Isolation and functional characterization of the FLOWERING LOCUS T homolog, the LsFT gene, in lettuce

High temperature-induced bolting of lettuce is undesirable agriculturally, making it important to find the mechanism governing the transition from vegetative to reproductive growth. FLOWERING LOCUS T (FT) genes play important roles in the induction of flowering in several plant species. To clarify floral induction in lettuce, we isolated the FT gene (LsFT) from lettuce. Sequence analysis and phylogenetic relationships of LsFT revealed considerable homology to FT genes of Arabidopsis, tomato, and other species. LsFT induced early flowering in transgenic Arabidopsis, but was not completely effective compared to AtFT. LsFT mRNA was abundant in the largest leaves under flowering-inducible conditions (higher temperatures). Gene expression was correlated with flower differentiation of the shoot apical meristem. Our results suggest that LsFT is a putative FT homolog in lettuce that regulates flower transition, similar to its homolog in Arabidopsis. This is the first information on the lettuce floral gene for elucidating regulation of the flowering transition in lettuce.     

Identification and functional characterization of SOC1-like genes in Pyrus bretschneideri

Flowering is a prerequisite for pear fruit production. Therefore, the development of flower buds and the control of flowering time are important for pear trees. However, the molecular mechanism of pear flowering is unclear. SOC1, a member of MADS-box family, is known as a flowering signal integrator in Arabidopsis. We identified eight SOC1-like genes in Pyrus bretschneideri and analyzed their basic information and expression patterns. Some pear SOC1-like genes were regulated by photoperiod in leaves. Moreover, the expression patterns were diverse during the development of pear flower buds. Two members of the pear SOC1-like genes, PbSOC1d and PbSOC1g, could lead to early flowering phenotype when overexpressed in Arabidopsis. PbSOC1d and PbSOC1g were identified as activators of the floral meristem identity genes AtAP1 and AtLFY and promote flowering time. These results suggest that PbSOC1d and PbSOC1g are promoters of flowering time and may be involved in flower bud development in pear.     

Molecular cloning and expression analyses of FaFT,FaTFL, and FaAP1 genes in cultivated strawberry: their correlation to flower bud formation

In this study, we cloned flowering-related genes FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1) from domesticated octaploid strawberries (Fragaria × ananassa) and analyzed their expression patterns in cultivars Tochiotome and Akihime. The floral meristem generation was induced under the short day and low temperature (SDLT), but not under the long day and high temperature (LDHT). We found that FaFT1, which is an orthologue of the Arabidopsis floral activator FT, was highly expressed in leaves under LDHT but not expressed in leaves under SDLT. On the other hand, the expression of FaTFL2, which belongs to the TFL1 family of flowering repressing genes, decreased in crowns (stem tissue including meristem) under SDLT. These results suggest that FaTFL2, as opposed to FvTFL1 in wild diploid strawberry Fragaria vesca, is related to flowering of the cultivated strawberry. Moreover, the FaTFL2 expression might be regulated by temperature rather than by photoperiod. We demonstrated that a reduction of the FaTFL2 expression is a key signal for flowering in domesticated strawberries.     

A dynamic role for sterols in embryogenesis of Pisum sativum

Molecular roles of sterols in plant development remain to be elucidated. To investigate sterol composition during embryogenesis, the occurrence of 25 steroid compounds in stages of developing seeds and pods of Pisum sativum was examined by GC-MS analysis. Immature seeds containing very young embryos exhibited the greatest concentrations of sterols. Regression models indicated that the natural log of seed or pod fr. wt was a consistent predictor of declining sterol content during embryonic development. Although total sterol levels were reduced in mature embryos, the composition of major sterols sitosterol and campesterol remained relatively constant in all 12 seed stages examined. In mature seeds, a significant decrease in isofucosterol was observed, as well as minor changes such as increases in cycloartenol branch sterols and campesterol derivatives. In comparison to seeds and pods, striking differences in composition were observed in sterol profiles of stems, shoots, leaves, flowers and flower buds, as well as cotyledons versus radicles. The highest levels of isofucosterol, a precursor to sitosterol, occurred in young seeds and flower buds, tissues that contain rapidly dividing cells and cells undergoing differentiation. Conversely, the highest levels of stigmasterol, a derivative of sitosterol, were found in fully-differentiated leaves while all seed stages exhibited low levels of stigmasterol. The observed differences in sterol content were correlated to mRNA expression data for sterol biosynthesis genes from Arabidopsis. These findings implicate the coordinated expression of sterol biosynthesis enzymes in gene regulatory networks underlying the embryonic development of flowering plants.     

Cloning and Characterization of FLOWERING LOCUS T-Like Genes from the Perennial Geophyte Saffron Crocus (Crocus sativus)

The transition to flowering is one of the most important developmental decisions made by plants. At the molecular level, many genes coordinate this transition. Among these, genes encoding for phosphatidylethanolamine-binding proteins (PEBPs) play important roles in regulating flower time and the fate of inflorescence meristem. To investigate the role of PEBPs in an industrially important crop cultivated for its nutritional and medicinal properties, the monocotyledonous species Crocus sativus L., we have isolated three FLOWERING LOCUS T (FT)-like genes designated as CsatFT1-like, CsatFT2-like, and CsatFT3-like. The isolated genes maintain the exon/intron organization of FT-like genes and encode proteins similar to the members of the PEBP family. Phylogenetic and amino acid analysis at critical positions confirmed that the isolated sequence belongs to the FT clade of the PEBP family phylogeny distinctly from the TERMINAL FLOWER 1 (TFL1) and MOTHER OF FT AND TFL1 clades. Expression analysis indicated differences in the expression of the three FT-like genes in different organs and different expressions during the day–night diurnal clock. Additionally, analysis of isolated promoter sequences using computational methods reveals the preservation of common binding motifs in FT-like promoters from other species, thus suggesting their importance among plant species.     

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

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

Natural genetic variation in <Emphasis Type="Italic">Brassica</Emphasis> homologs of <Emphasis Type="Italic">FLOWERING LOCUS T</Emphasis> and characterization of its expression domains

FLOWERING LOCUS T (FT), a major effect gene, regulates flowering time in Arabidopsis. We analyzed evolutionary changes distinguishing two FT homeologous loci in B. rapa, described genetic variation in homologs isolated and reported expression pattern of FT in B. juncea. Synteny analysis confirmed presence of two FT genomic copies in B. rapa ssp. pekinensis and resolved pre-existing anomalies regarding copy number in “AA” genome. Synteny analysis of B. rapa homeologous regions CR1 (129 kb) and CR2 (232 kb) revealed differential gene fractionation and wide-spread re-arrangements. Seven genomic DNA (gDNA) variants (2.1–2.2 kb) and 10 complementary DNA (cDNA) variants (528 bp) were isolated from 6 Brassica species. The gDNA variants shared 72–99 % similarity within Brassica and 58–60 % between Arabidopsis and Brassica. FT cDNA variants shared 92–100 % similarity within Brassica and 87 % between Arabidopsis and Brassica. Phylogenetic analysis of FT gDNA, cDNA and protein sequences revealed two major clades, differentiating homologs derived from species containing shared “BB” and “CC” genomes. Phylogram based on Brassica FT gDNA differentiated homeologs derived from AA-LF (Least fractioned) and AA-MF1 (Moderately fractioned) sub-genomes. Analysis of FT expression pattern in B. juncea revealed increasing levels correlating with attainment of physiological maturity; highest levels were detected in older leaves implying conservation in spatio-temporal expression pattern vis-à-vis Arabidopsis. In conclusion, our study reveals that polyploidy in Brassicas resulted in expansion of FT gene copies with homologs charting independent evolutionary course through accumulation of mutations. However, expression domains of FT remained conserved across Brassicaceae to preserve the critical function of FT in controlling flowering time.     

Cloning of FLOWERING LOCUS C ortholog in Wasabia japonica (Matsum.)

Wasabi (Wasabia japonica) is a commercially important crop in Japan. We isolated a FLC ortholog from wasabi and named as WjFLC. Predicted amino acid sequence encoded by WjFLC showed 89% identity with FLC of arabidopsis and conserved the MADS box motif. WjFLC was expressed in young and mature leaves, apical region of lateral bud, rhizome, and root. The expression of WjFLC was high in October and reduced in November when flower buds are formed in wasabi. WjFLC may be useful in monitoring the flowering response in wasabi.     

Initiation of Flowering in Protea compacta × Protea neriifolia Hybrid ‘Carnival’ Coincides with Expression of the FLOWERING LOCUS T Homologue

The Protea hybrid ‘Carnival’ (Protea compacta × Protea neriifolia) is responsive to seasonal change, arresting growth during the winter and producing flowers in the late summer after initiating flowering during the elongation of the spring flush. The large commercially attractive flower heads, consisting of over 200 florets, develop over a period of months. To begin to better understand the molecular factors that influence the transition to flowering in Protea, a homologue of the FLOWERING LOCUS T (FT) gene, ProFT, was isolated from ‘Carnival’, and its expression was analysed. ProFT showed increased expression in ‘Carnival’ leaves during October (13 h light/11 h dark; average daily temperature of 17 °C) at the time that floral organs were being pre-formed in the meristem. ProFT expression was fivefold higher in florally determined buds compared to that in leaves, and low levels were present in the vegetative meristems analysed. These results suggest that ProFT may act as a seasonally regulated floral inducer in ‘Carnival’, but based on spatial expression, data is also likely to play a role in inflorescence development and growth architecture.     

A promoter analysis of MOTHER OF FT AND TFL1 1 (JcMFT1), a seed-preferential gene from the biofuel plant Jatropha curcas

MOTHER OF FT AND TFL1 (MFT)-like genes belong to the phosphatidylethanoamine-binding protein (PEBP) gene family in plants. In contrast to their homologs FLOWERING LOCUS T (FT)-like and TERMINAL FLOWER 1 (TFL1)-like genes, which are involved in the regulation of the flowering time pathway, MFT-like genes function mainly during seed development and germination. In this study, a full-length cDNA of the MFT-like gene JcMFT1 from the biodiesel plant Jatropha curcas (L.) was isolated and found to be highly expressed in seeds. The promoter of JcMFT1 was cloned and characterized in transgenic Arabidopsis. A histochemical β-glucuronidase (GUS) assay indicated that the JcMFT1 promoter was predominantly expressed in both embryos and endosperms of transgenic Arabidopsis seeds. Fluorometric GUS analysis revealed that the JcMFT1 promoter was highly active at the mid to late stages of seed development. After seed germination, the JcMFT1 promoter activity decreased gradually. In addition, both the JcMFT1 expression in germinating Jatropha embryos and its promoter activity in germinating Arabidopsis embryos were induced by abscisic acid (ABA), possibly due to two ABA-responsive elements, a G-box and an RY repeat, in the JcMFT1 promoter region. These results show that the JcMFT1 promoter is seed-preferential and can be used to control transgene expression in the seeds of Jatropha and other transgenic plants.     

A possible role for flowering locus T‐encoding genes in interpreting environmental and internal cues affecting olive (Olea europaea L.) flower induction

Olive (Olea europaea L.) inflorescences, formed in lateral buds, flower in spring. However, there is some debate regarding time of flower induction and inflorescence initiation. Olive juvenility and seasonality of flowering were altered by overexpressing genes encoding flowering locus T (FT). OeFT1 and OeFT2 caused early flowering under short days when expressed in Arabidopsis. Expression of OeFT1/2 in olive leaves and OeFT2 in buds increased in winter, while initiation of inflorescences occurred i n late winter. Trees exposed to an artificial warm winter expressed low levels of OeFT1/2 in leaves and did not flower. Olive flower induction thus seems to be mediated by an increase in FT levels in response to cold winters. Olive flowering is dependent on additional internal factors. It was severely reduced in trees that carried a heavy fruit load the previous season (harvested in November) and in trees without fruit to which cold temperatures were artificially applied in summer. Expression analysis suggested that these internal factors work either by reducing the increase in OeFT1/2 expression or through putative flowering repressors such as TFL1. With expected warmer winters, future consumption of olive oil, as part of a healthy Mediterranean diet, should benefit from better understanding these factors.     

Relationship between seasonal progression of floral meristem development and <Emphasis Type="Italic">FLOWERING LOCUS T</Emphasis> expression in the deciduous tree <Emphasis Type="Italic">Fagus crenata</Emphasis>

Estimating the timing of flower bud formation in plants is essential to identify environmental factors that regulate floral transition. The presence of winter dormancy between the initiation of flowers and anthesis, characteristic of most trees in the temperate forests, hampers accurate estimation of the timing of floral transition. To overcome this difficulty, expression levels of flowering-time genes could be used as indicators of the timing of floral transition. Here, we evaluated the usefulness of molecular markers in estimating the timing of floral transition in Fagus crenata, a deciduous tree that shows intermittent and synchronized flowering at the population level. We selected FLOWERING LOCUS T (FT) as a candidate molecular marker and quantified the expression levels of its ortholog in F. crenata (FcFT). Subsequently, we analyzed the relationship between morphogenetic changes that occur between the vegetative state of the buds and the initiation of floral organs, and compared the FcFT expression levels in reproductive and vegetative buds, collected from spring to autumn. FcFT expression in leaves peaked at least two weeks before the morphological changes associated with flowering were visible in the buds in late July. FcFT expression levels were significantly higher in the reproductive buds than in the vegetative buds in July. These results suggest that the FcFT expression in July is a reliable indicator of the timing and occurrence of floral transition. This study highlights the utility of molecular tools in unraveling reproductive dynamics in plants, in combination with ecological and physiological approaches.