Flowering time regulation: photoperiod- and temperature-sensing in leaves

Methylation in DNA, histone, and RNA during flowering under stress condition: A review

Flowering is the most critical transition period in the whole lifecycle of plants, and it is a highly sensitive period to stress. New combinations of temperature, drought stress, carbon dioxide and other abiotic/biotic conditions resulting from contemporary climate change affect the flowering process. Plants have evolved several strategies to deal with environmental stresses, including epigenetic modifications. Numerous studies show that environmental stresses trigger methylation/demethylation during flowering to preserve/accelerate plant lifecycle. What's more, histone and DNA methylation can be induced to respond to stresses, resulting in changes of flowering gene expression and enhancing stress tolerance in plants. Furthermore, RNA methylation may influence stress-regulated flowering by regulating mRNA stability and antioxidant mechanism. Our review presents the involvement of methylation in stress-repressed and stress-induced flowering. The crosstalk between methylation and small RNAs, phytohormones and exogenous substances (such as salicylic acid, nitric oxide) during flowering under different stresses were discussed. The latest regulatory evidence of RNA methylation in stress-regulated flowering was collected for the first time. Meanwhile, the limited evidences of methylation in biotic stress-induced flowering were summarized. Thus, the review provides insights into understanding of methylation mechanism in stress-regulated flowering and makes use for the development of regulating plant flowering at epigenetic level in the future.

Transcriptome Analysis Reveals Putative Induction of Floral Initiation by Old Leaves in Tea-Oil Tree (Camellia oleifera ‘changlin53’)

Floral initiation is a major phase change in the spermatophyte, where developmental programs switch from vegetative growth to reproductive growth. It is a key phase of flowering in tea-oil trees that can affect flowering time and yield, but very little is known about the molecular mechanism of floral initiation in tea-oil trees. A 12-year-old Camellia oleifera (cultivar ‘changlin53’) was the source of experimental materials in the current study. Scanning electron microscopy was used to identify the key stage of floral initiation, and transcriptome analysis was used to reveal the transcriptional regulatory network in old leaves involved in floral initiation. We mined 5 DEGs related to energy and 55 DEGs related to plant hormone signal transduction, and we found floral initiation induction required a high level of energy metabolism, and the phytohormones signals in the old leaves regulate floral initiation, which occurred at stage I and II. Twenty-seven rhythm-related DEGs and 107 genes associated with flowering were also identified, and the circadian rhythm interacted with photoperiod pathways to induce floral initiation. Unigene0017292 (PSEUDO-RESPONSE REGULATOR), Unigene0046809 (LATE ELONGATED HYPOCOTYL), Unigene0009932 (GIGANTEA), Unigene0001842 (CONSTANS), and Unigene0084708 (FLOWER LOCUS T) were the key genes in the circadian rhythm-photoperiod regulatory network. In conjunction with morphological observations and transcriptomic analysis, we concluded that the induction of floral initiation by old leaves in C. oleifera ‘changlin53’ mainly occurred during stages I and II, floral initiation was completed during stage III, and rhythm–photoperiod interactions may be the source of the main signals in floral initiation induced by old leaves.

Dark secrets of phytomelatonin

Phytomelatonin is a newly identified plant hormone, and its primary functions in plant growth and development remain relatively poorly appraised. Phytomelatonin is a master regulator of reactive oxygen species (ROS) signaling and acts as a darkness signal in circadian stomatal closure. Plants exhibit at least three interrelated patterns of interaction between phytomelatonin and ROS production. Exogenous melatonin can induce flavonoid biosynthesis, which might be required for maintenance of antioxidant capacity under stress, after harvest, and in leaf senescence conditions. However, several genetic studies have provided direct evidence that phytomelatonin plays a negative role in the biosynthesis of flavonoids under non-stress conditions. Phytomelatonin delays flowering time in both dicot and monocot plants, probably via its receptor PMTR1 and interactions with the gibberellin, strigolactone, and ROS signaling pathways. Furthermore, phytomelatonin signaling also functions in hypocotyl and shoot growth in skotomorphogenesis and ultraviolet B (UV-B) exposure; the G protein α-subunit (Arabidopsis GPA1 and rice RGA1) and constitutive photomorphogenic1 (COP1) are important signal components during this process. Taken together, these findings indicate that phytomelatonin acts as a darkness signal with important regulatory roles in circadian stomatal closure, flavonoid biosynthesis, flowering, and hypocotyl and shoot growth.

Genomic analyses of rice bean landraces reveal adaptation and yield related loci to accelerate breeding

Rice bean (Vigna umbellata) is an underexploited domesticated legume crop consumed for dietary protein in Asia, yet little is known about the genetic diversity of this species. Here, we present a high-quality reference genome for a rice bean landrace (FF25) built using PacBio long-read data and a Hi-C chromatin interaction map, and assess the phylogenetic position and speciation time of rice bean within the Vigna genus. We sequence 440 landraces (two core collections), and GWAS based on data for growth sites at three widely divergent latitudes reveal loci associated with flowering and yield. Loci harboring orthologs of FUL (FRUITFULL), FT (FLOWERING LOCUS T), and PRR3 (PSEUDO-RESPONSE REGULATOR 3) contribute to the adaptation of rice bean from its low latitude center of origin towards higher latitudes, and the landraces which pyramid early-flowering alleles for these loci display maximally short flowering times. We also demonstrate that copy-number-variation for VumCYP78A6 can regulate seed-yield traits. Intriguingly, 32 landraces collected from a mountainous region in South-Central China harbor a recently acquired InDel in TFL1 (TERMINAL FLOWER1) affecting stem determinacy; these materials also have exceptionally high values for multiple human-desired traits and could therefore substantially advance breeding efforts to improve rice bean.

Rice bean is an underexploited legume crop that has many desirable properties against bio and abiotic stresses. Here, the authors report the genome assembly of this species, conduct population genetics studies and reveal the genetic variations associated with adaptation and yield traits.

The StBBX24 protein affects the floral induction and mediates salt tolerance in Solanum tuberosum

The transition from vegetative growth to reproductive development is a critical developmental switch in flowering plants to ensure a successful life cycle. However, while the genes controlling flowering are well-known in model plants, they are less well-understood in crops. In this work, we generated potato lines both silenced and overexpressed for the expression of StBBX24, a clock-controlled gene encoding a B-box protein located in the cytosol and nuclear chromatin fraction. We revealed that Solanum tuberosum lines silenced for StBBX24 expression displayed much earlier flowering than wild-type plants. Conversely, plants overexpressing StBBX24 mostly did not produce flower buds other than wild-type plants. In addition, RT-qPCR analyses of transgenic silenced lines revealed substantial modifications in the expression of genes functioning in flowering. Furthermore, S. tuberosum lines silenced for StBBX24 expression displayed susceptibility to high salinity with a lower capacity of the antioxidant system and strongly decreased expression of genes encoding Na+ transporters that mediate salt tolerance, contrary to the plants with StBBX24 overexpression. Altogether, these data reveal that StBBX24 participates in potato flowering repression and is involved in salt stress response.

Protein Kinase MpYAK1 Is Involved in Meristematic Cell Proliferation, Reproductive Phase Change and Nutrient Signaling in the Liverwort Marchantia polymorpha

Plant growth and development are regulated by environmental factors, including nutrient availability and light conditions, via endogenous genetic signaling pathways. Phosphorylation-dependent protein modification plays a major role in the regulation of cell proliferation in stress conditions, and several protein kinases have been shown to function in response to nutritional status, including dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs). Although DYRKs are widely conserved in eukaryotes, the physiological functions of DYRKs in land plants are still to be elucidated. In the liverwort Marchantia polymorpha, a model bryophyte, four putative genes encoding DYRK homologous proteins, each of which belongs to the subfamily yet another kinase 1 (Yak1), plant-specific DYRK, DYRK2, or pre-mRNA processing protein 4 kinase, were identified. MpYAK1-defective male and female mutant lines generated by the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9) system showed smaller sizes of thalli than did the wild-type plants and repressed cell divisions in the apical notch regions. The Mpyak1 mutants developed rhizoids from gemmae in the gemma cup before release. The Mpyak1 lines developed sexual organs even in non-inductive short-day photoperiod conditions supplemented with far-red light. In nitrogen (N)-deficient conditions, rhizoid elongation was inhibited in the Mpyak1 mutants. In conditions of aeration with 0.08% CO2 (v/v) and N depletion, Mpyak1 mutants accumulated higher levels of sucrose and lower levels of starch compared to the wild type. Transcriptomic analyses revealed that the expression of peroxidase genes was differentially affected by MpYAK1. These results suggest that MpYAK1 is involved in the maintenance of plant growth and developmental responses to light conditions and nutrient signaling.

A 215-bp indel at intron I of BoFLC2 affects flowering time in Brassica oleracea var. capitata during vernalization

In response to cold, a 215-bp deletion at intron I of BoFLC2 slows its silencing activity by feedback to the core genes of the PHD-PRC2 complex, resulting in late flowering in cabbage. Cabbage is a plant-vernalization-responsive flowering type. In response to cold, BoFLC2 is an important transcription factor, which allows cabbage plants to remain in the vegetative phase. However, there have been few reports on the detailed and functional effects of genetic variation in BoFLC2 on flowering time in cabbage. Herein, BoFLC2^E and BoFLC2^L, cloned from extremely early and extremely late flowering cabbages, respectively, exhibited a 215-bp indel at intron I, three non-synonymous SNPs and a 3-bp indel at exon II. BoFLC2^L was found to be related to late flowering, as verified in 40 extremely early/late flowering accessions, a diverse set of cabbage inbred lines and two F2 generations by using indel-FLC2 marker. Among the genetic variation of BoFLC2, the 215-bp deletion at intron I was the main reason for the delayed flowering time, as verified in the transgenic progenies of seed-vernalization-responsive Arabidopsis thaliana (Col) and rapid cycler B. oleracea (TO1000, boflc2). This is the first report to show that the intron I indel of BoFLC2 affects the flowering time of cabbage. Although the intron I 215-bp indel between BoFLC2^E and BoFLC2^L did not cause alternative splicing, it slowed BoFLC2^L silencing during vernalization and feedback to the core genes of the PHD-PRC2 complex, resulting in their lower transcription levels. Our study not only provides an effective molecular marker-assisted selective strategy for identifying bolting-resistant resources and breeding improved varieties in cabbage, but also provides an entry point for exploring the mechanisms of flowering time in plant-vernalization-responsive plants.

Clock-Controlled and Cold-Induced CYCLING DOF FACTOR6 Alters Growth and Development in Arabidopsis

The circadian clock represents a critical regulatory network, which allows plants to anticipate environmental changes as inputs and promote plant survival by regulating various physiological outputs. Here, we examine the function of the clock-regulated transcription factor, CYCLING DOF FACTOR 6 (CDF6), during cold stress in Arabidopsis thaliana. We found that the clock gates CDF6 transcript accumulation in the vasculature during cold stress. CDF6 mis-expression results in an altered flowering phenotype during both ambient and cold stress. A genome-wide transcriptome analysis links CDF6 to genes associated with flowering and seed germination during cold and ambient temperatures, respectively. Analysis of key floral regulators indicates that CDF6 alters flowering during cold stress by repressing photoperiodic flowering components, FLOWERING LOCUS T (FT), CONSTANS (CO), and BROTHER OF FT (BFT). Gene ontology enrichment further suggests that CDF6 regulates circadian and developmental-associated genes. These results provide insights into how the clock-controlled CDF6 modulates plant development during moderate cold stress.

Genomewide Identification and Characterization of the Genes Involved in the Flowering of Cotton

Flowering is a prerequisite for flowering plants to complete reproduction, and flowering time has an important effect on the high and stable yields of crops. However, there are limited reports on flowering-related genes at the genomic level in cotton. In this study, genomewide analysis of the evolutionary relationship of flowering-related genes in different cotton species shows that the numbers of flowering-related genes in the genomes of tetraploid cotton species Gossypium hirsutum and Gossypium barbadense were similar, and that these numbers were approximately twice as much as the number in diploid cotton species Gossypium arboretum. The classification of flowering-related genes shows that most of them belong to the photoperiod and circadian clock flowering pathway. The distribution of flowering-related genes on the chromosomes of the At and Dt subgenomes was similar, with no subgenomic preference detected. In addition, most of the flowering-related core genes in Arabidopsis thaliana had homologs in the cotton genome, but the copy numbers and expression patterns were disparate; moreover, flowering-related genes underwent purifying selection throughout the evolutionary and selection processes. Although the differentiation and reorganization of many key genes of the cotton flowering regulatory network occurred throughout the evolutionary and selection processes, most of them, especially those involved in the important flowering regulatory networks, have been relatively conserved and preferentially selected.

ORANGE negatively regulates flowering time in Arabidopsisthaliana

Floral transition is an important process in plant development, which is regulated by at least four flowering pathways: the photoperiod, vernalization, autonomous, and gibberellin (GA)-dependent pathways. The DnaJ-like zinc finger domain-containing protein ORANGE (OR) was originally cloned from the cauliflower or mutant, which has distinct phenotypes of the carotenoid-accumulating curd, the elongated petioles, and the delayed-flowering time. OR has been demonstrated to interact with phytoene synthase for carotenoid biosynthesis in plastids and with eukaryotic release factor 1-2 (eRF1-2) in the nucleus for the first two phenotypes, respectively. In this study, we showed that overexpression of OR in Arabidopsis thaliana resulted in a delayed-flowering phenotype resembling the cauliflower or mutant. Our results indicated that OR negatively regulates the expression of the flowering integrator genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1). Both GA₃ and vernalization treatments could not rescue the delayed-flowering phenotype of the OR-overexpressing seedlings, suggesting the repression of floral transition by OR does not depend on SOC1-mediated vernalization or GA-dependent pathways. Moreover, our analysis revealed that transcripts of OR and FT fluctuated in opposite directions diurnally, and the overexpression of OR repressed the accumulation of CONSTANS (CO), FT, and SOC1 transcripts in a 16 h/8 h light/dark long-day cycle. Our results indicated the possibility that OR represses flowering through the CO-FT-SOC1-mediated photoperiodic flowering pathway.

Transcriptome Analysis of Short-Day Photoperiod Inducement in Adzuki Bean (Vigna angularis L.) Based on RNA-Seq

The flowering characteristics of adzuki bean are influenced by several environmental factors. Light is an important ecological factor that induces flowering in adzuki bean, but to date, there have been few reports on the transcriptomic features of photoperiodic regulation of adzuki bean flowering. This study is based on RNA sequencing (RNA-seq) techniques to elucidate the expression of light-related regulatory genes under short-day photoperiod inducement of adzuki bean flowering, providing an important theoretical basis for its accelerated breeding. Short-day photoperiod inducement of 10 h was conducted for 5 day, 10 day, and 15 day periods on "Tang shan hong xiao dou" varieties, which are more sensitive to short-day photoperiod conditions than the other varieties. Plants grown under natural light (14.5 h) for 5 days, 10 days, and 15 days were used as controls to compare the progress of flower bud differentiation and flowering characteristics. The topmost unfolded functional leaves were selected for transcriptome sequencing and bioinformatics analysis. The short-day photoperiod inducement promoted flower bud differentiation and advanced flowering time in adzuki bean. Transcriptomic analysis revealed 5,608 differentially expressed genes (DEGs) for the combination of CK-5d vs. SD-5d, CK-10d vs. SD-10d, and CK-15d vs. SD-15d. The three groups of the DEGs were analyzed using the Gene Ontology (GO) and the Kyoto Encyclopedia of Genomes and Genomes (KEGG) databases; the DEGs were associated with flowering, photosystem, and the circadian rhythm and were mainly concentrated in the hormone signaling and metabolism, circadian rhythm, and antenna protein pathways; So, 13 light-related genes across the three pathways were screened for differential and expression characteristics. Through the functional annotations of orthologs, these genes were related to flowering, which were supposed to be good candidate genes in adzuki bean. The findings provide a deep understanding of the molecular mechanisms of adzuki bean flowering in response to short-day photoperiod inducement, which laid a foundation for the functional verification of genes in the next step, and provide an important reference for the molecular breeding of adzuki bean.

The rice transcription factor Nhd1 regulates root growth and nitrogen uptake by activating nitrogen transporters

Plants adjust root architecture and nitrogen (N) transporter activity to meet the variable N demand, but their integrated regulatory mechanism remains unclear. We have previously reported that a floral factor in rice (Oryza sativa), N-mediated heading date-1 (Nhd1), regulates flowering time. Here, we show that Nhd1 can directly activate the transcription of the high-affinity ammonium (NH4+) transporter 1;3 (OsAMT1;3) and the dual affinity nitrate (NO3-) transporter 2.4 (OsNRT2.4). Knockout of Nhd1 inhibited root growth in the presence of NO3- or a low concentration of NH4+. Compared to the wild-type (WT), nhd1 and osamt1;3 mutants showed a similar decrease in root growth and N uptake under low NH4+ supply, while nhd1 and osnrt2.4 mutants showed comparable root inhibition and altered NO3- translocation in shoots. The defects of nhd1 mutants in NH4+ uptake and root growth response to various N supplies were restored by overexpression of OsAMT1;3 or OsNRT2.4. However, when grown in a paddy field with low N availability, nhd1 mutants accumulated more N and achieved a higher N uptake efficiency (NUpE) due to the delayed flowering time and prolonged growth period. Our findings reveal a molecular mechanism underlying the growth duration-dependent NUpE.

The New Variation in the Promoter Region of FLOWERING LOCUS T Is Involved in Flowering in Brassica rapa

Flowering time is an important agronomic trait in Brassica rapa and has a wide range of variation. The change from vegetative to reproductive development is a major transition period, especially in flowering vegetable crops. In this study, two non-heading Chinese cabbage varieties with significantly different flowering times, Pak-choi (B. rapa var. communis Tesn et Lee) and Caitai (B. rapa var. tsaitai Hort.), were used to construct segregated F₂ populations. The bulk-segregant approach coupled with whole genome re-sequencing was used for QTL sequencing (QTL-seq) analysis to map flowering time traits. The candidate genes controlling flowering time in B. rapa were predicted by homologous gene alignment and function annotation. The major-effect QTL ft7.1 was detected on chromosome A07 of B. rapa, and the FT family gene BrFT was predicted as the candidate gene. Moreover, a new promoter regional difference of 1577 bp was revealed by analyzing the sequence of the BrFT gene. The promoter region activity analysis and divergent gene expression levels indicated that the difference in the promoter region may contribute to different flowering times. These findings provide insights into the mechanisms underlying the flowering time in Brassica and the candidate genes regulating flowering in production.

Proteolytic Activation of Plant Membrane-Bound Transcription Factors

Due to the presence of a transmembrane domain, the subcellular mobility plan of membrane-bound or membrane-tethered transcription factors (MB-TFs) differs from that of their cytosolic counterparts. The MB-TFs are mostly locked in (sub)cellular membranes, until they are released by a proteolytic cleavage event or when the transmembrane domain (TMD) is omitted from the transcript due to alternative splicing. Here, we review the current knowledge on the proteolytic activation mechanisms of MB-TFs in plants, with a particular focus on regulated intramembrane proteolysis (RIP), and discuss the analogy with the proteolytic cleavage of MB-TFs in animal systems. We present a comprehensive inventory of all known and predicted MB-TFs in the model plant Arabidopsis thaliana and examine their experimentally determined or anticipated subcellular localizations and membrane topologies. We predict proteolytically activated MB-TFs by the mapping of protease recognition sequences and structural features that facilitate RIP in and around the TMD, based on data from metazoan intramembrane proteases. Finally, the MB-TF functions in plant responses to environmental stresses and in plant development are considered and novel functions for still uncharacterized MB-TFs are forecasted by means of a regulatory network-based approach.

RAD-Seq-Based High-Density Linkage Maps Construction and Quantitative Trait Loci Mapping of Flowering Time Trait in Alfalfa (Medicago sativa L.)

Alfalfa (Medicago sativa L.) is a perennial forage crop known as the "Queen of Forages." To dissect the genetic mechanism of flowering time (FT) in alfalfa, high-density linkage maps were constructed for both parents of an F1 mapping population derived from a cross between Cangzhou (P1) and ZhongmuNO.1 (P2), consisting of 150 progenies. The FT showed a transgressive segregation pattern in the mapping population. A total of 13,773 single-nucleotide polymorphism markers was obtained by using restriction-site associated DNA sequencing and distributed on 64 linkage groups, with a total length of 3,780.49 and 4,113.45 cM and an average marker interval of 0.58 and 0.59 cM for P1 and P2 parent, respectively. Quantitative trait loci (QTL) analyses were performed using the least square means of each year as well as the best linear unbiased prediction values across 4 years. Sixteen QTLs for FT were detected for P1 and 22 QTLs for P2, accounting for 1.40-16.04% of FT variation. RNA-Seq analysis at three flowering stages identified 5,039, 7,058, and 7,996 genes that were differentially expressed between two parents, respectively. Based on QTL mapping, DEGs analysis, and functional annotation, seven candidate genes associated with flowering time were finally detected. This study discovered QTLs and candidate genes for alfalfa FT, making it a useful resource for breeding studies on this essential crop.

Characterization of Phytohormones and Transcriptomic Profiling of the Female and Male Inflorescence Development in Manchurian Walnut (Juglans mandshurica Maxim.)

Flowers are imperative reproductive organs and play a key role in the propagation of offspring, along with the generation of several metabolic products in flowering plants. In Juglans mandshurica, the number and development of flowers directly affect the fruit yield and subsequently its commercial value. However, owing to the lack of genetic information, there are few studies on the reproductive biology of Juglans mandshurica, and the molecular regulatory mechanisms underlying the development of female and male inflorescence remain unclear. In this study, phytohormones and transcriptomic sequencing analyses at the three stages of female and male inflorescence growth were performed to understand the regulatory functions underlying flower development. Gibberellin is the most dominant phytohormone that regulates flower development. In total, 14,579 and 7188 differentially expressed genes were identified after analyzing the development of male and female flowers, respectively, wherein, 3241 were commonly expressed. Enrichment analysis for significantly enriched pathways suggested the roles of MAPK signaling, phytohormone signal transduction, and sugar metabolism. Genes involved in floral organ transition and flowering were obtained and analyzed; these mainly belonged to the M-type MADS-box gene family. Three flowering-related genes (SOC1/AGL20, ANT, and SVP) strongly interacted with transcription factors in the co-expression network. Two key CO genes (CO3 and CO1) were identified in the photoperiod pathway. We also identified two GA20xs genes, one SVP gene, and five AGL genes (AGL8, AGL9, AGL15, AGL19, and AGL42) that contributed to flower development. The findings are expected to provide a genetic basis for the studies on the regulatory networks and reproductive biology in inflorescence development for J. mandshurica.

Different pruning level effects on flowering period and chlorophyll fluorescence parameters of Loropetalum chinense var. rubrum

"Pruning" is a simple and efficient way to control the flowering period, but it is rarely used in perennial woody ornamental plants. In this paper, Loropetalum chinense var. rubrum was pruned in different degrees, and the relationship between pruning intensity and flowering number, and flowering time and chlorophyll fluorescence parameters were compared. After statistics, it was found that pruning could advance blossoms of L. chinense var. rubrum; also, light and heavy cutting could both obtain a larger number of flowers. In addition, through correlation analysis, it was found that during the flowering period, the Rfd parameter of the unpruned treatment had a very significant positive correlation with the number of flowers FN, which was 0.81. In other pruning treatment groups, Rfd and FN also presented a certain positive correlation, indicating that the Rfd parameter can be used to predict the number of flowers during the flowering process of L. chinense var. rubrum. The research results provided a new idea for the regulation of the flowering period of L. chinense var. rubrum and other woody ornamental plants and laid the foundation for the diversified application of L. chinense var. rubrum.

Comparative transcriptome analysis provides insight into the molecular mechanisms of long-day photoperiod in Moringa oleifera

Moringa oleifera, is commonly cultivated as a vegetable in tropical and subtropical regions because of nutritional and medicinal benefits of its fruits, immature pods, leaves, and flowers. Flowering at the right time is one of the important traits for crop yield in M.oleifera. Under normal conditions, photoperiod is one of the key factors in determining when plant flower. However, the molecular mechanism underlying the effects of a long-day photoperiod on Moringa is not clearly understood. In the present study, deep RNA sequencing and sugar metabolome were conducted of Moringa leaves under long-day photoperiod. As a result, differentially expressed genes were significantly associated with starch and sucrose pathway and the circadian rhythm-plant pathway. In starch and sucrose pathway, sucrose, fructose, trehalose, glucose, and maltose exhibited pronounced rhythmicity over 24 h, and TPS (trehalose-6-phosphate synthase) genes constituted key regulatory genes. In an Arabidopsis overexpression line hosting the MoTPS1 or MoTPS2 genes, flowering occurred earlier under a short-day photoperiod. These results will support molecular breeding of Moringa and may help clarify to genetic architecture of long-day photoperiod related traits.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01186-4.

Assessing Flowering Time Under Different Photoperiods

Flowering time is one of the most important developmental transitions in plants, especially in annuals such as Arabidopsis thaliana. However, flowering is also a critical agronomic trait, as it impacts the level of vegetative biomass produced (e.g., leaves) or the amount of seed (grain) generated. Therefore, uncovering flowering phenotypes would help understand the impact of any regulatory network on the overall plant life cycle, since flowering integrates multiple cues, both environmental (e.g., photoperiod, temperature) and internal (e.g., induction/repression of specific genes, phytohormone accumulation, plant age). Although the photoperiod flowering pathway has been extensively studied, and its gene circuitry characterized in great detail, specific flowering time protocols are mostly accessible to specialized laboratories in this field. In this report, we address this knowledge gap by generating a reproducible, non-expensive, and step-by-step protocol to assess flowering time under different photoperiods. We provide a comprehensive description and highlight the major pitfalls in the process. Moreover, this protocol could be expanded to include temperature changes and thus contribute to assess the impact of both environmental conditions in the plant's decision to flower.

The tomato OST1-VOZ1 module regulates drought-mediated flowering

Flowering is a critical agricultural trait that substantially affects tomato fruit yield. Although drought stress influences flowering time, the molecular mechanism underlying drought-regulated flowering in tomato remains elusive. In this study, we demonstrated that loss of function of tomato OPEN STOMATA 1 (SlOST1), a protein kinase essential for abscisic acid (ABA) signaling and abiotic stress responses, lowers the tolerance of tomato plants to drought stress. slost1 mutants also exhibited a late flowering phenotype under both normal and drought stress conditions. We also established that SlOST1 directly interacts with and phosphorylates the NAC (NAM, ATAF and CUC)-type transcription factor VASCULAR PLANT ONE-ZINC FINGER 1 (SlVOZ1), at residue serine 67, thereby enhancing its stability and nuclear translocation in an ABA-dependent manner. Moreover, we uncovered several SlVOZ1 binding motifs from DNA affinity purification sequencing analyses and revealed that SlVOZ1 can directly bind to the promoter of the major flowering-integrator gene SINGLE FLOWER TRUSS to promote tomato flowering transition in response to drought. Collectively, our data uncover the essential role of the SlOST1-SlVOZ1 module in regulating flowering in response to drought stress in tomato and offer insights into a novel strategy to balance drought stress response and flowering.

Genetic characterization of flowering and phytochrome genes in peanut (Arachis hypogaea L.) for early maturity

BACKGROUND

Peanut (Arachis hypogaea L.) production and cropping pattern is highly influenced by the climatic factors including temperature and rain pattern fluctuations. It is one of the most important cash crop in the rain fed areas of Pakistan and its production, under changing climatic conditions, that can be improved by developing short duration varieties. The present study was based on the molecular characterization of the maturity associated gene families in the peanut under two light conditions.

METHODS AND RESULTS

Genomic analysis based on the in silico study of important gene families for early maturity associated attributes like flowering time, their pattern, duration and photoperiodism was done for a comprehensive mapping of maturity related genes. Phytochromes genes Phy A, Phy B and Phy E and flowering genes FT2a, Ft5a and COL2 were selected for in silico characterization for protein based analysis including Multiple Sequence Alignment (MSA), and Neighbor Joining (NJ) tree. MSA and NJ trees of the peanut with Arabidopsis thaliana and Glycine max showed a clear picture of the phylogenetic relationship on the basis of selected gene proteins. Expression profile of phytochrome and flowering genes revealed that photoperiod conditions i.e. short and long days, have great influence on the Phy A, Phy B and Phy E, Ft2a, FT5a and COL2 gene expression pattern. In current study, the relative expression of all studied genes was found higher in short day light condition at flower initiation stage of the plants than in the long light day condition with exception of COL2 gene protein.

CONCLUSIONS

The molecular characterization based on the in silico study of the particular genes and qPCR based gene expression profiling of the selected genes provided an evidence of the role of these genes and their comparative analysis under two photoperiodic conditions.

The chromatin remodelling ATPase BRAHMA interacts with GATA-family transcription factor GNC to regulate flowering time in Arabidopsis

BRAHMA (BRM) is the ATPase of the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodelling complex, which is indispensable for transcriptional inhibition and activation, associated with vegetative and reproductive development in Arabidopsis thaliana. Here, we show that BRM directly binds to the chromatin of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), which integrates multiple flowering signals to regulate floral transition, leading to flowering. In addition, genetic and molecular analysis showed that BRM interacts with GNC (GATA, NITRATE-INDUCIBLE, CARBON METABOLISM INVOLVED), a GATA transcription factor that represses flowering by directly repressing SOC1 expression. Furthermore, BRM is recruited by GNC to directly bind to the chromatin of SOC1. The transcript level of SOC1 is elevated in brm-3, gnc, and brm-3/gnc mutants, which is associated with increased histone H3 lysine 4 tri-methylation (H3K4Me3) but decreased DNA methylation. Taken together, our results indicate that BRM associates with GNC to regulate SOC1 expression and flowering time.

Identification and Functional Characterization of FLOWERING LOCUS T in Platycodon grandiflorus

Platycodon grandiflorus roots have been used as a foodstuff and traditional medicine for thousands of years in East Asia. In order to increase the root development of P. grandiflorus, cultivators removed the inflorescences, suggesting the possible negative effect of flowering on root development. This indicates that the genetic improvement of P. grandiflorus by late flowering is a potential approach to increase productivity. However, nothing is known about key genes integrating multiple flowering pathways in P. grandiflorus. In order to fill this gap, we identified potential homologs of the FLOWERING LOCUS T (FT) gene in P. grandiflorus. The alignment with other FT members and phylogenetic analysis revealed that the P. grandiflorus FT (PlgFT) protein contains highly conserved functional domains and belongs to the FT-like clade. The expression analysis revealed spatial variations in the transcription of PlgFT in different organs. In addition, the expression level of PlgFT was increased by high temperature but not by photoperiodic light input signals, presumably due to lacking the CONSTANS binding motif in its promoter region. Furthermore, PlgFT induced early flowering upon its overexpression in P. grandiflorus, suggesting the functional role of PlgFT in flowering. Taken together, we functionally characterized PlgFT as a master regulator of P. grandiflorus flowering under inductive high temperature, which will serve as an important target gene for improving the root productivity.

QTL mapping and genomic analyses of earliness and fruit ripening traits in a melon Recombinant Inbred Lines population supported by de novo assembly of their parental genomes

Earliness and ripening behavior are important attributes of fruits on and off the vine, and affect quality and preference of both growers and consumers. Fruit ripening is a complex physiological process that involves metabolic shifts affecting fruit color, firmness, and aroma production. Melon is a promising model crop for the study of fruit ripening, as the full spectrum of climacteric behavior is represented across the natural variation. Using Recombinant Inbred Lines (RILs) population derived from the parental lines "Dulce" (reticulatus, climacteric) and "Tam Dew" (inodorus, non-climacteric) that vary in earliness and ripening traits, we mapped QTLs for ethylene emission, fruit firmness and days to flowering and maturity. To further annotate the main QTL intervals and identify candidate genes, we used Oxford Nanopore long-read sequencing in combination with Illumina short-read resequencing, to assemble the parental genomes de-novo. In addition to 2.5 million genome-wide SNPs and short InDels detected between the parents, we also highlight here the structural variation between these lines and the reference melon genome. Through systematic multi-layered prioritization process, we identified 18 potential polymorphisms in candidate genes within multi-trait QTLs. The associations of selected SNPs with earliness and ripening traits were further validated across a panel of 177 diverse melon accessions and across a diallel population of 190 F1 hybrids derived from a core subset of 20 diverse parents. The combination of advanced genomic tools with diverse germplasm and targeted mapping populations is demonstrated as a way to leverage forward genetics strategies to dissect complex horticulturally important traits.

The emerging role of jasmonate in the control of flowering time

Plants dynamically synchronize their flowering time with changes in the internal and external environments through a variety of signaling pathways to maximize fitness. In the last two decades, the major pathways associated with flowering, including the photoperiod, vernalization, age, autonomous, gibberellin, and ambient temperature pathways, have been extensively analyzed. In recent years, an increasing number of signals, such as sugar, thermosensory, stress, and certain hormones, have been shown to be involved in fine-tuning flowering time. Among these signals, the jasmonate signaling pathway has a function in the determination of flowering time that has not been systematically summarized. In this review, we present an overview of current knowledge of jasmonate control of flowering and discuss jasmonate crosstalk with other signals (such as gibberellin, defense, and touch) during floral transition.

The RNA recognition motif-containing protein UBA2c prevents early flowering by promoting transcription of the flowering repressor FLM in Arabidopsis

FLOWERING LOCUS M (FLM) is a well-known MADS-box transcription factor that is required for preventing early flowering under low temperatures in Arabidopsis thaliana. Alternative splicing of FLM is involved in the regulation of temperature-responsive flowering. However, how the basic transcript level of FLM is regulated is largely unknown. Here, we conducted forward genetic screening and identified a previously uncharacterized flowering repressor gene, UBA2c. Genetic analyses indicated that UBA2c represses flowering at least by promoting FLM transcription. We further demonstrated that UBA2c directly binds to FLM chromatin and facilitates FLM transcription by inhibiting histone H3K27 trimethylation, a histone marker related to transcriptional repression. UBA2c encodes a protein containing two putative RNA recognition motifs (RRMs) and one prion-like domain (PrLD). We found that UBA2c forms speckles in the nucleus and that both the RRMs and PrLD are required not only for forming the nuclear speckles but also for the biological function of UBA2c. These results identify a previously unknown flowering repressor and provide insights into the regulation of flowering time.

A novel TFL1 gene induces flowering in the mast seeding alpine snow tussock, Chionochloa pallens (Poaceae)

Masting, the synchronous, highly variable flowering across years by a population of perennial plants, has been reported to be precipitated by various factors including nitrogen levels, drought conditions, and spring and summer temperatures. However, the molecular mechanism leading to the initiation of flowering in masting plants in particular years remains largely unknown, despite the potential impact of climate change on masting phenology. We studied genes controlling flowering in the alpine snow tussock Chionochloa pallens (Poaceae), a strongly masting perennial grass. We used a range of in situ and manipulated plants to obtain leaf samples from tillers (shoots) which subsequently remained vegetative or flowered. Here, we show that a novel orthologue of TERMINAL FLOWER 1 (TFL1; normally a repressor of flowering in other species) promotes the induction of flowering in C. pallens (hence Anti-TFL1), a conclusion supported by structural, functional and expression analyses. Global transcriptomic analysis indicated differential expression of CpTPS1, CpGA20ox1, CpREF6 and CpHDA6, emphasizing the role of endogenous cues and epigenetic regulation in terms of responsiveness of plants to initiate flowering. Our molecular-based study provides insights into the cellular mechanism of flowering in masting plants and will supplement ecological and statistical models to predict how masting will respond to global climate change.

Genome-wide in silico analysis of long intergenic non-coding RNAs from rice peduncles at the heading stage

Long intergenic non-coding RNAs (lincRNAs) belong to the category of long non-coding RNAs (lncRNAs), originated from intergenic regions, which do not code for proteins. LincRNAs perform prominent role in regulation of gene expression during plant development and stress response by directly interacting with DNA, RNA, or proteins, or triggering production of small RNA regulatory molecules. Here, we identified 2973 lincRNAs and investigated their expression dynamics during peduncle elongation in two Indian rice cultivars, Pokkali and Swarna, at the time of heading. Differential expression analysis revealed common and cultivar-specific expression patterns, which we utilized to infer the lincRNA candidates with potential involvement in peduncle elongation and panicle exsertion. Their putative targets were identified using in silico prediction methods followed by pathway mapping and literature-survey based functional analysis. Further, to infer the mechanism of action, we identified the lincRNAs which potentially act as miRNA precursors or target mimics.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01059-2.

ERF1 delays flowering through direct inhibition of FLOWERING LOCUS T expression in Arabidopsis

ETHYLENE RESPONSE FACTOR1 (ERF1) is a key component in ethylene signaling, playing crucial roles in both biotic and abiotic stress responses. Here, we demonstrate that ERF1 also has an important role during floral initiation in Arabidopsis thaliana. Knockdown or knockout of ERF1 accelerated floral initiation, whereas overexpression of ERF1 dramatically delayed floral transition. These contrasting phenotypes were correlated with opposite transcript levels of FLOWERING LOCUS T (FT). Chromatin immunoprecipitation (ChIP) assays revealed that ERF1 associates with genomic regions of the FT gene to repress its transcription. ft-10/ERF1RNAi plants showed a similar flowering phenotype to the ft-10 mutant, whereas the flowering of FTox/ERF1ox mimicked that of FTox plants, suggesting that ERF1 acts upstream of FT during floral initiation. Similarly, altered floral transition in ethylene-related mutants was also correlated with FT expression. Further analysis suggested that ERF1 also participates in delay in flowering-time control mediated by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. Thus, ERF1 may act as a negative modulator of flowering-time control by repressing FT transcription in Arabidopsis.

Mapping and identification of genetic loci affecting earliness of bolting and flowering in lettuce

KEY MESSAGE

Photoperiod and temperature conditions elicit different genetic regulation over lettuce bolting and flowering. This study identifies environment-specific QTLs and putative genes and provides information for genetic marker assay. Bolting, defined as stem elongation, marks the plant life cycle transition from vegetative to reproductive stage. Lettuce is grown for its leaf rosettes, and premature bolting may reduce crop quality resulting in economic losses. The transition to reproductive stage is a complex process that involves many genetic and environmental factors. In this study, the effects of photoperiod and ambient temperature on bolting and flowering regulation were studied by utilizing a lettuce mapping population to identify quantitative trait loci (QTL) and by gene expression analyses of genotypes with contrasting phenotypes. A recombinant inbred line (RIL) population, derived from a cross between PI 251246 (early bolting) and cv. Salinas (late bolting), was grown in four combinations of short (8 h) and long (16 h) days and low (20 °C) and high (35 °C) temperature. QTL models revealed both genetic (G) and environmental (E) effects, and GxE interactions. A major QTL for bolting and flowering time was found on chromosome 7 (qFLT7.2), and two candidate genes were identified by fine mapping, homology, and gene expression studies. In short days and high temperature conditions, qFLT7.2 had no effect on plant development, while several small-effect loci on chromosomes 2, 3, 6, 8, and 9 were associated with bolting and flowering. Of these, the QTL on chromosome 2, qBFr2.1, co-located with the Flowering Locus T (LsFT) gene. Polymorphisms between parent genotypes in the promotor region may explain identified gene expression differences and were used to design a genetic marker which may be used to identify the late bolting trait.

Is seasonal flowering time of Paphiopedilum species caused by differences in initial time of floral bud differentiation?

Members of the genus Paphiopedilum are world-famous for their large, colourful flowers, unique floral morphology and long floral lifespan. Most Paphiopedilum species bloom in spring or autumn. The control of flowering time is of great significance to the commercial production of floral crops, because it affects the sales and prices of flowers. However, the mechanism that regulates when Paphiopedilum species bloom is unclear. In the present study, floral bud initiation and development of P. micranthum (spring-flowering species with one flower per stalk), P. dianthum (autumn-flowering species with multiple flowers per stalk) and P. henryanum (autumn-flowering species with one flower per stalk) were investigated by morphological and anatomical methods. We divided Paphiopedilum floral bud differentiation into six phases: the initiation of differentiation, inflorescence primordium differentiation, flower primordium differentiation, sepal primordium differentiation, petal primordium differentiation and column primordium differentiation. We found that the timing of floral bud differentiation for the three species was synchronized when experiencing the same environment, while the period from initiation to flowering largely differed. In addition, initiation of floral bud differentiation in P. dianthum was earlier at a warmer environment. The difference in flowering time of three species was mainly caused by the duration of floral bud development, rather than the initiation time. The findings were of great significance for the cultivation and flowering regulation of Paphiopedilum species.

Light-induced mobile factors from shoots regulate rhizobium-triggered soybean root nodulation

Symbiotic nitrogen fixation is an energy-expensive process, and the light available to plants has been proposed to be a primary influencer. We demonstrate that the light-induced soybean TGACG-motif binding factor 3/4 (GmSTF3/4) and FLOWERING LOCUS T (GmFTs), which move from shoots to roots, interdependently induce nodule organogenesis. Rhizobium-activated calcium- and calmodulin-dependent protein kinase (CCaMK) phosphorylates GmSTF3, triggering GmSTF3–GmFT2a complex formation, which directly activates expression of nodule inception (NIN) and nuclear factor Y (NF-YA1 and NF-YB1). Accordingly, the CCaMK–STF–FT module integrates aboveground light signals with underground symbiotic signaling, ensuring that the host plant informs its roots that the aboveground environment is prepared to sustainably supply the carbohydrate necessary for symbiosis. These results suggest approaches that could enhance the balance of carbon and nitrogen in the biosphere.

Gibberellin Induced Transcriptome Profiles Reveal Gene Regulation of Loquat Flowering

Flowering is an integral part of the life cycle of flowering plants, which is essential for plant survival and crop production. Most woody fruit trees such as apples and pears bloom in spring, but loquat blooms in autumn and winter. Gibberellin (GA) plays a key role in the regulation of plant flower formation. In this study, we sprayed loquat plants with exogenous GA₃, which resulted in vigorous vegetative growth rather than floral bud formation. We then performed a comprehensive RNA-seq analysis on GA₃-treated and control-treated leaves and buds over three time periods to observe the effects of exogenous GA₃ application on floral initiation and development. The results showed that 111 differentially expressed genes (DEGs) and 563 DEGs were down-regulated, and 151 DEGs and 506 DEGs were up-regulated in buds and leaves, respectively, upon treatment with GA₃. Among those that are homologs of the DELLA-mediated GA signal pathway genes, some may be involved in the positive regulation of flower development, including EjWRKY75, EjFT, EjSOC1, EjAGL24, EjSPL, EjLFY, EjFUL, and EjAP1; while some may be involved in the negative regulation of flower development, including EjDELLA, EjMYC3, EjWRKY12, and EjWRKY13. Finally, by analyzing the co-expression of DEGs and key floral genes EjSOC1s, EjLFYs, EjFULs, EjAP1s, 330 candidate genes that may be involved in the regulation of loquat flowering were screened. These genes belong to 74 gene families, including Cyclin_C, Histone, Kinesin, Lipase_GDSL, MYB, P450, Pkinase, Tubulin, and ZF-HD_dimer gene families. These findings provide new insights into the regulation mechanism of loquat flowering.

EIN3-Mediated Ethylene Signaling Attenuates Auxin Response during Hypocotyl Thermomorphogenesis

The gaseous phytohormone ethylene plays vital roles in diverse developmental and environmental adaptation processes, such as fruit ripening, seedling establishment, mechanical stress tolerance and submergence escape. It is also known that in the light, ethylene promotes hypocotyl growth by stimulating the expression of PHYTOCHROME INTERACTING FACTOR3 (PIF3) transcription factor, which triggers microtubule reorganization during hypocotyl cell elongation. In particular, ethylene has been implicated in plant responses to warm temperatures in recent years. However, it is currently unclear how ethylene signals are functionally associated with hypocotyl thermomorphogenesis at the molecular level. Here, we show that ETHYLENE-INSENSITIVE3 (EIN3)-mediated ethylene signals attenuate hypocotyl thermomorphogenesis by suppressing auxin response. At warm temperatures, when the activity of the PIF4 thermomorphogenesis promoter is prominently high, the ethylene-activated EIN3 transcription factor directly induces the transcription of ARABIDOPSIS PP2C CLADE D7 (APD7) gene encoding a protein phosphatase that inactivates the plasma membrane (PM) H+-ATPase proton pumps. In conjunction with the promotive role of the PM H+-ATPases in hypocotyl cell elongation, our observations strongly support that the EIN3-directed induction of APD7 gene is linked with the suppression of auxin-induced cell expansion, leading to the reduction in thermomorphogenic hypocotyl growth. Our data demonstrate that APD7 acts as a molecular hub that integrates ethylene and auxin signals into hypocotyl thermomorphogenesis. We propose that the ethylene-auxin signaling crosstalks via the EIN3-APD7 module facilitate the fine-tuning of hypocotyl thermomorphogenesis under natural environments, which often fluctuate in a complex manner.

Isolation and Functional Characterization of Two SHORT VEGETATIVE PHASE Homologous Genes from Mango

The SHORT VEGETATIVE PHASE (SVP) gene is a transcription factor that integrates flowering signals and plays an important role in the regulation of flowering time in many plants. In this study, two full-length cDNA sequences of SVP homologous genes—MiSVP1 and MiSVP2—were obtained from ‘SiJiMi’ mango. Sequence analysis showed that the MiSVPs had typical MADS-box domains and were highly conserved between each other. The analysis of expression patterns showed that the MiSVPs were expressed during flower development and highly expressed in vegetative tissues, with low expression in flowers/buds. The MiSVPs could responded to low temperature, NaCl, and PEG treatment. Subcellular localization revealed that MiSVP1 and MiSVP2 were localized in the nucleus. Transformation of Arabidopsis revealed that overexpression of MiSVP1 delayed flowering time, overexpression of MiSVP2 accelerated flowering time, and neither MiSVP1 nor MiSVP2 had an effect on the number of rosette leaves. Overexpression of MiSVP1 increased the expression of AtFLC and decreased the expression of AtFT and AtSOC1, and overexpression of MiSVP2 increased the expression levels of AtSOC1 and AtFT and decreased the expression levels of AtFLC. Point-to-point and bimolecular fluorescence complementation (BiFC) assays showed that MiSVP1 and MiSVP2 could interact with SEP1-1, SOC1D, and AP1-2. These results suggest that MiSVP1 and MiSVP2 may play a significant roles in the flowering process of mango.

HD-ZIP Gene Family: Potential Roles in Improving Plant Growth and Regulating Stress-Responsive Mechanisms in Plants

Exploring the molecular foundation of the gene-regulatory systems underlying agronomic parameters or/and plant responses to both abiotic and biotic stresses is crucial for crop improvement. Thus, transcription factors, which alone or in combination directly regulated the targeted gene expression levels, are appropriate players for enlightening agronomic parameters through genetic engineering. In this regard, homeodomain leucine zipper (HD-ZIP) genes family concerned with enlightening plant growth and tolerance to environmental stresses are considered key players for crop improvement. This gene family containing HD and LZ domain belongs to the homeobox superfamily. It is further classified into four subfamilies, namely HD-ZIP I, HD-ZIP II, HD-ZIP III, and HD-ZIP IV. The first HD domain-containing gene was discovered in maize cells almost three decades ago. Since then, with advanced technologies, these genes were functionally characterized for their distinct roles in overall plant growth and development under adverse environmental conditions. This review summarized the different functions of HD-ZIP genes in plant growth and physiological-related activities from germination to fruit development. Additionally, the HD-ZIP genes also respond to various abiotic and biotic environmental stimuli by regulating defense response of plants. This review, therefore, highlighted the various significant aspects of this important gene family based on the recent findings. The practical application of HD-ZIP biomolecules in developing bioengineered plants will not only mitigate the negative effects of environmental stresses but also increase the overall production of crop plants.

Characterization of genes associated with TGA7 during the floral transition

BACKGROUND

The TGACG-binding (TGA) family has 10 members that play vital roles in Arabidopsis thaliana defense responses and development. However, their involvement in controlling flowering time remains largely unknown and requires further investigation.

RESULTS

To study the role of TGA7 during floral transition, we first investigated the tga7 mutant, which displayed a delayed-flowering phenotype under both long-day and short-day conditions. We then performed a flowering genetic pathway analysis and found that both autonomous and thermosensory pathways may affect TGA7 expression. Furthermore, to reveal the differential gene expression profiles between wild-type (WT) and tga7, cDNA libraries were generated for WT and tga7 mutant seedlings at 9 days after germination. For each library, deep-sequencing produced approximately 6.67 Gb of high-quality sequences, with the majority (84.55 %) of mRNAs being between 500 and 3,000 nt. In total, 325 differentially expressed genes were identified between WT and tga7 mutant seedlings. Among them, four genes were associated with flowering time control. The differential expression of these four flowering-related genes was further validated by qRT-PCR.

CONCLUSIONS

Among these four differentially expressed genes associated with flowering time control, FLC and MAF5 may be mainly responsible for the delayed-flowering phenotype in tga7, as TGA7 expression was regulated by autonomous pathway genes. These results provide a framework for further studying the role of TGA7 in promoting flowering.

GhLUX1 and GhELF3 Are Two Components of the Circadian Clock That Regulate Flowering Time of Gossypium hirsutum

Photoperiod is an important external factor that regulates flowering time, the core mechanism of which lies in the circadian clock-controlled expression of FLOWERING LOCUS T (FT) and its upstream regulators. However, the roles of the circadian clock in regulating cotton flowering time are largely unknown. In this study, we cloned two circadian clock genes in cotton, GhLUX1 and GhELF3. The physicochemical and structural properties of their putative proteins could satisfy the prerequisites for the interaction between them, which was proved by yeast two-hybrid (Y2H) and Bimolecular Fluorescent Complimentary (BiFC) assays. Phylogenetic analysis of LUXs and ELF3s indicated that the origin of LUXs was earlier than that of ELF3s, but ELF3s were more divergent and might perform more diverse functions. GhLUX1, GhELF3, GhCOL1, and GhFT exhibited rhythmic expression and were differentially expressed in the early flowering and late-flowering cotton varieties under different photoperiod conditions. Both overexpression of GhLUX1 and overexpression of GhELF3 in Arabidopsis delayed flowering probably by changing the oscillation phases and amplitudes of the key genes in the photoperiodic flowering pathway. Both silencing of GhLUX1 and silencing of GhELF3 in cotton increased the expression of GhCOL1 and GhFT and resulted in early flowering. In summary, the circadian clock genes were involved in regulating cotton flowering time and could be the candidate targets for breeding early maturing cotton varieties.

Transcriptome Analysis of Flower Development and Mining of Genes Related to Flowering Time in Tomato (Solanum lycopersicum)

Flowering is a morphogenetic process in which angiosperms shift from vegetative growth to reproductive growth. Flowering time has a strong influence on fruit growth, which is closely related to productivity. Therefore, research on crop flowering time is particularly important. To better understand the flowering period of the tomato, we performed transcriptome sequencing of early flower buds and flowers during the extension period in the later-flowering “Moneymaker” material and the earlier-flowering “20965” homozygous inbred line, and we analyzed the obtained data. At least 43.92 million clean reads were obtained from 12 datasets, and the similarity with the tomato internal reference genome was 92.86–94.57%. Based on gene expression and background annotations, 49 candidate genes related to flowering time and flower development were initially screened, among which the greatest number belong to the photoperiod pathway. According to the expression pattern of candidate genes, the cause of early flowering of “20965” is predicted. The modes of action of the differentially expressed genes were classified, and the results show that they are closely related to hormone regulation and participated in a variety of life activities in crops. The candidate genes we screened and the analysis of their expression patterns provide a basis for future functional verification, helping to explore the molecular mechanism of tomato flowering time more comprehensively.

Normal, novel or none: versatile regulation from alternative splicing

Pre-mRNA splicing is a vital step in the posttranscriptional regulation of gene expression. Splicing is catalyzed by the spliceosome, a multidalton RNA-protein complex, through two successive transesterifications to yield mature mRNAs. In Arabidopsis, more than 61% of all transcripts from intron-containing genes are alternatively spliced, thereby resulting in transcriptome and subsequent proteome diversities for cellular processes. Moreover, it is estimated that more alternative splicing (AS) events induced by adverse stimuli occur to confer stress tolerance. Recently, increasing AS variants encoding normal or novel proteins, or degraded by nonsense-mediated decay (NMD) and their corresponding splicing factors or regulators acting at the posttranscriptional level have been functionally characterized. This review comprehensively summarizes and highlights the advances in our understanding of the biological functions and underlying mechanisms of AS events and their regulators in Arabidopsis and provides prospects for further research on AS in crops.

Linkage mapping and QTL analysis of flowering time in faba bean

Flowering time marks the transition from vegetative to reproductive growth and is key for optimal yield in any crop. The molecular mechanisms controlling this trait have been extensively studied in model plants such as Arabidopsis thaliana and rice. While knowledge on the molecular regulation of this trait is rapidly increasing in sequenced galegoid legume crops, understanding in faba bean remains limited. Here we exploited translational genomics from model legume crops to identify and fine map QTLs linked to flowering time in faba bean. Among the 31 candidate genes relevant for flowering control in A. thaliana and Cicer arietinum assayed, 25 could be mapped in a segregating faba bean RIL population. While most of the genes showed conserved synteny among related legume species, none of them co-localized with the 9 significant QTL regions identified. The FT gene, previously implicated in the control of flowering time in numerous members of the temperate legume clade, mapped close to the most relevant stable and conserved QTL in chromosome V. Interestingly, QTL analysis suggests an important role of epigenetic modifications in faba bean flowering control. The new QTLs and candidate genes assayed here provide a robust framework for further genetic studies and will contribute to the elucidation of the molecular mechanisms controlling this trait.

Linkage mapping and QTL analysis of flowering time in faba bean

Flowering time marks the transition from vegetative to reproductive growth and is key for optimal yield in any crop. The molecular mechanisms controlling this trait have been extensively studied in model plants such as Arabidopsis thaliana and rice. While knowledge on the molecular regulation of this trait is rapidly increasing in sequenced galegoid legume crops, understanding in faba bean remains limited. Here we exploited translational genomics from model legume crops to identify and fine map QTLs linked to flowering time in faba bean. Among the 31 candidate genes relevant for flowering control in A. thaliana and Cicer arietinum assayed, 25 could be mapped in a segregating faba bean RIL population. While most of the genes showed conserved synteny among related legume species, none of them co-localized with the 9 significant QTL regions identified. The FT gene, previously implicated in the control of flowering time in numerous members of the temperate legume clade, mapped close to the most relevant stable and conserved QTL in chromosome V. Interestingly, QTL analysis suggests an important role of epigenetic modifications in faba bean flowering control. The new QTLs and candidate genes assayed here provide a robust framework for further genetic studies and will contribute to the elucidation of the molecular mechanisms controlling this trait.

Potential transceptor AtNRT1.13 modulates shoot architecture and flowering time in a nitrate-dependent manner

Compared with root development regulated by external nutrients, less is known about how internal nutrients are monitored to control plasticity of shoot development. In this study, we characterize an Arabidopsis thaliana transceptor, NRT1.13 (NPF4.4), of the NRT1/PTR/NPF family. Different from most NRT1 transporters, NRT1.13 does not have the conserved proline residue between transmembrane domains 10 and 11; an essential residue for nitrate transport activity in CHL1/NRT1.1/NPF6.3. As expected, when expressed in oocytes, NRT1.13 showed no nitrate transport activity. However, when Ser 487 at the corresponding position was converted back to proline, NRT1.13 S487P regained nitrate uptake activity, suggesting that wild-type NRT1.13 cannot transport nitrate but can bind it. Subcellular localization and β-glucuronidase reporter analyses indicated that NRT1.13 is a plasma membrane protein expressed at the parenchyma cells next to xylem in the petioles and the stem nodes. When plants were grown with a normal concentration of nitrate, nrt1.13 showed no severe growth phenotype. However, when grown under low-nitrate conditions, nrt1.13 showed delayed flowering, increased node number, retarded branch outgrowth, and reduced lateral nitrate allocation to nodes. Our results suggest that NRT1.13 is required for low-nitrate acclimation and that internal nitrate is monitored near the xylem by NRT1.13 to regulate shoot architecture and flowering time.

Fine-tuning Flowering Time via Genome Editing of Upstream Open Reading Frames of Heading Date 2 in Rice

Flowering time of rice (Oryza sativa L.) is among the most important agronomic traits for region adaptation and grain yield. In the process of rice breeding, efficient and slightly modulating the flowering time of an elite cultivar would be more popular with breeder. Hence, we are interested in slightly increasing the expression of flowering repressors by CRISPR/Cas9 genome editing system. It was predicated there were three uORFs in 5' leader sequence of Hd2. In this study, through editing Hd2 uORFs, we got four homozygous mutant lines. Phenotypic analysis showed that the hd2 urf edited lines flowered later by 4.6-11.2 days relative to wild type SJ2. Supporting the later flowering phenotype, the expression of Ehd1, Hd3a, and RFT1 is significantly decreased in hd2 urf than that in wild type. Moreover, we found that the transcription level of Hd2 is not affected, whereas the Hd2 protein level was increased in hd2 urf compared with wild type, which indicated that Hd2 uORFs indeed affect the translation of a downstream Hd2 pORF. In summary, we developed a efficient approach for delaying rice heading date based on editing uORF region of flowering repressor, which is time and labor saving compared to traditional breeding. In future, uORF of other flowering time related genes, including flowering promoter and flowering repressor genes, can also be used as targets to fine-tune the flowering time of varieties.

MYB106 is a negative regulator and a substrate for CRL3BPM E3 ligase in regulating flowering time in Arabidopsis thaliana

Flowering time is crucial for successful reproduction in plants, the onset and progression of which are strictly controlled. However, flowering time is a complex and environmentally responsive history trait and the underlying mechanisms still need to be fully characterized. Post-translational regulation of the activities of transcription factors (TFs) is a dynamic and essential mechanism for plant growth and development. CRL3^BPM E3 ligase is a CULLIN3-based E3 ligase involved in orchestrating protein stability via the ubiquitin proteasome pathway. Our study shows that the mutation of MYB106 induced early flowering phenotype while over-expression of MYB106 delayed Arabidopsis flowering. Transcriptome analysis of myb106 mutants reveals 257 differentially expressed genes between wild type and myb106-1 mutants, including Flowering Locus T (FT) which is related to flowering time. Moreover, in vitro electrophoretic mobility shift assays (EMSA), in vivo chromatin immunoprecipitation quantitative polymerase chain reaction (ChIP-qPCR) assays and dual luciferase assays demonstrate that MYB106 directly binds to the promoter of FT to suppress its expression. Furthermore, we confirm that MYB106 interacts with BPM proteins which are further identified by CRL3^BPM E3 ligases as the substrate. Taken together, we have identified MYB106 as a negative regulator in the control of flowering time and a new substrate for CRL3^BPM E3 ligases in Arabidopsis.

In Response to Abiotic Stress, DNA Methylation Confers EpiGenetic Changes in Plants

Epigenetics involves the heritable changes in patterns of gene expression determined by developmental and abiotic stresses, i.e., drought, cold, salinity, trace metals, and heat. Gene expression is driven by changes in DNA bases, histone proteins, the biogenesis of ncRNA, and changes in the nucleotide sequence. To cope with abiotic stresses, plants adopt certain changes driven by a sophisticated biological system. DNA methylation is a primary mechanism for epigenetic variation, which can induce phenotypic alterations in plants under stress. Some of the stress-driven changes in plants are temporary, while some modifications may be stable and inheritable to the next generations to allow them to cope with such extreme stress challenges in the future. In this review, we discuss the pivotal role of epigenetically developed phenotypic characteristics in plants as an evolutionary process participating in adaptation and tolerance responses to abiotic and biotic stresses that alter their growth and development. We emphasize the molecular process underlying changes in DNA methylation, differential variation for different species, the roles of non-coding RNAs in epigenetic modification, techniques for studying DNA methylation, and its role in crop improvement in tolerance to abiotic stress (drought, salinity, and heat). We summarize DNA methylation as a significant future research priority for tailoring crops according to various challenging environmental issues.

Phosphorylation-mediated signalling in flowering: prospects and retrospects of phosphoproteomics in crops

Protein phosphorylation is a major post-translational modification, regulating protein function, stability, and subcellular localization. To date, annotated phosphorylation data are available mainly for model organisms and humans, despite the economic importance of crop species and their large kinomes. Our understanding of the phospho-regulation of flowering in relation to the biology and interaction between the pollen and pistil is still significantly lagging, limiting our knowledge on kinase signalling and its potential applications to crop production. To address this gap, we bring together relevant literature that were previously disconnected to present an overview of the roles of phosphoproteomic signalling pathways in modulating molecular and cellular regulation within specific tissues at different morphological stages of flowering. This review is intended to stimulate research, with the potential to increase crop productivity by providing a platform for novel molecular tools.

Beyond the Genetic Pathways, Flowering Regulation Complexity in Arabidopsis thaliana

Flowering is one of the most critical developmental transitions in plants’ life. The irreversible change from the vegetative to the reproductive stage is strictly controlled to ensure the progeny’s success. In Arabidopsis thaliana, seven flowering genetic pathways have been described under specific growth conditions. However, the evidence condensed here suggest that these pathways are tightly interconnected in a complex multilevel regulatory network. In this review, we pursue an integrative approach emphasizing the molecular interactions among the flowering regulatory network components. We also consider that the same regulatory network prevents or induces flowering phase change in response to internal cues modulated by environmental signals. In this sense, we describe how during the vegetative phase of development it is essential to prevent the expression of flowering promoting genes until they are required. Then, we mention flowering regulation under suboptimal growing temperatures, such as those in autumn and winter. We next expose the requirement of endogenous signals in flowering, and finally, the acceleration of this transition by long-day photoperiod and temperature rise signals allowing A. thaliana to bloom in spring and summer seasons. With this approach, we aim to provide an initial systemic view to help the reader integrate this complex developmental process.