Leaf and shoot apical meristem transcriptomes of quinoa (Chenopodium quinoa Willd.) in response to photoperiod and plant development

Understanding the regulation of flowering time is crucial for adaptation of crops to new environment. In this study, we examined the timing of floral transition and analysed transcriptomes in leaf and shoot apical meristems of photoperiod-sensitive and -insensitive quinoa accessions. Histological analysis showed that floral transition in quinoa initiates 2-3 weeks after sowing. We found four groups of differentially expressed genes in quinoa genome that responded to plant development and floral transition: (i) 222 genes responsive to photoperiod in leaves, (ii) 1812 genes differentially expressed between accessions under long-day conditions in leaves, (iii) 57 genes responding to developmental changes under short-day conditions in leaves and (iv) 911 genes responding to floral transition within the shoot apical meristem. Interestingly, among numerous candidate genes, two putative FT orthologs together with other genes (e.g. SOC1, COL, AP1) were previously reported as key regulators of flowering time in other species. Additionally, we used coexpression networks to associate novel transcripts to a putative biological process based on the annotated genes within the same coexpression cluster. The candidate genes in this study would benefit quinoa breeding by identifying and integrating their beneficial haplotypes in crossing programs to develop adapted cultivars to diverse environmental conditions.

Exogenous hydrogen sulphide promotes plant flowering through the Arabidopsis splicing factor AtU2AF65a

Alternative splicing (AS) is an important regulatory mode at the post-transcriptional level, through which many flowering genes regulate floral transition by producing multiple transcripts, and splicing factors have essential roles in this process. Hydrogen sulphide (H2S) is a newly found gasotransmitter that has critical physiological roles in plants, and one of its potential modes of action is via persulfidation of target proteins at specific cysteine sites. Previously, it has been shown that both the splicing factor AtU2AF65a and H2S are involved in the regulation of plant flowering. This study found that, in Arabidopsis, the promoting effect of H2S on flowering was abolished in atu2af65a-4 mutants. Transcriptome analyses showed that when AtU2AF65a contained mutations, the regulatory function of H2S during the AS of many flowering genes (including SPA1, LUH, LUG and MAF3) was inhibited. The persulfidation assay showed that AtU2AF65a can be persulfidated by H2S, and the RNA immunoprecipitation data indicated that H2S could alter the binding affinity of AtU2AF65a to the precursor messenger RNA of the above-mentioned flowering genes. Overall, our results suggest that H2S may regulate the AS of flowering-related genes through persulfidation of splicing factor AtU2AF65a and thus lead to early flowering in plants.

Inhibition of flowering by gibberellins in the woody plant Jatropha curcas is restored by overexpression of JcFT

Jatropha curcas is a perennial oil-seed plant with vigorous vegetative growth but relatively poor reproductive growth and low seed yield. Gibberellins (GA) promote flowering in most annual plants, but inhibit flower initiation in many woody plants, including J. curcas. However, the underlying mechanisms of GA inhibition on flowering in perennial woody plants remain unclear. Here, we found that overexpression of the gibberellin biosynthesis gene JcGA20ox1 delays flowering in J. curcas and in the J. curcas × J. integerrima hybrids. Consistent with this finding, overexpression of the GA catabolic gene JcGA2ox6 induces early flowering in J. curcas. qRTPCR revealed that the inhibition of floral transition by overexpressing JcGA20ox1 resulted from the decreased of JcFT and other flowering related genes, which was restored by overexpressing JcFT in J. curcas. Overexpression of JcGA20ox1 or JcGA2ox6 reduced seed yield, but overexpression of JcFT significantly increased seed yield. Furthermore, hybridization experiments showed that the reduction in seed yield caused by overexpression of JcGA20ox1 or JcGA2ox6 was partially restored by the overexpression of JcFT. In addition, JcGA20ox1, JcGA2ox6 and JcFT were also found to be involved in the regulation of seed oil content and endosperm development. In conclusion, our study revealed the inhibitiory effect of GA on flowering in J. curcas is mediated through JcFT and demonstratd the effects of JcGA20ox1, JcGA2ox6 and JcFT on agronomic traits. This study also indicates the potential value of GA metabolism genes and JcFT in breeding new varieties of woody oil-seed plants.

Genetic and epigenetic basis of phytohormones control of floral transition in plants

The timing of the developmental transition from the vegetative to the reproductive stages is critical for angiosperm and fine-tuned by the integration of endogenous factors and external environmental cues to ensure proper and successful reproduction. Plants have evolved sophisticated mechanisms to response to diverse environmental or stress signals, which may be mediated by plant hormones which coordinate their flowering time. Endogenous and exogenous phytohormones such as gibberellin (GA), auxin, cytokinin (CK), jasmonate (JA), abscisic acid (ABA), ethylene (ET), brassinosteroids (BR) and the cross-talk among them are critical for the precise regulating of flowering time. Recent studies on the model flowering plant Arabidopsis thaliana revealed that diverse transcription factors and epigenetic regulators play key roles in the phytohormones that regulate floral transition. This review aims to summarize current knowledge on the genetic and epigenetic mechanisms that underlying the phytohormone control of floral transition in Arabidopsis, offering insights into how these processes are regulated and their implications for plant biology.

The RAV transcription factor TEMPRANILLO1 involved in ethylene-mediated delay of chrysanthemum flowering

TEMPRANILLO1 (TEM1) is a transcription factor belonging to related to ABI3 and VP1 family, which is also known as ethylene response DNA-binding factor 1 and functions as a repressor of flowering in Arabidopsis. Here, a putative homolog of AtTEM1 was isolated and characterized from chrysanthemum, designated as CmTEM1. Exogenous application of ethephon leads to an upregulation in the expression of CmTEM1. Knockdown of CmTEM1 promotes floral initiation, while overexpression of CmTEM1 retards floral transition. Further phenotypic observations suggested that CmTEM1 involves in the ethylene-mediated inhibition of flowering. Transcriptomic analysis established that expression of the flowering integrator CmAFL1, a member of the APETALA1/FRUITFULL subfamily, was downregulated significantly in CmTEM1-overexpressing transgenic plants compared with wild-type plants but was verified to be upregulated in amiR-CmTEM1 lines by quantitative RT-PCR. In addition, CmTEM1 is capable of binding to the promoter of the CmAFL1 gene to inhibit its transcription. Moreover, the genetic evidence supported the notion that CmTEM1 partially inhibits floral transition by targeting CmAFL1. In conclusion, these findings demonstrate that CmTEM1 acts as a regulator of ethylene-mediated delayed flowering in chrysanthemum, partly through its interaction with CmAFL1.

Differential gene expression during floral transition in pineapple

Pineapple (Ananas comosus var. comosus) and ornamental bromeliads are commercially induced to flower by treatment with ethylene or its analogs. The apex is transformed from a vegetative to a floral meristem and shows morphological changes in 8 to 10 days, with flowers developing 8 to 10 weeks later. During eight sampling stages ranging from 6 h to 8 days after treatment, 7961 genes were found to exhibit differential expression (DE) after the application of ethylene. In the first 3 days after treatment, there was little change in ethylene synthesis or in the early stages of the ethylene response. Subsequently, three ethylene response transcription factors (ERTF) were up-regulated and the potential gene targets were predicted to be the positive flowering regulator CONSTANS-like 3 (CO), a WUSCHEL gene, two APETALA1/FRUITFULL (AP1/FUL) genes, an epidermal patterning gene, and a jasmonic acid synthesis gene. We confirm that pineapple has lost the flowering repressor FLOWERING LOCUS C. At the initial stages, the SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) was not significantly involved in this transition. Another WUSCHEL gene and a PHD homeobox transcription factor, though not apparent direct targets of ERTF, were up-regulated within a day of treatment, their predicted targets being the up-regulated CO, auxin response factors, SQUAMOSA, and histone H3 genes with suppression of abscisic acid response genes. The FLOWERING LOCUS T (FT), TERMINAL FLOWER (TFL), AGAMOUS-like APETELAR (AP2), and SEPETALA (SEP) increased rapidly within 2 to 3 days after ethylene treatment. Two FT genes were up-regulated at the apex and not at the leaf bases after treatment, suggesting that transport did not occur. These results indicated that the ethylene response in pineapple and possibly most bromeliads act directly to promote the vegetative to flower transition via APETALA1/FRUITFULL (AP1/FUL) and its interaction with SPL, FT, TFL, SEP, and AP2. A model based on AP2/ERTF DE and predicted DE target genes was developed to give focus to future research. The identified candidate genes are potential targets for genetic manipulation to determine their molecular role in flower transition.

Application value of protein phase separation mechanism of flowering regulation in de novo domestication

Global climate change and population growth pose a serious threat to world food security. The current crops varieties will be insufficient to meet food needs in the future, and there is an urgent need for high yielding and quality crops varieties with strong environmental adaptability. The rapid de novo domestication of wild species to create new germplasm that can be applied to crop breeding is a new strategy for ensuring food security. The flowering time is an important factor in determining the crop planting area and yield, and is a trait that is often selected in crop domestication. At present, the modification of flowering traits by de novo domestication is usually achieved by direct editing of the major genes that control flowering in crop, which are very limited in number and relatively homogeneous in function. Floral transition is regulated by the complex network of environmental and endogenous signals. Here, we propose a new strategy that using genome editing to precisely modify protein function by changing protein phase separation capacity of important proteins that regulate expression of flowering genes, which may provide new options for the design of flowering traits in de novo domestication.

TM3 and STM3 Promote Flowering Together with FUL2 and MBP20, but Act Antagonistically in Inflorescence Branching in Tomato

The moment at which a plant transitions to reproductive development is paramount to its life cycle and is strictly controlled by many genes. The transcription factor SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) plays a central role in this process in Arabidopsis. However, the role of SOC1 in tomato (Solanum lycopersicum) has been sparsely studied. Here, we investigated the function of four tomato SOC1 homologs in the floral transition and inflorescence development. We thoroughly characterized the SOC1-like clade throughout the Solanaceae and selected four tomato homologs that are dynamically expressed upon the floral transition. We show that of these homologs, TOMATO MADS 3 (TM3) and SISTER OF TM3 (STM3) promote the primary and sympodial transition to flowering, while MADS-BOX PROTEIN 23 (MBP23) and MBP18 hardly contribute to flowering initiation in the indeterminate cultivar Moneyberg. Protein-protein interaction assays and whole-transcriptome analysis during reproductive meristem development revealed that TM3 and STM3 interact and share many targets with FRUITFULL (FUL) homologs, including cytokinin regulators. Furthermore, we observed that mutating TM3/STM3 affects inflorescence development, but counteracts the inflorescence-branching phenotype of ful2 mbp20. Collectively, this indicates that TM3/STM3 promote the floral transition together with FUL2/MBP20, while these transcription factors have opposite functions in inflorescence development.

ZmSPL13 and ZmSPL29 act together to promote vegetative and reproductive transition in maize

Flowering time is a key agronomic trait determining environmental adaptation and yield potential of crops. The regulatory mechanisms of flowering in maize still remain rudimentary. In this study, we combine expressional, genetic, and molecular studies to identify two homologous SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors ZmSPL13 and ZmSPL29 as positive regulators of juvenile-to-adult vegetative transition and floral transition in maize. We show that both ZmSPL13 and ZmSPL29 are preferentially expressed in leaf phloem, vegetative and reproductive meristem. We show that vegetative phase change and flowering time are moderately delayed in the Zmspl13 and Zmspl29 single knockout mutants and more significantly delayed in the Zmspl13/29 double mutants. Consistently, the ZmSPL29 overexpression plants display precocious vegetative phase transition and floral transition, thus early flowering. We demonstrate that ZmSPL13 and ZmSPL29 directly upregulate the expression of ZmMIR172C and ZCN8 in the leaf, and of ZMM3 and ZMM4 in the shoot apical meristem, to induce juvenile-to-adult vegetative transition and floral transition. These findings establish a consecutive signaling cascade of the maize aging pathway by linking the miR156-SPL and the miR172-Gl15 regulatory modules and provide new targets for genetic improvement of flowering time in maize cultivars.

CRISPR/Cas9 mutants delineate roles of Populus FT and TFL1/CEN/BFT family members in growth, dormancy release and flowering

Vegetative and reproductive phase change and phenology are economically and ecologically important traits. Trees typically require several years of growth before flowering and, once mature, seasonal control of the transition to flowering and flower development is necessary to maintain vegetative meristems and for reproductive success. Members of two related gene subfamilies, FLOWERING LOCUST (FT) and TERMINAL FLOWER1 (TFL1)/CENTRORADIALIS (CEN)/BROTHER OF FT AND TFL1 (BFT), have antagonistic roles in flowering in diverse species and roles in vegetative phenology in trees, but many details of their functions in trees have yet to be resolved. Here, we used CRISPR/Cas9 to generate single and double mutants involving the five Populus FT and TFL1/CEN/BFT genes. The ft1 mutants exhibited wild-type-like phenotypes in long days and short days, but after chilling, to release dormancy, they showed delayed bud flush and GA3 could compensate for the ft1 mutation. After rooting and generating some phytomers in tissue culture, both cen1 and cen1ft1 mutants produced terminal as well as axillary flowers, indicating that the cen1 flowering phenotype is independent of FT1. The CEN1 showed distinct circannual expression patterns in vegetative and reproductive tissues and comparison with the expression patterns of FT1 and FT2 suggests that the relative levels of CEN1 compared with FT1 and FT2 regulate multiple phases of vegetative and reproductive seasonal development.

FLK is an mRNA m6A reader that regulates floral transition by modulating the stability and splicing of FLC in Arabidopsis

N⁶-methyladenosine (m⁶A), which is added, removed, and interpreted by m⁶A writers, erasers, and readers, respectively, is the most abundant modification in eukaryotic mRNAs. The m⁶A marks play a pivotal role in the regulation of floral transition in plants. FLOWERING LOCUS K (FLK), an RNA-binding protein harboring K-homology (KH) motifs, is known to regulate floral transition by repressing the levels of a key floral repressor FLOWERING LOCUS C (FLC) in Arabidopsis. However, the molecular mechanism underlying FLK-mediated FLC regulation remains unclear. In this study, we identified FLK as a novel mRNA m⁶A reader protein that directly binds the m⁶A site in the 3'-untranslated region of FLC transcripts to repressing FLC levels by reducing its stability and splicing. Importantly, FLK binding of FLC transcripts was abolished in vir-1, an m⁶A writer mutant, and the late-flowering phenotype of the flk mutant could not be rescued by genetic complementation using the mutant FLK^m gene, in which the m⁶A reader encoding function was eliminated, indicating that FLK binds and regulates FLC expression in an m⁶A-dependent manner. Collectively, our study has addressed a long-standing question of how FLK regulates FLC transcript levels and established a molecular link between the FLK-mediated recognition of m⁶A modifications on FLC transcripts and floral transition in Arabidopsis.

Replication protein RPA2A regulates floral transition by cooperating with PRC2 in Arabidopsis

RPA2A is a subunit of the conserved heterotrimeric replication protein A (RPA) in Arabidopsis, which is an essential replisome component that binds to single-stranded DNA during DNA replication. RPA2A controls a set of developmental processes, but the underlying mechanism is largely unknown. Here we show that RPA2A represses key flowering genes including FLOWERING LOCUS T (FT), AGAMOUS (AG) and AGAMOUS LIKE 71 (AGL71) to suppress floral transition by cooperating with the PRC2 complex. RPA2A is vigorously expressed in dividing cells and required for correct DNA replication. Mutation of RPA2A leads to early flowering, which is dependent on ectopic expression of key flowering genes including FT molecularly and genetically. RPA2A and PRC2 have common target genes including FT, AG and AGL71 supported using genetic analysis, transcriptome profiling and H3K27me3 ChIP-seq analysis. Furthermore, RPA2A physically interacts with PRC2 components CLF, EMF2 and MSI1, which recruits CLF to the chromatin loci of FT, AG and AGL71. Together, our results show that the replication protein RPA2A recruits PRC2 to key flowering genes through physical protein interaction, thereby repressing the expression of these genes to suppress floral transition in Arabidopsis.

Genome-wide binding analysis of transcription factor Rice Indeterminate 1 reveals a complex network controlling rice floral transition

RICE INDETERMINATE 1 (RID1) plays a critical role in controlling floral transition in rice (Oryza sativa). However, the molecular basis for this effect, particularly the target genes and regulatory specificity, remains largely unclear. Here, we performed chromatin immunoprecipitation followed by sequencing (ChIP-seq) in young leaves at the pre-floral-transition stage to identify the target genes of RID1, identifying 2,680 genes associated with RID1 binding sites genome-wide. RID1 binding peaks were highly enriched for TTTGTC, the direct binding motif of the INDETERMINATE DOMAIN protein family that includes RID1. Interestingly, CACGTG and GTGGGCCC, two previously uncharacterized indirect binding motifs, were enriched through the interactions of RID1 with the novel flowering-promoting proteins OsPIL12 and OsTCP11, respectively. Moreover, the ChIP-seq data demonstrated that RID1 bound to numerous rice heading-date genes, such as HEADING DATE 1 (HD1) and FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (OsFKF1). Notably, transcriptome sequencing (RNA-seq) analysis revealed roles of RID1 in diverse developmental pathways. Genetic analysis combined with genome-wide ChIP-seq and RNA-seq results showed that RID1 directly binds to the promoter of OsERF#136 (a repressor of rice flowering) and negatively regulates its expression. Overall, our findings provide new insights into the molecular and genetic mechanisms underlying rice floral transition and characterize OsERF#136 as a previously unrecognized direct target of RID1.

Verticillium dahliae Secretes Small RNA to Target Host MIR157d and Retard Plant Floral Transition During Infection

Bidirectional trans-kingdom RNA silencing [or RNA interference (RNAi)] plays a key role in plant-pathogen interactions. It has been shown that plant hosts export specific endogenous miRNAs into pathogens to inhibit their virulence, whereas pathogens deliver small RNAs (sRNAs) into plant cells to disturb host immunity. Here, we report a trans-kingdom fungal sRNA retarding host plant floral transition by targeting a miRNA precursor. From Arabidopsis plants infected with Verticillium dahliae, a soil-borne hemibiotrophic pathogenic fungus that causes wilt diseases in a wide range of plant hosts, we obtained a number of possible trans-kingdom V. dahliae sRNAs (VdsRNAs) by sequencing AGO1-immunoprecipitated sRNAs. Among these, a 24-nt VdsRNA derived from V. dahliae rRNA, VdrsR-1, was shown to be an actual trans-kingdom VdsRNA that targets the miR157d precursor MIR157d, resulting in increased rather than reduced miR157d accumulation in V. dahliae-infected plants. Consistent with the miR157 family in the regulation of vegetative and floral transitions by targeting SPL genes in several plant species, we detected two SPL genes, SPL13A/B, that were notably reduced in V. dahliae-infected and VdrsR-1-expressing plants compared with control plants. Furthermore, V. dahliae-infected and VdrsR-1-expressing plants also displayed delayed vegetative phase change and floral transition compared to control plants. Taken together, we disclosed a novel mode of action for a trans-kingdom fungal sRNA, VdrsR-1, which was secreted into host cells to modulate plant floral transition by employing the miR157d/SPL13A/B regulatory module, leading to prolonged host vegetative growth that would undoubtedly benefit fungal propagation.

BcSOC1 Promotes Bolting and Stem Elongation in Flowering Chinese Cabbage

Flowering Chinese cabbage is one of the most economically important stalk vegetables. However, the molecular mechanisms underlying bolting, which is directly related to stalk quality and yield, in this species remain unknown. Previously, we examined five key stem development stages in flowering Chinese cabbage. Here, we identified a gene, BcSOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1), in flowering Chinese cabbage using transcriptome analysis, whose expression was positively correlated with bolting. Exogenous gibberellin (GA3) and low-temperature treatments significantly upregulated BcSOC1 and promoted early bolting and flowering. Additionally, BcSOC1 overexpression accelerated early flowering and stem elongation in both Arabidopsis and flowering Chinese cabbage, whereas its knockdown dramatically delayed bolting and flowering and inhibited stem elongation in the latter; the inhibition of stem elongation was more notable than delayed flowering. BcSOC1 overexpression also induced cell expansion by upregulating genes encoding cell wall structural proteins, such as BcEXPA11 (cell wall structural proteins and enzymes) and BcXTH3 (xyloglucan endotransglycosidase/hydrolase), upon exogenous GA3 and low-temperature treatments. Moreover, the length of pith cells was correlated with stem height, and BcSOC1 interacted with BcAGL6 (AGAMOUS-LIKE 6) and BcAGL24 (AGAMOUS-LIKE 24). Thus, BcSOC1 plays a vital role in bolting and stem elongation of flowering Chinese cabbage and may play a novel role in regulating stalk development, apart from the conserved function of Arabidopsis SOC1 in flowering alone.

Insights Into the Molecular Mechanisms of Late Flowering in Prunus sibirica by Whole-Genome and Transcriptome Analyses

Freezing during the flowering of Prunus sibirica is detrimental to fruit production. The late flowering (LF) type, which is delayed by 7-15 days compared with the normal flowering (NF) type, avoids damages at low temperature, but the molecular mechanism of LF remains unclear. Therefore, this study was conducted to comprehensively characterize floral bud differentiation. A histological analysis showed that initial floral bud differentiation was delayed in the LF type compared to the NF type. Genome-wide associated studies (GWAS) showed that a candidate gene (PaF106G0600023738.01) was significantly associated with LF type. It was identified as trehalose-6-phosphate phosphatase (PsTPPF), which is involved in trehalose-6-phosphate (Tre6P) signaling pathway and acts on floral transition. A whole-transcriptome RNA sequencing analysis was conducted, and a total of 6,110 differential expression (DE) mRNAs, 1,351 DE lncRNAs, and 148 DE miRNAs were identified. In addition, 24 DE mRNAs related with floral transition were predicted, and these involved the following: three interactions between DE lncRNAs and DE mRNAs of photoperiod pathway with two mRNAs (COP1, PaF106G0400018289.01 and CO3, MXLOC_025744) and three lncRNAs (CCLR, LTCONS_00031803, COCLR1, LTCONS_00046726, and COCLR2, LTCONS_00046731); one interaction between DE miRNAs and DE mRNAs with one mRNA, encoding trehalose-6-phosphate synthase (PsTPS1, PaF106G0100001132.01), and one miRNA (miRNA167h). Combined with the expression profiles and Tre6P levels, functions of PsTPPF and PsTPS1 in Tre6P regulation were considered to be associated with flowering time. A new network of ceRNAs correlated with LF was constructed, and it consisted of one mRNA (PsTPS1), one lncRNA (TCLR, LTCONS_00034157), and one miRNA (miR167h). This study provided insight into the molecular regulatory mechanism of LF in Prunus sibirica.

OsbZIP62/OsFD7, a functional ortholog of FLOWERING LOCUS D, regulates floral transition and panicle development in rice

We have characterized a rice bZIP protein-coding gene OsbZIP62/OsFD7 that is expressed preferentially in the shoot apical meristem and during early panicle developmental stages in comparison with other OsFD genes characterized to date. Surprisingly, unlike OsFD1, OsFD7 interacts directly and more efficiently with OsFTLs; the interaction is strongest with OsFTL1 followed by Hd3a and RFT1, as confirmed by fluorescence lifetime imaging-Förster resonant energy transfer (FLIM-FRET) analysis. In addition, OsFD7 is phosphorylated at its C-terminal end by OsCDPK41 and OsCDPK49 in vitro, and this phosphorylated moiety is recognized by OsGF14 proteins. OsFD7 RNAi transgenics were late flowering; the transcript levels of some floral meristem identity genes (e.g. OsMADS14, OsMADS15, and OsMADS18) were also down-regulated. RNAi lines also exhibited dense panicle morphology with an increase in the number of primary and secondary branches resulting in longer panicles and more seeds, probably due to down-regulation of SEPALLATA family genes. In comparison with other FD-like proteins previously characterized in rice, it appears that OsFD7 may have undergone diversification during evolution, resulting in the acquisition of newer functions and thus playing a dual role in floral transition and panicle development in rice.

VDAC1 Negatively Regulates Floral Transition in Arabidopsis thaliana

Voltage-dependent anion channels (VDACs) are the most important proteins in mitochondria. They localize to the outer mitochondrial membrane and contribute to the metabolite transport between the mitochondria and cytoplasm, which aids plant growth regulation. Here, we report that Arabidopsis thaliana VDAC1 is involved in the floral transition, with the loss of AtVDAC1 function, resulting in an early-flowering phenotype. AtVDAC1 is expressed ubiquitously in Arabidopsis. To identify the flowering pathway integrators that may be responsible for AtVDAC1′s function during the floral transition, an RNA-seq analysis was performed. In total, 106 differentially expressed genes (DEGs) were identified between wild-type and atvdac1-5 mutant seedlings. However, none were involved in flowering-related pathways. In contrast, AtVDAC1 physically associated with FLOWERING LOCUS T. Thus, in the floral transition, AtVDAC1 may function partly through the FLOWERING LOCUS T protein.

Morphological, Physiological, and Molecular Responses of Sweetly Fragrant Luculia gratissima During the Floral Transition Stage Induced by Short-Day Photoperiod

Photoperiod-regulated floral transition is vital to the flowering plant. Luculia gratissima "Xiangfei" is a flowering ornamental plant with high development potential economically and is a short-day woody perennial. However, the genetic regulation of short-day-induced floral transition in L. gratissima is unclear. To systematically research the responses of L. gratissima during this process, dynamic changes in morphology, physiology, and transcript levels were observed and identified in different developmental stages of long-day- and short-day-treated L. gratissima plants. We found that floral transition in L. gratissima occurred 10 d after short-day induction, but flower bud differentiation did not occur at any stage under long-day conditions. A total of 1,226 differentially expressed genes were identified, of which 146 genes were associated with flowering pathways of sugar, phytohormones, photoperiod, ambient temperature, and aging signals, as well as floral integrator and meristem identity genes. The trehalose-6-phosphate signal positively modulated floral transition by interacting with SQUAMOSA PROMOTER-BINDING-LIKE PROTEIN 4 (SPL4) in the aging pathway. Endogenous gibberellin, abscisic acid, cytokinin, and jasmonic acid promoted floral transition, whereas strigolactone inhibited it. In the photoperiod pathway, FD, CONSTANS-LIKE 12, and nuclear factors Y positively controlled floral transition, whereas PSEUDO-RESPONSE REGULATOR 7, FLAVIN-BINDING KELCH REPEAT F-BOX PROTEIN 1, and LUX negatively regulated it. SPL4 and pEARLI1 positively affected floral transition. Suppressor of Overexpression of Constans 1 and AGAMOUSLIKE24 integrated multiple flowering signals to modulate the expression of FRUITFULL/AGL8, AP1, LEAFY, SEPALLATAs, SHORT VEGETATIVE PHASE, and TERMINAL FLOWER 1, thereby regulating floral transition. Finally, we propose a regulatory network model for short-day-induced floral transition in L. gratissima. This study improves our understanding of flowering time regulation in L. gratissima and provides knowledge for its production and commercialization.

Haplotype variations of major flowering time genes in quinoa unveil their role in the adaptation to different environmental conditions

Response to photoperiod is of major importance in crop production. It defines the adaptation of plants to local environments. Quinoa is a short-day plant which had been domesticated in the Andeans regions. We wanted to understand the adaptation to long-day conditions by studying orthologues of two major flowering time regulators of Arabidopsis, FLOWERING LOCUS T (FT) and CONSTANS (CO) in quinoa accessions with contrasting photoperiod response. By searching the quinoa reference genome sequence, we identified 24 FT and six CO homologs. CqFT genes displayed remarkably different expression patterns between long- and short-day conditions, whereas the influence of the photoperiod on CqCOL expressions was moderate. Cultivation of 276 quinoa accessions under short- and long-day conditions revealed great differences in photoperiod sensitivity. After sequencing their genomes, we identified large sequence variations in 12 flowering time genes. We found non-random distribution of haplotypes across accessions from different geographical origins, highlighting the role of CqFT and CqCOL genes in the adaptation to different day-length conditions. We identified five haplotypes causing early flowering under long days. This study provides assets for quinoa breeding because superior haplotypes can be assembled in a predictive breeding approach to produce well-adapted early flowering lines under long-day photoperiods.

The transition to flowering in winter rapeseed during vernalization

Flowering time is a major determinant of adaptation, fitness and yield in the allopolyploid species rapeseed (Brassica napus). Despite being a close relative to Arabidopsis thaliana, little is known about the timing of floral transition and the genes that govern this process. Winter, semi-winter and spring type plants have important life history characteristics that differ in vernalization requirements for flowering and are important for growing rapeseed in different regions of the world. In this study, we investigated the timing of vernalization-driven floral transition in winter rapeseed and the effect of photoperiod and developmental age on flowering time and vernalization responsiveness. Microscopy and whole transcriptome analyses at the shoot apical meristems of plants grown under controlled conditions showed that floral transition is initiated within few weeks of vernalization. Certain Bna.SOC1 and Bna.SPL5 homeologs were among the induced genes, suggesting that they are regulating the timing of cold-induced floral transition. Moreover, the flowering response of plants with shorter pre-vernalization period correlated with a delayed expression of Bna.SOC1 and Bna.SPL5 genes. In essence, this study presents a detailed analysis of vernalization-driven floral transition and the aspects of juvenility and dormancy and their effect on flowering time in rapeseed.

Regulation of Early Plant Development by Red and Blue Light: A Comparative Analysis Between Arabidopsis thaliana and Solanum lycopersicum

In vertical farming, plants are grown in multi-layered growth chambers supplied with energy-efficient LEDs that produce less heat and can thus be placed in close proximity to the plants. The spectral quality control allowed by LED lighting potentially enables steering plant development toward desired phenotypes. However, this requires detailed knowledge on how light quality affects different developmental processes per plant species or even cultivar, and how well information from model plants translates to horticultural crops. Here we have grown the model dicot Arabidopsis thaliana (Arabidopsis) and the crop plant Solanum lycopersicum (tomato) under white or monochromatic red or blue LED conditions. In addition, seedlings were grown in vitro in either light-grown roots (LGR) or dark-grown roots (DGR) LED conditions. Our results present an overview of phenotypic traits that are sensitive to red or blue light, which may be used as a basis for application by tomato nurseries. Our comparative analysis showed that young tomato plants were remarkably indifferent to the LED conditions, with red and blue light effects on primary growth, but not on organ formation or flowering. In contrast, Arabidopsis appeared to be highly sensitive to light quality, as dramatic differences in shoot and root elongation, organ formation, and developmental phase transitions were observed between red, blue, and white LED conditions. Our results highlight once more that growth responses to environmental conditions can differ significantly between model and crop species. Understanding the molecular basis for this difference will be important for designing lighting systems tailored for specific crops.

Complementary Transcriptome and Proteome Analyses Provide Insight into the Floral Transition in Bamboo (Dendrocalamus latiflorus Munro)

Most woody bamboos bloom only once after long vegetative growth phases and die immediately afterwards. It is difficult, however, to determine the timing of the floral transition, as little information is available on the molecular mechanism of plant maturity in bamboos. To uncover the bamboo floral transition mechanism, its morpho-physiological characteristics, transcriptomes and large-scale quantitative proteomes were investigated in leaves which were collected at different stages during floral transition in a woody bamboo, Dendrocalamus latiflorus. We identified many flowering time-associated genes and the continued increase and decrease genes were screened as flowering biomarker genes (e.g., the MADS14 and bHLH13 genes). These different genes were assigned to specific metabolic pathways by the Kyoto Encyclopedia of Genes and Genomes (KEGG). And the photoperiod pathways depending on the circadian rhythm may play an essential role in the bamboo floral transition. In addition, a total of 721 differently expressed proteins of leaves from the vegetative-to-reproductive stages were identified. Fifty-five genes were specifically differentially expressed at both the transcriptomic and proteomic levels, including genes related to photosynthesis and nucleotide sugar, which may be involved in the floral transition. This work provides insights into bamboo flowers and the management of forest breeding.

dlf1 promotes floral transition by directly activating ZmMADS4 and ZmMADS67 in the maize shoot apex

The floral transition of the maize (Zea mays ssp. mays) shoot apical meristem determines leaf number and flowering time, which are key traits influencing local adaptation and yield potential. dlf1 (delayed flowering1) encodes a basic leucine zipper protein that interacts with the florigen ZCN8 to mediate floral induction in the shoot apex. However, the mechanism of how dlf1 promotes floral transition remains largely unknown. We demonstrate that dlf1 underlies qLB7-1, a quantitative trait locus controlling leaf number and flowering time that was identified in a BC₂ S₃ population derived from a cross between maize and its wild ancestor, teosinte (Zea mays ssp. parviglumis). Transcriptome sequencing and chromatin immunoprecipitation sequencing demonstrated that DLF1 binds the core promoter of two AP1/FUL subfamily MADS-box genes, ZmMADS4 and ZmMADS67, to activate their expression. Knocking out ZmMADS4 and ZmMADS67 both increased leaf number and delayed flowering, indicating that they promote the floral transition. Nucleotide diversity analysis revealed that dlf1 and ZmMADS67 were targeted by selection, suggesting that they may have played important roles in maize flowering time adaptation. We show that dlf1 promotes maize floral transition by directly activating ZmMADS4 and ZmMADS67 in the shoot apex, providing novel insights into the mechanism of maize floral transition.

Evidence that AGL17 is a significant downstream target of CLF in floral transition control

Polycomb-group (PcG) proteins are evolutionarily conserved in higher organisms and play essential roles in many developmental processes by catalyzing the trimethylation of histone H3 lysine 27 (H3K27me3), a key repressive histone mark. In Arabidopsis (Arabidopsis thaliana), histone methyltransferase CURLY LEAF (CLF) is one of the major PcG catalytic components, playing critical roles in plant growth and development, especially the floral transition. We have recently profiled the genome-wide occupancy of CLF by chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq). Interestingly, AGAMOUS-LIKE 17 (AGL17) that encodes a known flowering activator was found to be a CLF direct target. Based on this observation, we set out to examine to what extent this genetic module regulates the flowering. First, we validated the ChIP-seq results by ChIP-qPCR and show that CLF indeed targets AGL17, and the level of H3K27me3 is decreased when CLF is lost. Further, we show that the expression of AGL17 is significantly up-regulated in the clf-29 mutant compared to wild-type (WT). Finally, our clf agl17 double mutant analysis suggests that AGL17 is a significant downstream target of CLF in floral transition control.

The Important Function of Mediator Complex in Controlling the Developmental Transitions in Plants

Developmental transitions in plants are tightly associated with changes in the transcriptional regulation of gene expression. One of the most important regulations is conferred by cofactors of RNA polymerase II including the mediator complex, a large complex with a modular organization. The mediator complex recruits transcription factors to bind to the specific sites of genes including protein-coding genes and non-coding RNA genes to promote or repress the transcription initiation and elongation using a protein-protein interaction module. Mediator complex subunits have been isolated and identified in plants and the function of most mediator subunits in whole life cycle plants have been revealed. Studies have shown that the Mediator complex is indispensable for the regulation of plant developmental transitions by recruiting age-, flowering-, or hormone-related transcription factors. Here, we first overviewed the Mediator subunits in plants, and then we summarized the specific Mediator subunits involved in developmental transitions, including vegetative phase change and floral transition. Finally, we proposed the future directions to further explore their roles in plants. The link between Mediator subunits and developmental transitions implies the necessity to explore targets of this complex as a potential application in developing high quality crop varieties.

CONSTANS-FKBP12 interaction contributes to modulation of photoperiodic flowering in Arabidopsis

Flowering time is a key process in plant development. Photoperiodic signals play a crucial role in the floral transition in Arabidopsis thaliana, and the protein CONSTANS (CO) has a central regulatory function that is tightly regulated at the transcriptional and post-translational levels. The stability of CO protein depends on a light-driven proteasome process that optimizes its accumulation in the evening to promote the production of the florigen FLOWERING LOCUS T (FT) and induce seasonal flowering. To further investigate the post-translational regulation of CO protein we have dissected its interactome network employing in vivo and in vitro assays and molecular genetics approaches. The immunophilin FKBP12 has been identified in Arabidopsis as a CO interactor that regulates its accumulation and activity. FKBP12 and CO interact through the CCT domain, affecting the stability and function of CO. fkbp12 insertion mutants show a delay in flowering time, while FKBP12 overexpression accelerates flowering, and these phenotypes can be directly related to a change in accumulation of FT protein. The interaction is conserved between the Chlamydomonas algal orthologs CrCO-CrFKBP12, revealing an ancient regulatory step in photoperiod regulation of plant development.

Molecular Characterization and Expression Profile of PaCOL1, a CONSTANS-like Gene in Phalaenopsis Orchid

CONSTANS (CO) and CONSTANS-like (COL) genes play important roles in coalescing signals from photoperiod and temperature pathways. However, the mechanism of CO and COLs involved in regulating the developmental stage transition and photoperiod/temperature senescing remains unclear. In this study, we identified a COL ortholog gene from the Taiwan native orchid Phalaenopsis aphrodite. The Phalaenopsis aphrodite CONSTANS-like 1 (PaCOL1) belongs to the B-box protein family and functions in the nucleus and cytosol. Expression profile analysis of Phalaenopsis aphrodite revealed that PaCOL1 was significantly expressed in leaves, but its accumulation was repressed during environmental temperature shifts. We found a differential profile for PaCOL1 accumulation, with peak accumulation at late afternoon and at the middle of the night. Arabidopsis with PaCOL1 overexpression showed earlier flowering under short-day (SD) conditions (8 h/23 °C light and 16 h/23 °C dark) but similar flowering time under long-day (LD) conditions (16 h/23 °C light and 8 h/23 °C dark). Transcriptome sequencing revealed several genes upregulated in PaCOL1-overexpressing Arabidopsis plants that were previously involved in flowering regulation of the photoperiod pathway. Yeast two-hybrid (Y2H) analysis and bimolecular fluorescence complementation (BiFC) analysis revealed that PaCOL1 could interact with a crucial clock-associated regulator, AtCCA1, and a flowering repressor, AtFLC. Furthermore, expressing PaCOL1 in cca1.lhy partially reversed the mutant flowering time under photoperiod treatment, which confirms the role of PaCOL1 function in the rhythmic associated factors for modulating flowering.

Identification and characterisation of a novel FT orthologous gene in London plane with a distinct expression response to environmental stimuli compared to PaFT

FLOWERING LOCUS T (FT) is a key integrator of environmental signals and internal cues, and codes for florigen-like activity which regulates the transition from vegetative to reproductive growth in flowering plants. Unlike annual plants, perennial tree species undergo several years of vegetative growth prior to the transition to the reproductive stage, as characterised by the ability to form flower buds. Thereafter, trees in temperate regions typically display an annual growth cycle involving distinct vegetative growth, flowering and dormancy stages. In London plane (Platanus acerifolia Willd.), a FT-like gene has previously been identified. Here, we report the isolation of a novel FT orthologous gene, PaFTL, and investigate the functions of PaFT and PaFTL through the analysis of expression profiles and transgenic phenotypes. PaFT displayed the highest levels of expression during tree dormancy, and similarly elevated expression levels were seen under conditions of low temperature and short days (LT/SD). In contrast, PaFTL transcripts were up-regulated during the floral transition phase, the early stages of inflorescence development and throughout the main flowering period, whereas expression levels were low and variable during dormancy and in response to LT/SD treatments. Ectopic expression of 35s::PaFTL in tobacco produced a phenotype similar to that with PaFT, namely, advanced floral initiation. Overall, the results suggest that PaFT and PaFTL have both conserved and diverse functions in floral initiation, floral development and dormancy regulation.

Strigolactone represses the synthesis of melatonin, thereby inducing floral transition in Arabidopsis thaliana in an FLC-dependent manner

The transition from vegetative to reproductive growth is a key developmental event in a plant's life cycle. The process is mediated by a combination of phytohormones, including melatonin (MT) and strigolactone (SL). Here, the Arabidopsis mutants, d14-1 and max4-1, which are compromised with respect to either SL synthesis or signaling, were shown to flower earlier than wild types. The tissue MT content in both mutants was higher than in wild types, as a result of the up-regulation of various genes encoding enzymes involved in MT synthesis. The abundance in the mutants of transcripts derived from each of the genes SPLs, AP1, and SOC1 was reduced with exogenously supplied MT, while FLC was induced. Plants exposed to a high concentration of MT did not flower earlier than wild types. The tissue MT content of a mutant unable to synthesize caffeic acid O-methyltransferase was less than that of wild type and flowered earlier than did wild types. The suggestion is that the flowering time of Arabidopsis is altered if the tissue content of MT is either higher than ~ 8 ng/g F.W, or lower than ~ 0.9 ng/g. Within this range, SL acts to determine flowering time by its regulation of SPL genes. The application of exogenous SL reduces tissue MT content. The flowering time of the flc-3 mutant was unaffected by exogenously supplying either MT or/and SL. It is proposed that MT acts downstream of SL to activate FLC, inducing a delay to flowering if its concentration lies outside a certain range.

Genome-wide analysis of MADS-box family genes during flower development in lettuce

Lettuce (Lactuca sativa L.) is an important leafy vegetable consumed worldwide. Heat-induced bolting and flowering greatly limit lettuce production during the summer. Additionally, MADS-box transcription factors are important for various aspects of plant development and architecture (e.g., flowering and floral patterning). However, there has been no comprehensive study of lettuce MADS-box family genes. In this study, we identified 82 MADS-box family genes in lettuce, including 23 type I genes and 59 type II genes. Transcriptome profiling revealed that LsMADS gene expression patterns differ among the various floral stages and organs. Moreover, heat-responsive cis-elements were detected in the promoter regions of many LsMADS genes. An in situ hybridization assay of 10 homologs of flower-patterning genes and a yeast two-hybrid assay of the encoded proteins revealed that the ABC model is conserved in lettuce. Specifically, the APETALA1 (AP1) homolog in lettuce, LsMADS55, is responsive to heat and is specifically expressed in the inflorescence meristem and pappus bristles. The overexpression of LsMADS55 results in early flowering in Arabidopsis thaliana. Furthermore, we observed that the heat shock factor LsHSFB2A-1 can bind to the LsMADS55 promoter in lettuce. Therefore, a model was proposed for the LsMADS-regulated floral organ specification and heat-induced flowering in lettuce.

Dissecting the Function of MADS-Box Transcription Factors in Orchid Reproductive Development

The orchid family (Orchidaceae) represents the second largest angiosperm family, having over 900 genera and 27,000 species in almost all over the world. Orchids have evolved a myriad of intriguing ways in order to survive extreme weather conditions, acquire nutrients, and attract pollinators for reproduction. The family of MADS-box transcriptional factors have been shown to be involved in the control of many developmental processes and responses to environmental stresses in eukaryotes. Several findings in different orchid species have elucidated that MADS-box genes play critical roles in the orchid growth and development. An in-depth understanding of their ecological adaptation will help to generate more interest among breeders and produce novel varieties for the floriculture industry. In this review, we summarize recent findings of MADS-box transcription factors in regulating various growth and developmental processes in orchids, in particular, the floral transition and floral patterning. We further discuss the prospects for the future directions in light of new genome resources and gene editing technologies that could be applied in orchid research and breeding.

Floral Induction and Flower Development of Orchids

Orchids comprise one of the largest, most highly evolved angiosperm families, and form an extremely peculiar group of plants. Various orchids are available through traditional breeding and micro-propagation since they are valuable as potted plants and/or cut flowers in horticultural markets. The flowering of orchids is generally influenced by environmental signals such as temperature and endogenous developmental programs controlled by genetic factors as is usual in many flowering plant species. The process of floral transition is connected to the flower developmental programs that include floral meristem maintenance and floral organ specification. Thanks to advances in molecular and genetic technologies, the understanding of the molecular mechanisms underlying orchid floral transition and flower developmental processes have been widened, especially in several commercially important orchids such as Phalaenopsis, Dendrobium and Oncidium. In this review, we consolidate recent progress in research on the floral transition and flower development of orchids emphasizing representative genes and genetic networks, and also introduce a few successful cases of manipulation of orchid flowering/flower development through the application of molecular breeding or biotechnology tools.

Overexpression of Lilium formosanumMADS-box (LFMADS) Causing Floral Defects While Promoting Flowering in Arabidopsis thaliana, Whereas Only Affecting Floral Transition Time in Nicotiana tabacum

The Formosa lily (Lilium formosanum) is one of the most common horticultural species in Taiwan. To explore gene regulation involved in this species, we used transcriptome analysis to generate PH-FB (mixed floral buds) and PH-LF (mature leaves) datasets. Combination of the PH-FB and PH-LF constructed a de novo assembly of the ALL dataset, including 18,041 contigs and 23,807 unigenes by Nr, GO, COG, and KEGG databases. The differential gene expression (DGE) analysis revealed 9937 genes were upregulated while 10,383 genes were downregulated in the developing floral buds compared to mature leaves. Seven putative genes (LFMADS1 to 7) encoding floral organ identity proteins were selected for further analysis. LFMADS1-6 genes were specifically expressed in the floral organ, while LFMADS7 in the floral buds and mature leaves. Phylogenetic analysis revealed that LFMADS1-3 is classified into B-class, LFMADS4 into C-class, LFMADS5 into D-class, and LFMADS6-7 into E-class, respectively. LFMADS-GFP fusion proteins appeared to localize in the nucleus, supporting their roles as transcription factors (TFs). Overexpression of the LFMADS2, LFMADS4, and LFMADS6 genes in Arabidopsis resulted in early flowering and floral defect, however, only early flowering in transgenic tobacco was observed. Highly expressed floral integrator genes, including AtFT, AtLFY, and AtFUL in transgenic Arabidopsis and NtFUL and NtSOC1 in transgenic tobacco, resulted in early flowering phenotype through qRT-PCR analysis. Yeast two-hybrid analysis suggested that LFMADSs may form higher order complexes with the B-, C-, D, and/or E-class proteins to determine the floral organ identity. Furthermore, E-class LFMADS proteins may function as a glue to mediate and strengthen the protein-protein interactions. Therefore, our de novo datasets would provide information for investigating other differentially expressed candidate transcripts. In addition, functional conservation of LFMADSs appears to be vital in floral transition and floral organ identity.

Array of MADS-Box Genes: Facilitator for Rapid Adaptation?

In a world of global warming, the question emerges whether all plants have suitable mechanisms to keep pace with the rapidly changing environment. Most previous studies have focused on either the ability of plants to rapidly acclimatize via physiological and developmental plasticity, or long-term adaptation over thousands of years. However, we wonder whether plants can also adapt to changes in the environment within only a few generations. We hypothesize that rapidly evolving clusters of tandemly duplicated developmental control genes represent a source for fast adaptation. Specifically, we propose that a tandem cluster of FLC-like MADS-box genes involved in the transition to flowering in Arabidopsis functions as a facilitator for rapid adaptation to changes in ambient temperature.

Control of plant growth and development by overexpressing MAP3K17, an ABA-inducible MAP3K, in Arabidopsis

Abscisic acid (ABA) plays an important role in plant growth, development, and stress responses. ABA regulates many aspects of plant growth and development, including seed maturation, dormancy, germination, the transition from vegetative to reproductive growth, leaf senescence and responses to environmental stresses, such as drought, high salinity and cold. It is also known that mitogen-activated protein kinase (MAPK) cascades function in ABA signaling. Recently, we and another group have identified the ABA-inducible MAP3Ks MAP3K17 and MAP3K18 as the upstream MAP3Ks of MKK3, implicating the MAP3K17/18-MKK3-MPK1/2/7/14 cascade in ABA signaling. It has also been reported that overexpression of MAP3K18 in Arabidopsis causes an early leaf senescence phenotype, ABA hypersensitive stomata closing, and drought tolerance. In this study, we generated transgenic plants overexpressing MAP3K17 (35S:MAP3K17) and its kinase-inactive form (35S:MAP3K17KN). The bolting of 35S:MAP3K17 was earlier than WT, and the fresh weights of the seedlings were smaller, whereas 35S:MAP3K17KN showed the opposite phenotype. These results indicate that the transition from vegetative to reproductive growth can be regulated by overexpression of MAP3K17 and its kinase-inactive form. Moreover, 35S:MAP3K17 showed lower sensitivity to ABA during post-germinated growth, whereas 35S:MAP3K17 KN showed the opposite phenotype, suggesting the negative roles of MAP3K17 in the response to ABA. Our work provides the possibility to regulate plant growth and development by the genetic manipulation of ABA-induced MAPK cascades, leading to improved crop growth and productivity.

Histone lysine methyltransferases BnaSDG8.A and BnaSDG8.C are involved in the floral transition in Brassica napus

Although increasing experimental evidence demonstrates that histone methylations play important roles in Arabidopsis plant growth and development, little information is available regarding Brassica napus. In this study, we characterized two genes encoding homologues of the Arabidopsis histone 3 lysine 36 (H3K36) methyltransferase SDG8, namely, BnaSDG8.A and BnaSDG8.C. Although no duplication of SDG8 homologous genes had been previously reported to occur during the evolution of any sequenced species, a domain-duplication was uncovered in BnaSDG8.C. This duplication led to the identification of a previously unknown NNH domain in the SDG8 homologues, providing a useful reference for future studies and revealing the finer mechanism of SDG8 function. One NNH domain is present in BnaSDG8.A, while two adjacent NNH domains are present in BnaSDG8.C. Reverse transcriptase-quantitative polymerase chain reaction analysis revealed similar patterns but with varied levels of expression of BnaSDG8.A/C in different plant organs/tissues. To directly investigate their function, BnaSDG8.A/C cDNA was ectopically expressed to complement the Arabidopsis mutant. We observed that the expression of either BnaSDG8.A or BnaSDG8.C could rescue the Arabidopsis sdg8 mutant to the wild-type phenotype. Using RNAi and CRISPR/Cas9-mediated gene editing, we obtained BnaSDG8.A/C knockdown and knockout mutants with the early flowering phenotype as compared with the control. Further analysis of two types of the mutants revealed that BnaSDG8.A/C are required for H3K36 m2/3 deposition and prevent the floral transition of B. napus by directly enhancing the H3K36 m2/3 levels at the BnaFLC chromatin loci. This observation on the floral transition by epigenetic modification in B. napus provides useful information for breeding early-flowering varieties.

High Constitutive Overexpression of Glycosyl Hydrolase Family 17 Delays Floral Transition by Enhancing FLC Expression in Transgenic Arabidopsis

Vitis vinifera glycosyl hydrolase family 17 (VvGHF17) is a grape apoplasmic β-1,3-glucanase, which belongs to glycosyl hydrolase family 17 in grapevines. β-1,3-glucanase is not only involved in plant defense response but also has various physiological functions in plants. Although VvGHF17 expression is negatively related to the length of inflorescence in grapevines, the physiological functions of VvGHF17 are still uncertain. To clarify the physiological functions of VvGHF17, we conducted a phenotypic analysis of VvGHF17-overexpressing Arabidopsis plants. VvGHF17-overexpressing Arabidopsis plants showed short inflorescence, similar to grapevines. These results suggested that VvGHF17 might negatively regulate the length of inflorescence in plants. VvGHF17 expression induced a delay of floral transition in Arabidopsis plants. The expression level of FLOWERING LOCUS C (FLC), known as a floral repressor gene, in inflorescence meristem of transgenic plants were increased by approximately 10-fold as compared with wild plants. These results suggest that VvGHF17 induces a delay of floral transition by enhancing FLC expression and concomitantly decreases the length of plant inflorescence.

The SBP-Box Gene VpSBP11 from Chinese Wild Vitis Is Involved in Floral Transition and Affects Leaf Development

Flowering occurs in angiosperms during a major developmental transition from vegetative growth to the reproductive phase. Squamosa promoter binding protein (SBP)-box genes have been found to play critical roles in regulating flower and fruit development, but their roles in grapevine have remained unclear. To better understand the functions of the grape SBP-box genes in both vegetative and reproductive growth phases, a full-length complementary DNA (cDNA) sequence of the putative SBP-box transcription factor gene, VpSBP11, was obtained from Chinese wild grapevine Vitis pseudoreticulata Wen Tsai Wang (W. T. Wang) clone ‘Baihe-35-1’. VpSBP11 encoded a putative polypeptide of 170 amino acids with a highly conserved SBP-domain with two zinc-binding sites of the Cx2C-x3-H-x11-C-x6-H (C2HCH) type and a nuclear localization signal. We confirmed that the VpSBP11 protein was targeted to the nucleus and possessed transcriptional activation activity by subcellular localization and trans-activation assay. Over-expression of VpSBP11 in Arabidopsis thaliana was shown to activate the FUL gene, and subsequently the AP1 and LFY genes, all of which were floral meristem identity genes, and to cause earlier flowering than in wild type (WT) plants. The pattern of vegetative growth was also different between the transgenic and WT plants. For example, in the VpSBP11 over-expressing transgenic plants, the number of rosette leaves was less than that of WT; the petiole was significantly elongated; and the rosette and cauline leaves curled upwards or downwards. These results were consistent with VpSBP11 acting as a transcription factor during the transition from the vegetative stage to the reproductive stage.

ERECTA-family receptor kinase genes redundantly prevent premature progression of secondary growth in the Arabidopsis hypocotyl

Secondary growth is driven by continuous cell proliferation and differentiation of the cambium that acts as vascular stem cells, producing xylem and phloem to expand vascular tissues laterally. During secondary growth of hypocotyls in Arabidopsis thaliana, the xylem undergoes a drastic phase transition from a parenchyma-producing phase to a fiber-producing phase at the appropriate time. However, it remains to be fully elucidated how progression of secondary growth is properly controlled. We focused on phenotypes of hypocotyl vasculatures caused by double mutation in ERECTA (ER) and ER-LIKE1 (ERL1) receptor-kinase genes to elucidate their roles in secondary growth. ER and ERL1 redundantly suppressed excessive radial growth of the hypocotyl vasculature during secondary growth. ER and ERL1 also prevented premature initiation of the fiber differentiation process mediated by the NAC SECONDARY WALL THICKENING PROMOTING FACTORs in the hypocotyl xylem. Upon floral transition, the hypocotyl xylem gained a competency to respond to GA in a BREVIPEDICELLUS-dependent manner, which was a prerequisite for fiber differentiation. However, even after the floral transition, ER and ERL1 prevented precocious initiation of the GA-mediated fiber formation. Collectively, our findings reveal that ER and ERL1 redundantly prevent premature progression of sequential events in secondary growth.

Floral transitions in wheat and barley: interactions between photoperiod, abiotic stresses, and nutrient status

The timing of plant reproduction has a large impact on yield in crop plants. Reproductive development in temperate cereals comprises two major developmental transitions. During spikelet initiation, the identity of the shoot meristem switches from the vegetative to the reproductive stage and spikelet primordia are formed on the apex. Subsequently, floral morphogenesis is initiated, a process strongly affected by environmental variation. Recent studies in cereal grasses have suggested that this later phase of inflorescence development controls floret survival and abortion, and is therefore crucial for yield. Here, we provide a synthesis of the early morphological and the more recent genetic studies on shoot development in wheat and barley. The review explores how photoperiod, abiotic stress, and nutrient signalling interact with shoot development, and pinpoints genetic factors that mediate development in response to these environmental cues. We anticipate that research in these areas will be important in understanding adaptation of cereal grasses to changing climate conditions.

Small RNA-Sequencing Links Physiological Changes and RdDM Process to Vegetative-to-Floral Transition in Apple

Transition from vegetative to floral buds is a critical physiological change during flower induction that determines fruit productivity. Small non-coding RNAs (sRNAs) including microRNAs (miRNAs) and small interfering RNAs (siRNAs) are pivotal regulators of plant growth and development. Although the key role of sRNAs in flowering regulation has been well-described in Arabidopsis and some other annual plants, their relevance to vegetative-to-floral transition (hereafter, referred to floral transition) in perennial woody trees remains under defined. Here, we performed Illumina sequencing of sRNA libraries prepared from vegetative and floral bud during flower induction of the apple trees. A large number of sRNAs exemplified by 33 previously annotated miRNAs and six novel members display significant differential expression (DE) patterns. Notably, most of these DE-miRNAs in floral transition displayed opposite expression changes in reported phase transition in apple trees. Bioinformatics analysis suggests most of the DE-miRNAs targeted transcripts involved in SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) gene regulation, stress responses, and auxin and gibberellin (GA) pathways, with further suggestion that there is an inherent link between physiological stress response and metabolism reprogramming during floral transition. We also observed significant changes in 24 nucleotide (nt) sRNAs that are hallmarks for RNA-dependent DNA methylation (RdDM) pathway, suggestive of the correlation between epigenetic modifications and the floral transition. The study not only provides new insight into our understanding of fundamental mechanism of poorly studied floral transition in apple and other woody plants, but also presents important sRNA resource for future in-depth research in the apple flowering physiology.

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.

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.

Leaf-Like Sepals Induced by Ectopic Expression of a SHORT VEGETATIVE PHASE (SVP)-Like MADS-Box Gene from the Basal Eudicot Epimedium sagittatum

Epimedium L. (Berberidaceae, Ranales), a perennial traditional Chinese medicinal herb, has become a new popular landscape plant for ground cover and pot culture in many countries based on its excellent ornamental characteristics and, distinctive and diverse floral morphology. However, little is known about the molecular genetics of flower development in Epimedium sagittatum. Here, we describe the characterization of EsSVP that encodes a protein sharing 68, 54, and 35% similarity with SVP, AGAMOUS-like 24 (AGL24) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) in Arabidopsis, respectively. Quantitative RT-PCR (qRT-PCR) indicated that EsSVP transcripts were principally found in petiole and leaf tissues, with little expression in roots and flowers and no in fruits. The highest EsSVP expression was observed in leaves. The flowering time of 35S::EsSVP in most Arabidopsis thaliana and in all petunia plants was not affected in both photoperiod conditions, but 35S::EsSVP 5# and 35S::EsSVP 1# Arabidopsis lines induced late and early flowering under long day (LD, 14 h light/10 h dark) and short day (SD, 10 h light/14 h dark) conditions, respectively. The 35S::EsSVP Arabidopsis produced extra secondary inflorescence or floral meristems in the axils of the leaf-like sepals with excrescent trichomes, and leaf-like sepals not able to enclose the inner three whorls completely. Moreover, almost all transgenic Arabidopsis plants showed persistent sepals around the completely matured fruits. Upon ectopic expression of 35S::EsSVP in Petunia W115, sepals were enlarged, sometimes to the size of leaves; corollas were greenish and did not fully open. These results suggest that EsSVP is involved in inflorescence meristem identity and flowering time regulation in some conditions. Although, the SVP homologs might have suffered functional diversification among diverse species between core and basal eudicots, the protein functions are conserved between Arabidopsis/Petunia and Epimedium.

CHD3 chromatin-remodeling factor PICKLE regulates floral transition partially via modulating LEAFY expression at the chromatin level in Arabidopsis

PICKLE (PKL), a putative CHD3 chromatin remodeling factor, has been suggested to be involved in multiple processes in Arabidopsis. Here, we confirmed the late-flowering phenotype caused by pkl mutation with pkl mutants in two different ecotypes, and investigated the possible mechanisms that account for PKL regulation of flowering time. Quantitative RT-PCR and RNA-seq assays showed that expression of the LEAFY gene (LFY) and a number of LFY-regulated floral homeotic genes were down-regulated in seedlings of the pkl mutants. As predicted, overexpression of LFY restored normal flowering time of pkl mutants. Our results suggest that PKL may be involved in regulating flowering time via LFY expression. To uncover the underlying mechanism, ChIP-PCR using anti-PKL was performed on materials from three developmental stages of seedlings. Our results showed that PKL associated with the genomic sequences of LFY, particularly at 10-day and 25-day after germination. We also showed that loss of PKL affected H3K27me3 level at the promoter of LFY. Taken together, our data suggest that transcriptional regulation of LFY at the chromatin level by PKL may at least partially account for the late-flowering phenotype of pkl mutants.

Melatonin mediates the stabilization of DELLA proteins to repress the floral transition in Arabidopsis

Precise floral transition from vegetative growth phase to reproductive growth phase is very important in plant life cycle. In flowering genetic pathways, DELLA proteins are master transcriptional regulators of gibberelic acid (GA) pathway, and FLOWERING LOCUS C (FLC) is a core repressor of vernalization pathway as well as downstream of DELLAs. As a crucial messenger in plants, the possible involvement of melatonin (N-acetyl-5-methoxytryptamine) in flowering and underlying molecular mechanism are unknown in Arabidopsis. In this study, we found that exogenous melatonin treatment delayed floral transition in Arabidopsis. Exogenous melatonin treatment conferred protein stabilizations of DELLAs [REPRESSOR of ga1-3 (RGA) and RGA-LIKE3 (RGL3)], without regulating the transcripts of DELLAs and endogenous GA level. Notably, exogenous melatonin delayed plant flowering and DELLA-activated transcripts of FLC were alleviated in della mutants, and those were exacerbated in DELLA overexpressing plants. Taken together, this study provides direct link between melatonin and floral transition, and indicates the novel involvement of DELLAs-activated FLC in melatonin-mediated flowering in Arabidopsis.

Molecular Characterization of FT and FD Homologs from Eriobotrya deflexa Nakai forma koshunensis

In angiosperms, regulation of flowering is a vital process for successful reproduction. To date, the molecular mechanism of flowering is well-studied in the model plant, Arabidopsis, in which key genes such as FLOWERING LOCUST (FT) or FD have been identified to regulate flowering. However, the flowering mechanisms are still largely unknown in fruit trees like loquat. To this end, we first cloned one FT- and two FD-like genes from the loquat (Eriobotrya deflexa Nakai f. koshunensis) and referred to as EdFT, EdFD1, and EdFD2, respectively. Phylogenetic analysis has shown that EdFT, EdFD1, and EdFD2 are conserved during the evolution process. EdFT is mainly expressed in reproductive tissues (e.g., flower buds, flowers, and fruits), while EdFD1 and EdFD2 are mainly expressed in apical buds including leaf buds and flower buds. EdFT is localized in the whole cell, while EdFD1 or EdFD2 is localized in the nucleus. Ectopic expression of EdFT, EdFD1, and EdFD2 in Arabidopsis results in early flowering. In addition, we have also revealed that the EdFT interacts with both EdFD1 and EdFD2. Overall, these data suggest that the EdFT, EdFD1, and EdFD2 are the functional homologs of FT and FD, respectively, which might act together to regulate loquat flowering through a similar mechanism found in Arabidopsis.