechanism of apricot flower hormone abortion. Using normal and abortive buds as materials, the pistil abortion of apricot was observed, and the genes related to flowering regulation were identified by RNA sequencing. As compared with normal flowers, the pistil appearance of abortive flower is normal, but the ovary development is abnormal. Microstructure showed that the pollen grains of abortive flowers were decreased sharply, the ovaries shrunk and the ovule primordia developed stagnately. The hormone levels of ZR, IAA and CTK in abortive flowers decreased significantly. But the content of ABA and GA was significantly higher than normal flowers. Through RNA-Seq, it revealed that AUX1, TIR1, ARF, GH3 and SAUR, vital genes displayed identical differential expression profiles to auxin transduction pathway, and ABF, SnRK2, PP2C to abscisic acid, JAZ, MYC2 to jasmonic acid. Mutations in ARF, which encodes auxin response factor, can lead to serious defects in pistils. ARF promotes ovary formation mainly by inhibiting the expression of SPT in this domain. AG regulates early pistil development through CTK-AUXIN, and directly or indirectly affects these hormone pathways at different levels. Once the pistil begins to develop, SPT and ind connect CTK and AUXIN to realize the development of pistil structure. Other factors affecting hormone transport also showed apical and basal hyperplasia and ovary development.

Green revolution to grain revolution: Florigen in the frontiers

Burgeoning human population dents, globally, the brimming buffer stock as well as gain in food grain production. However, an imminent global starvation was averted through precise scientific intervention and pragmatic policy changes in the 1960s and was eulogized as the “Green Revolution”. Miracle rice and wheat obtained through morphometric changes in the ideotype of these two crops yielded bumper harvest that nucleated in Asia and translated into Latin America. The altered agronomic traits in these two crops were the result of tinkering with the phyto-hormone “Gibberellin’. Recently, another plant hormone ‘Cytokinin’ has gained prominence for its involvement in the grain revolution in rice and other field crops. Suo moto homeostasis of CK by the cytokinin oxidase enzyme governs the cardinal shoot apical meristem that produces new flowering primordia thereby enhancing grain number. Similarly, the flowering hormone ‘Florigen’ impacts sympodia formation, flowering, and fruit production in tomato. The role of heterozygosity induced heterosis by florigen in revolutionizing tomato production and cellular homeostasis of CK by CK oxidising enzyme (CKX) in enhancing rice production has been path-breaking. This review highlights role of phytohormones in grain revolution and crop specific fine-tuning of gibberellins, cytokinins and florigen to accomplish maximum yield potential in field crops.

The flowering hormone florigen accelerates secondary cell wall biogenesis to harmonize vascular maturation with reproductive development

Florigen, a proteinaceous hormone, functions as a universal long-range promoter of flowering and concurrently as a generic growth-attenuating hormone across leaf and stem meristems. In flowering plants, the transition from the vegetative phase to the reproductive phase entails the orchestration of new growth coordinates and a global redistribution of resources, signals, and mechanical loads among organs. However, the ultimate cellular processes governing the adaptation of the shoot system to reproduction remain unknown. We hypothesized that if the mechanism for floral induction is universal, then the cellular metabolic mechanisms underlying the conditioning of the shoot system for reproduction would also be universal and may be best regulated by florigen itself. To understand the cellular basis for the vegetative functions of florigen, we explored the radial expansion of tomato stems. RNA-Seq and complementary genetic and histological studies revealed that florigen of endogenous, mobile, or induced origins accelerates the transcription network navigating secondary cell wall biogenesis as a unit, promoting vascular maturation and thereby adapting the shoot system to the developmental needs of the ensuing reproductive phase it had originally set into motion. We then demonstrated that a remarkably stable and broadly distributed florigen promotes MADS and MIF genes, which in turn regulate the rate of vascular maturation and radial expansion of stems irrespective of flowering or florigen level. The dual acceleration of flowering and vascular maturation by florigen provides a paradigm for coordinated regulation of independent global developmental programs.

Overexpression of RICE FLOWERING LOCUS T 1 (RFT1) Induces Extremely Early Flowering in Rice.

RICE FLOWERING LOCUS T 1 (RFT1) is a major florigen that functions to induce reproductive development in the shoot apical meristem (SAM). To further our study of RFT1, we overexpressed the gene and examined the expression patterns of major regulatory genes during floral transition and inflorescence development. Overexpression induced extremely early flowering in the transgenics, and a majority of those calli directly formed spikelets with a few spikelets, thus bypassing normal vegetative development. FRUITFULL (FUL)-clade genes OsMADS14, OsMADS15, and OsMADS18 were highly induced in the RFT1-expressing meristems. OsMADS34 was also induced in the meristems. This indicated that RFT1 promotes the expression of major regulatory genes that are important for inflorescence development. RFT1 overexpression also induced SEPALLATA (SEP)-clade genes OsMADS1, OsMADS5, and OsMADS7 in the greening calli before floral transition occurred. This suggested their possible roles at the early reproductive stages. We found it interesting that expression of OsFD1 as well as OsFD2 and OsFD3 was strongly increased in the RFT1-expressing calli and spikelets. At a low frequency, those calli produced plants with a few leaves that generated a panicle with a small number of spikelets. In the transgenic leaves, the FUL-clade genes and OsMADS34 were induced, but SEP-clade gene expression was not increased. This indicated that OsMADS14, OsMADS15, OsMADS18, and OsMADS34 act immediately downstream of RFT1.

Histone Deacetylase 701 (HDT701) Induces Flowering in Rice by Modulating Expression of OsIDS1.

Rice is a facultative short-day (SD) plant in which flowering is induced under SD conditions or by other environmental factors and internal genetic programs. Overexpression of Histone Deacetylase 701 (HDT701) accelerates flowering in hybrid rice. In this study, mutants defective in HDT701 flowered late under both SD and long-day conditions. Expression levels of florigens Heading date 3a (Hd3a) and Rice Flowering Locus T1 (RFT1), and their immediate upstream floral activator Early heading date 1 (Ehd1), were significantly decreased in the hdt701 mutants, indicating that HDT701 functions upstream of Ehd1 in controlling flowering time. Transcript levels of OsINDETERMINATE SPIKELET 1 (OsIDS1), an upstream repressor of Ehd1, were significantly increased in the mutants while those of OsGI and Hd1 were reduced. Chromatin-immunoprecipitation assays revealed that HDT701 directly binds to the promoter region of OsIDS1. These results suggest that HDT701 induces flowering by suppressing OsIDS1.

Identification of the Regulatory Region Responsible for Vascular Tissue-Specific Expression in the Rice Hd3a Promoter.

Flowering time is determined by florigens. These genes include, Heading date 3a (Hd3a) and Rice FT 1 (RFT1) in rice, which are specifically expressed in the vascular tissues of leaves at the floral transition stage. To study the cis-regulatory elements present in the promoter region of Hd3a, we generated transgenic plants carrying the 1.75-kb promoter fragment of Hd3a that was fused to the β-glucuronidase (GUS) reporter gene. Plants expressing this construct conferred a vascular cell-specific expression pattern for the reporter gene. However, GUS was expressed in leaves at all developmental stages, including the early seedling stage when Hd3a was not detected. Furthermore, the reporter was expressed in roots at all stages. This suggests that the 1.75-kb region lackings cis-elements that regulate leaf-specific expression at the appropriate developmental stages. Deletion analyses of the promoter region indicated that regulatory elements determining vascular cell-specific expression are present in the 200-bp region between -245 bp and -45 bp from the transcription initiation site. By transforming the Hd3a–GUS construct to rice cultivar ‘Taichung 65’ which is defective in Ehd1, we observed that Ehd1 is the major regulatory element that controls Hd3a promoter activity.

Synthesis and evaluation of locostatin-based chemical probes towards PEBP-proteins

Phosphatidyl ethanolamine-binding proteins (PEBPs) are implicated in various critical physiological processes in all eukaryotes. Among them is Flowering Locus T (FT), the protein recently discovered as the vital flowering hormone in plants. Small molecule inhibitors and activators of FT could provide control over plant flowering and are therefore an interesting target for industrial agriculture. No small molecule inhibitors or activators are known for FT, but for a structurally similar PEBP, RKIP, an inhibitor called locostatin has been reported to covalently bind in the RKIP ligand binding pocket. Herein, we report the synthesis of novel locostatin-based chemical PEBP probes and evaluate their ability and selectivity towards the binding of FT and RKIP.

IDENTIFICATION OF BULBING HORMONE GENES IN ONION (Allium cepa)

<p>Initiation of onion (Allium cepa L.) bulb formation seems to require “bulbing hormone” which has been considered to be produced in leaf blades in response to the stimulus of long day conditions. But “bulbing hormone” is not yet identified. Previous study revealed a protein called Flowering locusT (FT) as flowering hormone, florigen. FT act for flowering by change on the day length on the higher plants. Objective of this study is identify”bulbing hormone” in onion plants. Method used in this study are cloning gene and gene expression analysis of the FT in onion plants.Full length of cDNA was cloned by the degenerate PCR and 5’­and 3’-RACE method.As a result, six kinds of full length cDNA clones for FThomologs in onion plants were obtained.These genes were named AcFT1 to6. By expression analysis of these genes, AcFT4, 5 and 6,expression increased as it got closer to a condition in long days in association with the bulbing of onion. Furthermore, in order to investigate the functions of these genes, we optimize transformation methods for onion plants. Medium containing 2,4-Dand kinetin showed high efficient plant regeneration from seed-derived callus of onion. Medium containing 2,4-D and kinetin as plant growth regulators is effective for induction of shoot-inducible callus, and advance shoots were developed from the callus on the shoot induction medium which contained thidiazuron, benzyl adenine or trans-zeatin as cytokinins. In conclusion, bulbing hormone in onion plants were possibly gene AcFT4, 5,6.</p><p><br /><strong>Keywords</strong> : Allium cepa L, bulbing hormone gene, Flowering locusT</p>

A Conserved Cytochrome P450 Evolved in Seed Plants Regulates Flower Maturation

Global inspection of plant genomes identifies genes maintained in low copies across taxa and under strong purifying selection, which are likely to have essential functions. Based on this rationale, we investigated the function of the low-duplicated CYP715 cytochrome P450 gene family that appeared early in seed plants and evolved under strong negative selection. Arabidopsis CYP715A1 showed a restricted tissue-specific expression in the tapetum of flower buds and in the anther filaments upon anthesis. cyp715a1 insertion lines showed a strong defect in petal development, and transient alteration of pollen intine deposition. Comparative expression analysis revealed the downregulated expression of genes involved in pollen development, cell wall biogenesis, hormone homeostasis, and floral sesquiterpene biosynthesis, especially TPS21 and several key genes regulating floral development such as MYB21, MYB24, and MYC2. Accordingly, floral sesquiterpene emission was suppressed in the cyp715a1 mutants. Flower hormone profiling, in addition, indicated a modification of gibberellin homeostasis and a strong disturbance of the turnover of jasmonic acid derivatives. Petal growth was partially restored by the active gibberellin GA3 or the functional analog of jasmonoyl-isoleucine, coronatine. CYP715 appears to function as a key regulator of flower maturation, synchronizing petal expansion and volatile emission. It is thus expected to be an important determinant of flower–insect interaction.

Effect of a Hormone Containing Nitrobenzene in Combination with Fertilizers on Early Flower Induction of Ixora coccinea Hybrids under Outdoor and Shaded Conditions

Ixora or Jungle Geranium (Ixora coccinea) is an invaluable plant in tropical landscapes for its beautiful foliage, easiness of growing and fabulous flowers freely formed and artfully presented. Sri Lanka exports Ixora hybrids as planting materials after rooting the Ixora cuttings on a coir dust media. Exporting potted Ixora plants with flowers is more beneficial than that without flowers as flowers increase product quality and gain customer attraction. In commercial floriculture venture, flower induction at young age using plant growth regulators and fertilizers is a popular practice. An experiment was conducted to investigate the most effective combination of a plant growth regulator and fertilizers for early flower initiation of Ixora hybrids; Vulcanus, Chanmai, Nora grant and Kontiki under outdoor and shaded conditions. Six-month-aged healthy plants of Ixora hybrids grown in containers were treated with a flowering hormone containing Nitrobenzene in combination with two fertilizer types: F1 – Bloom special and F2 – Krista K 44. Flowering hormone was sprayed in four concentrations; H1= 0.075% (V/V), H2 = 0.100% (V/V), H3 = 0.125% (V/V) and H4 = 0.150% (V/V) once in two weeks. Both fertilizers were applied as a liquid spray in same concentration (1 g/L of water) once a week. All the cultural practices were similarly applied to the plants under two light levels (outdoor light ‘L1’and shaded conditions ‘L2’). There were 16 treatment combinations and control, each with five replicates. Each treatment combination consisted of three plants. The number of flower-initiated plants per treatment combination was recorded as a percentage for five weeks. Percentage values after five weeks of treatments were transformed into log values and used for the analysis of variance. All the hybrids except Vulcanus showed a significant flower induction compared to the control. Outdoor light and shaded conditions were not significantly different to each other showing the potentiality of early flower initiation of hybrids under shaded condition. However, no significant difference in flower induction was observed among four levels of the flowering hormone and the two types of fertilizers (p> 0.05) in all four Ixora hybrids despite differences among the hybrids. Therefore, it could be concluded that the least expensive combination of hormone and fertilizer type could play a profitable and a positive role in early flower induction of Ixora at an age of six months.

The Importance of the Plant Circadian Clock to Confer Heat Tolerance

Most eukaryotic organisms display specialized cellular and behavioral oscillations with a period of approximately 24 hours, which are called circadian rhythms. The biological clock generates a rhythm that conveys temporal information over a day. Through this system, most eukaryotic organisms appropriately respond to daily or seasonal environmental changes by regulating their physiology and development in a time-dependent manner, conferring the organism with an adaptive advantage. In plants, the endogenous timing system also controls many important physiological processes including flower opening, hormone synthesis, metabolic pathways and gene expression so that these sessile species may survive efficiently in changing environments. Temperature compensation (TC) is one of the defining features of the clock mechanism. Under this mechanism, the pace of the clock, or period, remains stable over a broad range of physiologically relevant temperatures, which is unlikely to happen in other biochemical reactions. Thus, this mechanism allows organisms to sustain their ordinary life in various thermal environments by providing an accurate measure of the passage of time, regardless of the ambient temperature. Considering the current global climate changes our planet is undergoing, understanding the fundamental mechanism underlying TC cannot be overemphasized. In this review, we discuss the molecular organization of the plant circadian clock and the concept of TC, as well as the significance of plant TC in conferring fitness under the current increasing thermal environments.

Genetic Variation for Life History Sensitivity to Seasonal Warming inArabidopsis thaliana

Climate change has altered life history events in many plant species; however, little is known about genetic variation underlying seasonal thermal response. In this study, we simulated current and three future warming climates and measured flowering time across a globally diverse set of Arabidopsis thaliana accessions. We found that increased diurnal and seasonal temperature (1°–3°) decreased flowering time in two fall cohorts. The early fall cohort was unique in that both rapid cycling and overwintering life history strategies were revealed; the proportion of rapid cycling plants increased by 3–7% for each 1° temperature increase. We performed genome-wide association studies (GWAS) to identify the underlying genetic basis of thermal sensitivity. GWAS identified five main-effect quantitative trait loci (QTL) controlling flowering time and another five QTL with thermal sensitivity. Candidate genes include known flowering loci; a cochaperone that interacts with heat-shock protein 90; and a flowering hormone, gibberellic acid, a biosynthetic enzyme. The identified genetic architecture allowed accurate prediction of flowering phenotypes (R2 > 0.95) that has application for genomic selection of adaptive genotypes for future environments. This work may serve as a reference for breeding and conservation genetic studies under changing environments.

Tomato yield heterosis is triggered by a dosage sensitivity of the florigen pathway that fine-tunes shoot architecture.

The superiority of hybrids has long been exploited in agriculture, and although many models explaining “heterosis” have been put forth, direct empirical support is limited. Particularly elusive have been cases of heterozygosity for single gene mutations causing heterosis under a genetic model known as overdominance. In tomato (Solanum lycopersicum), plants carrying mutations in SINGLE FLOWER TRUSS (SFT) encoding the flowering hormone florigen are severely delayed in flowering, become extremely large, and produce few flowers and fruits, but when heterozygous, yields are dramatically increased. Curiously, this overdominance is evident only in the background of “determinate” plants, in which the continuous production of side shoots and inflorescences gradually halts due to a defect in the flowering repressor SELF PRUNING (SP). How sp facilitates sft overdominance is unclear, but is thought to relate to the opposing functions these genes have on flowering time and shoot architecture. We show that sft mutant heterozygosity (sft/+) causes weak semi-dominant delays in flowering of both primary and side shoots. Using transcriptome sequencing of shoot meristems, we demonstrate that this delay begins before seedling meristems become reproductive, followed by delays in subsequent side shoot meristems that, in turn, postpone the arrest of shoot and inflorescence production. Reducing SFT levels in sp plants by artificial microRNAs recapitulates the dose-dependent modification of shoot and inflorescence production of sft/+ heterozygotes, confirming that fine-tuning levels of functional SFT transcripts provides a foundation for higher yields. Finally, we show that although flowering delays by florigen mutant heterozygosity are conserved in Arabidopsis, increased yield is not, likely because cyclical flowering is absent. We suggest sft heterozygosity triggers a yield improvement by optimizing plant architecture via its dosage response in the florigen pathway. Exploiting dosage sensitivity of florigen and its family members therefore provides a path to enhance productivity in other crops, but species-specific tuning will be required.

Structure and function of florigen and the receptor complex

In the 1930s, the flowering hormone, florigen, was proposed to be synthesized in leaves under inductive day length and transported to the shoot apex, where it induces flowering. More recently, generated genetic and biochemical data suggest that florigen is a protein encoded by the gene, FLOWERING LOCUS T (FT). A rice (Oryza sativa) FT homolog, Hd3a, interacts with the rice FD homolog, OsFD1, via a 14-3-3 protein. Formation of this tri-protein complex is essential for flowering promotion by Hd3a in rice. In addition, the multifunctionality of FT homologs, other than for flowering promotion, is an emerging concept. Here we review the structural and biochemical features of the florigen protein complex and discuss the molecular basis for the multifunctionality of FT proteins.

Functional Diversification of FD Transcription Factors in Rice, Components of Florigen Activation Complexes

Florigen, a protein encoded by the FLOWERING LOCUS T (FT) in Arabidopsis and Heading date 3a (Hd3a) in rice, is the universal flowering hormone in plants. Florigen is transported from leaves to the shoot apical meristem and initiates floral evocation. In shoot apical cells, conserved cytoplasmic 14-3-3 proteins act as florigen receptors. A hexameric florigen activation complex (FAC) composed of Hd3a, 14-3-3 proteins, and OsFD1, a transcription factor, activates OsMADS15, a rice homolog of Arabidopsis APETALA1, leading to flowering. Because FD is a key component of the FAC, we characterized the FD gene family and their functions. Phylogenetic analysis of FD genes indicated that this family is divided into two groups: (i) canonical FD genes that are conserved among eudicots and non-Poaceae monocots; and (ii) Poaceae-specific FD genes that are organized into three subgroups: Poaceae FD1, FD2 and FD3. The Poaceae FD1 group shares a small sequence motif, T(A/V)LSLNS, with FDs of eudicots and non-Poaceae monocots. Overexpression of OsFD2, a member of the Poaceae FD2 group, produced smaller leaves with shorter plastochrons, suggesting that OsFD2 controls leaf development. In vivo subcellular localization of Hd3a, 14-3-3 and OsFD2 suggested that in contrast to OsFD1, OsFD2 is restricted to the cytoplasm through its interaction with the cytoplasmic 14-3-3 proteins, and interaction of Hd3a with 14-3-3 facilitates nuclear translocation of the FAC containing OsFD2. These results suggest that FD function has diverged between OsFD1 and OsFD2, but formation of a FAC is essential for their function.