Gibberellic Acid Reduces Flowering Intensity in Sweet Orange [Citrus sinensis (L.) Osbeck] by Repressing CiFT Gene Expression

In Citrus, gibberellic acid (GA₃) applied at the floral bud inductive period significantly reduces flowering intensity. This effect is being used to improve the fruit set of parthenocarpic cultivars that tend to flower profusely. However, the molecular mechanisms involved in the process remain unclear. To contribute to the knowledge of this phenomenon, adult trees of ‘Salustiana’ sweet orange were sprayed at the floral bud inductive period with 40?mg?L^?1 of GA₃ and the expression pattern of flowering genes was examined up to the onset of bud sprouting. Trees sprayed with paclobutrazol (PBZ, 2,000?mg L^?1), a gibberellin biosynthesis inhibitor, were used to confirm the effects, and untreated trees served as control. Bud sprouting, flowering intensity, and developed shoots were evaluated in the spring. GA₃ significantly reduced the number of flowers per 100 nodes by 72% compared to the control, whereas PBZ increased the number by 123%. Data of the expression pattern of flowering genes in leaves of GA₃-treated trees revealed that this plant growth regulator inhibited flowering by repressing relative expression of the homolog of FLOWERING LOCUS T, CiFT, whereas PBZ increased flowering by boosting its expression. The activity of the homologs TERMINAL FLOWER 1, FLOWERING LOCUS C, SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1, and APETALA1 was not affected by the treatments. The number of flowers per inflorescence, in both leafy and leafless inflorescences, was not altered by GA₃ but increased with PBZ; the latter paralleled LEAFY relative expression. These results suggest that GA₃ inhibits flowering in Citrus by repressing CiFT expression in leaves.     

Reference gene selection for RT-qPCR analysis of Chrysanthemum lavandulifolium during its flowering stages

Quantitative real-time RT-PCR (RT-qPCR) is a technology that can be used to analyze the abundance of gene expression. Reference genes, which are assumed to remain at constant levels in different tissues at various developmental stages and photoperiodic treatments, were selected to analyze the expression levels of flowering time genes and floral development genes. Using digital gene expression technology, nine reference genes with moderate expression in the leaves of Chrysanthemum lavandulifolium at the juvenile phase (CK1) and the squaring stage (W1) were selected as the candidate reference genes for further study. A total of 115 biological samples of C. lavandulifolium were analyzed, including different tissues under various developmental stages and leaves with varied photoperiodic treatments. The stability of the nine reference genes was slightly variable across the samples, but MTP, SKIP16 and PGK were the most stable genes overall. In addition, the relative expression level of ClFT in different tissues of plants with the competence to flower was analyzed to verify the reference genes selected in this study. These studies provide a guide for selecting reference genes for analyzing the expression pattern of flowering time genes and floral development genes in C. lavandulifolium.     

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.     

Genetic and physical mapping of flowering time loci in canola (Brassica napus L.)

We identified quantitative trait loci (QTL) underlying variation for flowering time in a doubled haploid (DH) population of vernalisation—responsive canola (Brassica napus L.) cultivars Skipton and Ag-Spectrum and aligned them with physical map positions of predicted flowering genes from the Brassica rapa genome. Significant genetic variation in flowering time and response to vernalisation were observed among the DH lines from Skipton/Ag-Spectrum. A molecular linkage map was generated comprising 674 simple sequence repeat, sequence-related amplified polymorphism, sequence characterised amplified region, Diversity Array Technology, and candidate gene based markers loci. QTL analysis indicated that flowering time is a complex trait and is controlled by at least 20 loci, localised on ten different chromosomes. These loci each accounted for between 2.4 and 28.6 % of the total genotypic variation for first flowering and response to vernalisation. However, identification of consistent QTL was found to be dependant upon growing environments. We compared the locations of QTL with the physical positions of predicted flowering time genes located on the sequenced genome of B. rapa. Some QTL associated with flowering time on A02, A03, A07, and C06 may represent homologues of known flowering time genes in Arabidopsis; VERNALISATION INSENSITIVE 3, APETALA1, CAULIFLOWER, FLOWERING LOCUS C, FLOWERING LOCUS T, CURLY LEAF, SHORT VEGETATIVE PHASE, GA3 OXIDASE, and LEAFY. Identification of the chromosomal location and effect of the genes influencing flowering time may hasten the development of canola varieties having an optimal time for flowering in target environments such as for low rainfall areas, via marker-assisted selection.     

Characterization of Chrysanthemum ClSOC1-1 and ClSOC1-2, homologous genes of SOC1

The MADS-box gene SOC1/TM3 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1/ Tomato MADS-box gene 3) is a main integrator in the Arabidopsis flowering pathway; its structure and function are highly conserved in many plant species. SOC1-like genes have been isolated in chrysanthemum, one of the most well-known ornamental plants, but it has not been well characterized thus far. We isolated and characterized ClSOC1-1 and ClSOC1-2, two putative orthologs of Arabidopsis SOC1, from the wild diploid chrysanthemum, Chrysanthemum lavandulifolium, to investigate the regulatory mechanisms of flowering time control in chrysanthemum. Expression analysis indicated that ClSOC1-1 and ClSOC1-2 were expressed in all examined organs/tissues (leaves, shoot apices, petioles, stems and roots) with different expression levels, and with high expression in the shoot apices and leaves during the early stage of floral transition. The expression levels of ClSOC1-1 and ClSOC1-2 in the shoot apices increased at different developmental stages with the highest expression levels after 7 days of short-day treatment. Overexpression of ClSOC1-1 and ClSOC1-2 in wild-type Arabidopsis resulted in early flowering, which was coupled with the upregulation of one of the flowering promoter genes LEAFY. Our results suggested that the ClSOC1-1 and ClSOC1-2 genes play an evolutionarily conserved role in promoting flowering in Chrysanthemum lavandulifolium and could serve as a vital target for the genetic manipulation of flowering time in the chrysanthemum.     

FLC调控植物成花的分子机制研究新进展

FLC是植物成花关键抑制因子, 主要通过结合到其下游2个关键的成花促进基因(FT和SOC1)启动子上而抑制二者的表达。此外, 还可以与其它调控因子结合调控开花。然而, 关于FLC在成花调控中的具体分子机制仍需深入研究。该文主要结合8条成花调控遗传途径, 梳理近年来与FLC相关的新进展, 并展望了未来的研究方向。     

The E3 Ubiquitin Ligase HOS1 Regulates Arabidopsis Flowering by Mediating CONSTANS Degradation Under Cold Stress

The timing of flowering is coordinated by a web of gene regulatory networks that integrates developmental and environmental cues in plants. Light and temperature are two major environmental determinants that regulate flowering time. Although prolonged treatment with low nonfreezing temperatures accelerates flowering by stable repression of FLOWERING LOCUS C (FLC), repeated brief cold treatments delay flowering. Here, we report that intermittent cold treatments trigger the degradation of CONSTANS (CO), a central activator of photoperiodic flowering; daily treatments caused suppression of the floral integrator FLOWERING LOCUS T (FT) and delayed flowering. Cold-induced CO degradation is mediated via a ubiquitin/proteasome pathway that involves the E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 1 (HOS1). HOS1-mediated CO degradation occurs independently of the well established cold response pathways. It is also independent of the light signaling repressor CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) E3 ligase and light wavelengths. CO has been shown to play a key role in photoperiodic flowering. Here, we demonstrated that CO served as a molecular hub, integrating photoperiodic and cold stress signals into the flowering genetic pathways. We propose that the HOS1-CO module contributes to the fine-tuning of photoperiodic flowering under short term temperature fluctuations, which often occur during local weather disturbances.     

Cadmium induces early flowering in Arabidopsis

We found that cadmium promoted flowering in Arabidopsis and suppressed nitric oxide accumulation in leaves. Supplementation with NO donor SNP delayed flowering, whereas application of NO scavenger cPTIO further promoted the transition from vegetative to reproductive stage under Cd stress. Semi-quantitative RT-PCR showed that Cd treatment up-regulated the expression of CONSTANS and FLOWERING LOCUS T, whereas down-regulated the expression of FLOWERING LOCUS C.     

Potent induction of Arabidopsis thaliana flowering by elevated growth temperature

The transition to flowering is an important event in the plant life cycle and is modulated by several environmental factors including photoperiod, light quality, vernalization, and growth temperature, as well as biotic and abiotic stresses. In contrast to light and vernalization, little is known about the pathways that mediate the responses to other environmental variables. A mild increase in growth temperature, from 23 °C to 27 °C, is equally efficient in inducing flowering of Arabidopsis plants grown in 8-h short days as is transfer to 16-h long days. There is extensive natural variation in this response, and we identify strains with contrasting thermal reaction norms. Exploiting this natural variation, we show that FLOWERING LOCUS C potently suppresses thermal induction, and that the closely related floral repressor FLOWERING LOCUS M is a major-effect quantitative trait locus modulating thermosensitivity. Thermal induction does not require the photoperiod effector CONSTANS, acts upstream of the floral integrator FLOWERING LOCUS T, and depends on the hormone gibberellin. Analysis of mutants defective in salicylic acid biosynthesis suggests that thermal induction is independent of previously identified stress-signaling pathways. Microarray analyses confirm that the genomic responses to floral induction by photoperiod and temperature differ. Furthermore, we report that gene products that participate in RNA splicing are specifically affected by thermal induction. Above a critical threshold, even small changes in temperature can act as cues for the induction of flowering. This response has a genetic basis that is distinct from the known genetic pathways of floral transition, and appears to correlate with changes in RNA processing.     

FLC: A key regulator of flowering time in Arabidopsis

The timing of floral transition has significant consequences for reproductive success in plants. The molecular genetic dissection of flowering time control in Arabidopsis identified an integrated network of pathways that quantitatively control this developmental switch. A central player in this process is the FLOWERING LOCUS C gene (FLC), which blocks flowering by inhibiting the genes required to switch the meristem from vegetative to floral development. Three systems (the FRIGIDA gene, vernalization, and the autonomous pathway) all influence the state of FLC. Last years many new genes have been identified that regulate FLC expression, and most of them are involved in the modification of FLC chromatin. This review focuses on recent insights in FLC regulation.     

Brahma is required for proper expression of the floral repressor FLC in Arabidopsis

Background

BRAHMA (BRM) is a member of a family of ATPases of the SWI/SNF chromatin remodeling complexes from Arabidopsis. BRM has been previously shown to be crucial for vegetative and reproductive development.

Methodology/Principal Findings

Here we carry out a detailed analysis of the flowering phenotype of brm mutant plants which reveals that, in addition to repressing the flowering promoting genes CONSTANS (CO), FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), BRM also represses expression of the general flowering repressor FLOWERING LOCUS C (FLC). Thus, in brm mutant plants FLC expression is elevated, and FLC chromatin exhibits increased levels of histone H3 lysine 4 tri-methylation and decreased levels of H3 lysine 27 tri-methylation, indicating that BRM imposes a repressive chromatin configuration at the FLC locus. However, brm mutants display a normal vernalization response, indicating that BRM is not involved in vernalization-mediated FLC repression. Analysis of double mutants suggests that BRM is partially redundant with the autonomous pathway. Analysis of genetic interactions between BRM and the histone H2A.Z deposition machinery demonstrates that brm mutations overcome a requirement of H2A.Z for FLC activation suggesting that in the absence of BRM, a constitutively open chromatin conformation renders H2A.Z dispensable.

Conclusions/Significance

BRM is critical for phase transition in Arabidopsis. Thus, BRM represses expression of the flowering promoting genes CO, FT and SOC1 and of the flowering repressor FLC. Our results indicate that BRM controls expression of FLC by creating a repressive chromatin configuration of the locus.