The Arabidopsis thaliana CONSTANS-LIKE 4 (COL4) - A Modulator of Flowering Time

The B-box protein BBX13/COL15 suppresses photoperiodic flowering by attenuating the action of CONSTANS in Arabidopsis

The optimal timing of transition from vegetative to floral reproductive phase is critical for plant productivity and agricultural yields. Light plays a decisive role in regulating this transition. The B-box (BBX) family of transcription factors regulates several light-mediated developmental processes in plants, including flowering. Here, we identify a previously uncharacterized group II BBX family member, BBX13/COL15, as a negative regulator of flowering under long-day conditions. BBX13 is primarily expressed in the leaf vasculature, buds, and flowers, showing a similar spatial expression pattern to the major flowering time regulators CO and FT. bbx13 mutants flower early, while BBX13-overexpressors exhibit delayed flowering under long days. Genetic analyses showed that BBX13 acts upstream to CO and FT and negatively regulates their expression. BBX13 physically interacts with CO and inhibits the CO-mediated transcriptional activation of FT. In addition, BBX13 directly binds to the CORE2 motif on the FT promoter, where CO also binds. Chromatin immunoprecipitation data indicates that BBX13 reduces the in vivo binding of CO on the FT promoter. Through luciferase assay, we found that BBX13 inhibits the CO-mediated transcriptional activation of FT. Together, these findings suggest that BBX13/COL15 represses flowering in Arabidopsis by attenuating the binding of CO on the FT promoter.

More than flowering: CONSTANS plays multifaceted roles in plant development and stress responses

Plants have evolved a remarkable ability to sense and respond to changes in photoperiod, allowing adjustments to their growth and development based on seasonal and environmental cues. The floral transition is a pivotal stage in plant growth and development, signifying a shift from vegetative to reproductive growth. CONSTANS (CO), a central photoperiodic response factor conserved in various plants, mediates day-length signals to control the floral transition, although its mechanisms of action vary among plants with different day-length requirements. In addition, recent studies have uncovered roles for CO in organ development and stress responses. These pleiotropic roles in model plants and crops make CO a potentially fruitful target for molecular breeding aimed at modifying crop agronomic traits. This review systematically traces research on CO, from its discovery and functional studies to the exploration of its regulatory mechanisms and newly discovered functions, providing important insight into the roles of CO and laying a foundation for future research.

Genome-Wide Identification and Expression Analysis of the COL Gene Family in Hemerocallis citrina Baroni

Hemerocallis citrina Baroni (H. citrina) is an important specialty vegetable that is not only edible and medicinal but also has ornamental value. However, much remains unknown about the regulatory mechanisms associated with the growth, development, and flowering rhythm of this plant. CO, as a core regulatory factor in the photoperiod pathway, coordinates light and circadian clock inputs to transmit flowering signals. We identified 18 COL genes (HcCOL1-HcCOL18) in the H. citrina cultivar 'Mengzihua' and studied their chromosomal distribution, phylogenetic relationships, gene and protein structures, collinearity, and expression levels in the floral organs at four developmental stages. The results indicate that these genes can be classified into three groups based on phylogenetic analysis. The major expansion of the HcCOL gene family occurred via segmental duplication, and the Ka/Ks ratio indicated that the COL genes of Arabidopsis thaliana, Oryza sativa, Phalaenopsis equestris, and H. citrina were under purifying selection. Many cis-elements, including light response elements, abiotic stress elements, and plant hormone-inducible elements, were distributed in the promoter sequences of the HcCOL genes. Expression analysis of HcCOL genes at four floral developmental stages revealed that most of the HcCOL genes were expressed in floral organs and might be involved in the growth, development, and senescence of the floral organs of H. citrina. This study lays a foundation for the further elucidation of the function of the HcCOL gene in H. citrina and provides a theoretical basis for the molecular design breeding of H. citrina.

Two B-box proteins orchestrate vegetative and reproductive growth in summer chrysanthemum

Floral transition, the switch from vegetative to reproductive growth, is extremely important for the growth and development of flowering plants. In the summer chrysanthemum, CmBBX8, a member of the subgroup II B-box (BBX) family, positively regulates the transition by physically interacting with CmERF3 to inhibit CmFTL1 expression. In this study, we show that CmBBX5, a B-box subgroup I member comprising two B-boxes and a CCT domain, interacts with CmBBX8. This interaction suppresses the recruitment of CmBBX8 to the CmFTL1 locus without affecting its transcriptional activation activity. CmBBX5 overexpression led to delayed flowering under both LD (long-day) and SD (short-day) conditions, while lines expressing the chimeric repressor gene-silencing (CmBBX5-SRDX) exhibited the opposite phenotype. Subsequent genetic evidence indicated that in regulating flowering, CmBBX5 is partially dependent on CmBBX8. Moreover, during the vegetative growth period, levels of CmBBX5 expression were found to exceed those of CmBBX8. Collectively, our findings indicate that both CmERF3 and CmBBX5 interact with CmBBX8 to dampen the regulation of CmFTL1 via distinct mechanisms, which contribute to preventing the premature flowering of summer chrysanthemum.

Genome-wide identification and expression analysis of the CONSTANS-like family in potato (Solanum tuberosum L.)

The CONSTANS-like (COL) gene plays important roles in plant growth, development, and abiotic stress. A total of 15 COL genes are unevenly distributed on eight chromosomes in the potato genome. The amino acid length of the family members was 347-453 aa, the molecular weight was 38.65-49.92 kD, and the isoelectric point was 5.13-6.09. The StCOL family can be divided into three subfamilies by evolutionary tree analysis, with conserved motifs and similar gene structure positions in each subfamily. The analysis of promoter cis-acting elements showed 17 cis-acting elements related to plant hormones, stress, and light response. Collinearity analysis of COL genes of tomato, potato, and Arabidopsis showed that 13 StCOL genes in the different species may have a common ancestor. A total of 10 conserved motifs and six kinds of post-translational modifications in the 15 StCOL proteins were identified. The 15 StCOL genes exhibit a genomic structure consisting of exons and introns, typically ranging from two to four in number. The results showed that 10 genes displayed significant expression across all potato tissues, while the remaining five genes were down-expressed in potato transcriptome data. The quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis exhibited differential expression of 8 StCOL genes in the potato leaves and tubers at different growth stages, as well as 7 StCOL genes under 2°C treatment conditions. These results suggested that the StCOL gene family may play an important role in regulating potato tuberization and responding to cold stress.

Evolution of major flowering pathway integrators in Orchidaceae

The Orchidaceae is a mega-diverse plant family with ca. 29,000 species with a large variety of life forms that can colonize transitory habitats. Despite this diversity, little is known about their flowering integrators in response to specific environmental factors. During the reproductive transition in flowering plants a vegetative apical meristem (SAM) transforms into an inflorescence meristem (IM) that forms bracts and flowers. In model grasses, like rice, a flowering genetic regulatory network (FGRN) controlling reproductive transitions has been identified, but little is known in the Orchidaceae. In order to analyze the players of the FRGN in orchids, we performed comprehensive phylogenetic analyses of CONSTANS-like/CONSTANS-like 4 (COL/COL4), FLOWERING LOCUS D (FD), FLOWERING LOCUS C/FRUITFULL (FLC/FUL) and SUPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) gene lineages. In addition to PEBP and AGL24/SVP genes previously analyzed, here we identify an increase of orchid homologs belonging to COL4, and FUL gene lineages in comparison with other monocots, including grasses, due to orchid-specific gene lineage duplications. Contrariwise, local duplications in Orchidaceae are less frequent in the COL, FD and SOC1 gene lineages, which points to a retention of key functions under strong purifying selection in essential signaling factors. We also identified changes in the protein sequences after such duplications, variation in the evolutionary rates of resulting paralogous clades and targeted expression of isolated homologs in different orchids. Interestingly, vernalization-response genes like VERNALIZATION1 (VRN1) and FLOWERING LOCUS C (FLC) are completely lacking in orchids, or alternatively are reduced in number, as is the case of VERNALIZATION2/GHD7 (VRN2). Our findings point to non-canonical factors sensing temperature changes in orchids during reproductive transition. Expression data of key factors gathered from Elleanthus auratiacus, a terrestrial orchid in high Andean mountains allow us to characterize which copies are actually active during flowering. Altogether, our data lays down a comprehensive framework to assess gene function of a restricted number of homologs identified more likely playing key roles during the flowering transition, and the changes of the FGRN in neotropical orchids in comparison with temperate grasses.

Genome-Wide Identification of Peanut B-Boxs and Functional Characterization of AhBBX6 in Salt and Drought Stresses

The B-box (BBX) gene family includes zinc finger protein transcription factors that regulate a multitude of physiological and developmental processes in plants. While BBX gene families have been previously determined in various plants, the members and roles of peanut BBXs are largely unknown. In this research, on the basis of the genome-wide identification of BBXs in three peanut species (Arachis hypogaea, A. duranensis, and A. ipaensis), we investigated the expression profile of the BBXs in various tissues and in response to salt and drought stresses and selected AhBBX6 for functional characterization. We identified a total of 77 BBXs in peanuts, which could be grouped into five subfamilies, with the genes from the same branch of the same subgroup having comparable exon-intron structures. In addition, a significant number of cis-regulatory elements involved in the regulation of responses to light and hormones and abiotic stresses were found in the promoter region of peanut BBXs. Based on the analysis of transcriptome data and qRT-PCR, we identified AhBBX6, AhBBX11, AhBBX13, and AhBBX38 as potential genes associated with tolerance to salt and drought. Silencing AhBBX6 using virus-induced gene silencing compromised the tolerance of peanut plants to salt and drought stresses. The results of this study provide knowledge on peanut BBXs and establish a foundation for future research into their functional roles in peanut development and stress response.

MiCOL6, MiCOL7A and MiCOL7B isolated from mango regulate flowering and stress response in transgenic Arabidopsis

The CONSTANS/CONSTANS-Like (CO/COL) family has been shown to play important roles in flowering, stress tolerance, fruit development and ripening in higher plants. In this study, three COL genes, MiCOL6, MiCOL7A and MiCOL7B, which each contain only one CCT domain, were isolated from mango (Mangifera indica), and their functions were investigated. MiCOL7A and MiCOL7B were expressed mainly at 20 days after flowering (DAF), and all three genes were highly expressed during the flowering induction period. The expression levels of the three genes were affected by light conditions, but only MiCOL6 exhibited a clear circadian rhythm. Overexpression of MiCOL6 promoted earlier flowering, while overexpression of MiCOL7A or MiCOL7B delayed flowering compared to that in the control lines of Arabidopsis thaliana under long-day (LD) and short-day (SD) conditions. Overexpressing MiCOL6, MiCOL7A or MiCOL7B in transgenic plants increased superoxide dismutase (SOD) and proline levels, decreased malondialdehyde (MAD) levels, and improved survival under drought and salt stress. In addition, yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) analyses showed that the MiCOL6, MiCOL7A and MiCOL7B proteins interact with several stress- and flower-related proteins. This work demonstrates the functions of MiCOL6, MiCOL7A and MiCOL7B and provides a foundation for further research on the role of mango COL genes in flowering regulation and the abiotic stress response.

Genome-Wide Association Studies Prioritize Genes Controlling Seed Size and Reproductive Period Length in Soybean

Hundred-seed weight (HSW) and reproductive period length (RPL) are two major agronomic traits critical for soybean production and adaptation. However, both traits are quantitatively controlled by multiple genes that have yet to be comprehensively elucidated due to the lack of major genes; thereby, the genetic basis is largely unknown. In the present study, we conducted comprehensive genome-wide association analyses (GWAS) of HSW and RPL with multiple sets of accessions that were phenotyped across different environments. The large-scale analysis led to the identification of sixty-one and seventy-four significant QTLs for HSW and RPL, respectively. An ortholog-based search analysis prioritized the most promising candidate genes for the QTLs, including nine genes (TTG2, BZR1, BRI1, ANT, KLU, EOD1/BB, GPA1, ABA2, and ABI5) for HSW QTLs and nine genes (such as AGL8, AGL9, TOC1, and COL4) and six known soybean flowering time genes (E2, E3, E4, Tof11, Tof12, and FT2b) for RPL QTLs. We also demonstrated that some QTLs were targeted during domestication to drive the artificial selection of both traits towards human-favored traits. Local adaptation likely contributes to the increased genomic diversity of the QTLs underlying RPL. The results provide additional insight into the genetic basis of HSW and RPL and prioritize a valuable resource of candidate genes that merits further investigation to reveal the complex molecular mechanism and facilitate soybean improvement.

Genome-wide identification of the Pinus tabuliformis CONSTANS-like gene family and their potential roles in reproductive cone development

The CONSTANS-like (COL) genes, as a core transcription factor in the photoperiod regulation pathway, play a key role in plant reproduction development. However, their molecular characterization has rarely been studied in Pinus tabuliformis. Here, 10 PtCOL genes were identified in the P. tabuliformis genome and multiple sequence alignments have indicated that the PtCOL proteins contained highly conserved B-BOX1 and CCT domains. Sequence similarity analysis showed that PtCOL1 and PtCOL3 had the higher similarity with Norway spruce COLs (PaCOL2 and PaCOL1) and Arabidopsis COLs (AtCOL3, 4 and 5), respectively. Phylogeny and gene structure analyses revealed that PtCOLs were divided into three subgroups, each with identical or similar distributions of exons, introns, and motifs. Moreover, 10 PtCOLs were distributed on 6 chromosomes and PtCOL9 has syntenic gene pairs in both Ginkgo biloba and Sequoiadendron giganteum. Interestingly, in transcriptome profiles, most PtCOLs exhibited a diurnal oscillation pattern under both long (LD) and short (SD) day conditions. Additionally, PtCOLs were highly expressed in needles and female cones, and showed different spatial expression patterns. Among the ten PtCOLs, PtCOL1/3 heterologous overexpression Arabidopsis displayed a delayed-flowering phenotype under SD, indicating that they are likely to play a crucial role in the reproductive development. Additionally, PtCOL1 and PtCOL3 were not only capable of interacting with each other, but they were each capable of interacting with themselves. Furthermore, PtCOL1 and PtCOL3 were also involved in the MADS-box protein-protein interaction (PPI) network in P. tabuliformis cone development. Direct interactions of PtDAL11 with PtCOL1/3 impeded PtCOL1/3 translocation into the nucleus. In summary, this study provided comprehensive understanding for the functions of the PtCOL gene family and revealed their biological roles in the photoperiod-dependent P. tabuliformis cone development.

Integrated metabolic profiling and transcriptome analysis of Lonicera japonica flowers for chlorogenic acid, luteolin and endogenous hormone syntheses

The active ingredients of many medicinal plants are the secondary metabolites associated with the growth period. Lonicera japonica Thunb. is an important traditional Chinese medicine, and the flower development stage is an important factor that influences the quality of medicinal ingredients. In this study, transcriptomics and metabolomics were performed to reveal the regulatory mechanism of secondary metabolites during flowering of L. japonica. The results showed that the content of chlorogenic acid (CGA) and luteolin gradually decreased from green bud stage (Sa) to white flower stage (Sc), especially from white flower bud stage (Sb) to Sc. Most of the genes encoding the crucial rate-limiting enzymes, including PAL, C4H, HCT, C3'H, F3'H and FNSII, were down-regulated in three comparisons. Correlation analysis identified some members of the MYB, AP2/ERF, bHLH and NAC transcription factor families that are closely related to CGA and luteolin biosynthesis. Furthermore, differentially expressed genes (DEGs) involved in hormone biosynthesis, signalling pathways and flowering process were analysed in three flower developmental stage.

Epigenetic regulation of photoperiodic flowering in plants

In response to changeable season, plants precisely control the initiation of flowering in appropriate time of the year to ensure reproductive success. Day length (photoperiod) acts as the most important external cue to determine flowering time. Epigenetics regulates many major developmental stages in plant life, and emerging molecular genetics and genomics researches reveal their essential roles in floral transition. Here, we summarize the recent advances in epigenetic regulation of photoperiod-mediated flowering in Arabidopsis and rice, and discuss the potential of epigenetic regulation in crops improvement, and give the brief prospect for future study trends.

Genome-wide association study reveals loci and candidate genes of flowering time in jute (Corchorus L.)

Suitable flowering time can improve fiber yield and quality, which is of great significance for jute biological breeding. In this study, 242 jute accessions were planted in Fujian for 2 consecutive years, and 244,593 SNPs distributed in jute genome were used for genome-wide association analysis of flowering time. A total of 19 candidate intervals (P < 0.0001) were identified by using GLM and FaST-LMM and were significantly associated with flowering time, with phenotypic variation explained (PVE) ranging from 5.8 to 18.61%. Six stable intervals that were repeatedly detected in different environments were further identified by the linkage disequilibrium heatmap. The most likely 7 candidate genes involved to flowering time were further predicted according to the gene functional annotations. Notably, functional analysis of the candidate gene CcPRR7 of the major loci qFT-3-1, a key factor in circadian rhythm in the photoperiodic pathway, was evaluated by linkage, haplotype, and transgenic analysis. β-glucuronidase (GUS) and luciferase (LUC) activity assay of the promoters with two specific haplotypes confirmed that the flowering time can be controlled by regulating the expression of CcPRR7. The model of CcPRR7 involved in the photoperiod regulation pathway under different photoperiods was proposed. These findings provide insights into genetic loci and genes for molecular marker-assisted selection in jute and valuable information for genetically engineering PRR7 homologs in plants.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01435-8.

Involvement of CONSTANS-like Proteins in Plant Flowering and Abiotic Stress Response

The process of flowering in plants is a pivotal stage in their life cycle, and the CONSTANS-like (COL) protein family, known for its photoperiod sensing ability, plays a crucial role in regulating plant flowering. Over the past two decades, homologous genes of COL have been identified in various plant species, leading to significant advancements in comprehending their involvement in the flowering pathway and response to abiotic stress. This article presents novel research progress on the structural aspects of COL proteins and their regulatory patterns within transcription complexes. Additionally, we reviewed recent information about their participation in flowering and abiotic stress response, aiming to provide a more comprehensive understanding of the functions of COL proteins.

Identification of BBX gene family and its function in the regulation of microtuber formation in yam

BBX proteins play important roles in all of the major light-regulated developmental processes. However, no systematic analysis of BBX gene family regarding the regulation of photoperiodic microtuber formation has been previously performed in yam. In this study, a systematic analysis on the BBX gene family was conducted in three yam species, with the results, indicating that this gene plays a role in regulating photoperiodic microtuber formation. These analyses included identification the BBX gene family in three yam species, their evolutionary relationships, conserved domains, motifs, gene structure, cis-acting elements, and expressional patterns. Based on these analyses, DoBBX2/DoCOL5 and DoBBX8/DoCOL8 showing the most opposite pattern of expression during microtuber formation were selected as candidate genes for further investigation. Gene expression analysis showed DoBBX2/DoCOL5 and DoBBX8/DoCOL8 were highest expressed in leaves and exhibited photoperiod responsive expression patterns. Besides, the overexpression of DoBBX2/DoCOL5 and DoBBX8/DoCOL8 in potato accelerated tuber formation under short-day (SD) conditions, whereas only the overexpression of DoBBX8/DoCOL8 enhanced the accelerating effect of dark conditions on tuber induction. Tuber number was increased in DoBBX8/DoCOL8 overexpressing plants under dark, as well as in DoBBX2/DoCOL5 overexpressing plants under SD. Overall, the data generated in this study may form the basis of future functional characterizations of BBX genes in yam, especially regarding their regulation of microtuber formation via the photoperiodic response pathway.

BBX7 interacts with BBX8 to accelerate flowering in chrysanthemum

The quantitative control of FLOWERING LOCUS T (FT) activation is important for the floral transition in flowering plants. However, the flowering regulation mechanisms in the day-neutral, summer-flowering chrysanthemum plant remain unclear. In this study, the chrysanthemum BBX7 homolog CmBBX7 was isolated and its flowering function was identified. The expression of CmBBX7 showed a diurnal rhythm and CmBBX7 exhibited higher expression levels than CmBBX8. Overexpression of CmBBX7 in transgenic chrysanthemum accelerated flowering, whereas lines transfected with a chimeric repressor (pSRDX-CmBBX7) exhibited delayed flowering. Yeast single hybridization, luciferase, electrophoretic mobility shift, and chromatin immunoprecipitation assays showed that CmBBX7 directly targets CmFTL1. In addition, we found that CmBBX7 and CmBBX8 interact to positively regulate the expression of CmFTL1 through binding to its promoter. Collectively, these results highlight CmBBX7 as a key cooperator in the BBX8-FT module to control chrysanthemum flowering.

Identifying Genes Associated with Female Flower Development of Phellodendron amurense Rupr. Using a Transcriptomics Approach

Phellodendron amurense Rupr., a species of Rutaceae, is a nationally protected and valuable medicinal plant. It is generally considered to be dioecious. With the discovery of monoecious P. amurense, the phenomenon that its sex development is regulated by epigenetics has been revealed, but the way epigenetics affects the sex differentiation of P. amurense is still unclear. In this study, we investigated the effect of DNA methylation on the sexual development of P. amurense. The young inflorescences of male plants were treated with the demethylation agent 5-azaC, and the induced female flowers were obtained. The induced female flowers' morphological functions and transcriptome levels were close to those of normally developed plants. Genes associated with the development of female flowers were studied by comparing the differences in transcriptome levels between the male and female flowers. Referring to sex-related genes reported in other plants, 188 candidate genes related to the development of female flowers were obtained, including sex-regulating genes, genes related to the formation and development of sexual organs, genes related to biochemical pathways, and hormone-related genes. RPP0W, PAL3, MCM2, MCM6, SUP, PIN1, AINTEGUMENTA, AINTEGUMENTA-LIKE6, AGL11, SEUSS, SHI-RELATED SEQUENCE 5, and ESR2 were preliminarily considered the key genes for female flower development. This study has demonstrated that epigenetics was involved in the sex regulation of P. amurense, with DNA methylation as one of its regulatory modes. Moreover, some candidate genes related to the sexual differentiation of P. amurense were obtained with analysis. These results are of great significance for further exploring the mechanism of sex differentiation of P. amurense and studying of sex differentiation of plants.

Overexpression of two CONSTANS-like 2 (MiCOL2) genes from mango delays flowering and enhances tolerance to abiotic stress in transgenic Arabidopsis

The CO/COL gene family plays an important role in regulating photoperiod-dependent flowering time in plants. In this study, two COL2 gene homologs, MiCOL2A and MiCOL2B, were isolated from 'SiJiMi' mango, and their expression patterns and functions were characterized. The MiCOL2A and MiCOL2B genes both belonged to the group Ⅰ of CO/COL gene family. MiCOL2A and MiCOL2B exhibited distinct circadian rhythms and were highly expressed in leaves during the flowering induction period. Subcellular localization analysis revealed that MiCOL2A and MiCOL2B are localized in the nucleus. The overexpression of MiCOL2A and MiCOL2B significantly delayed flowering time in Arabidopsis under both long-day (LD) and short-day (SD) conditions. The MiCOL2A and MiCOL2B overexpression Arabidopsis plants exhibited more tolerance to slat and drought stress after abiotic stress treatments, with greater ROS scavenging capacity and protective enzyme activity, less cell damage and death and higher expression of stress response genes than wild type plants. Bimolecular fluorescence complementation (BiFC) analysis showed that MiCOL2A and MiCOL2B interacted with several stress-related proteins, including zinc finger protein 4 (MiZFP4), MYB30-INTERACTING E3 LIGASE 1 (MiMIEL1) and RING zinc finger protein 34 (MiRZFP34). The results indicate that MiCOL2A and MiCOL2B are not only involved in flowering time but also play a positive role in abiotic stress responses in plants.

Multi-layered roles of BBX proteins in plant growth and development

Light and phytohormone are external and internal cues that regulate plant growth and development throughout their life cycle. BBXs (B-box domain proteins) are a group of zinc finger proteins that not only directly govern the transcription of target genes but also associate with other factors to create a meticulous regulatory network to precisely regulate numerous aspects of growth and developmental processes in plants. Recent studies demonstrate that BBXs play pivotal roles in light-controlled plant growth and development. Besides, BBXs have been documented to regulate phytohormone-mediated physiological procedures. In this review, we summarize and highlight the multi-faced role of BBXs, with a focus in photomorphogenesis, photoperiodic flowering, shade avoidance, abiotic stress, and phytohormone-mediated growth and development in plant.

Genome-wide identification and expression pattern analysis of quinoa BBX family

BBX is a transcription factor encoding zinc finger protein that plays a key role in plant growth and development as well as in responding to abiotic stresses. However, in quinoa, which is known as a "super grain" and has extremely high nutritional value, this gene family has not yet been thoroughly studied. In this study, in order to fully understand the family function of the BBX in quinoa, a total of 31 BBX members were identified by bioinformatics methods. These BBX members were mainly acidic proteins, and most of their secondary structures were random coil s, 31 CqBBX members were unevenly distributed on 17 chromosomes, and the analysis of replication events found that quinoa BBX genes produced a total of 14 pairs of gene replication. The BBX genes were divided into five subfamilies according to phylogenetics, and its gene structure and conserved motif were basically consistent with the classification of its phylogenetic tree. In addition, a total of 43 light response elements, hormone response elements, tissue-specific expression response elements, and abiotic stress response elements were found in the promoter region, involving stress elements such as drought and low temperature. Finally, the expression patterns of CqBBX genes in different tissues and abiotic stresses were studied by combining transcriptome data and qRT-PCR , and all 13 genes responded to drought, salt, and low-temperature stress to varying degrees. This study is the first comprehensive study of the BBX family of quinoa, and its results provide important clues for further analysis of the function of the abiotic stress response.

Involvement of PtCOL5-PtNF-YC4 in reproductive cone development and gibberellin signaling in Chinese pine

It is well documented that the CO/NF-YB/NF-YC trimer (NF-Y-CO) binds and regulates the FT promoter. However, the FT/TFL1-like (FLOWERING LOCUS T/TERMINALFLOWER1-like) genes in gymnosperms are all flowering suppressors, and the regulation model of NF-Y in gymnosperms is different from that in angiosperms. Here, using Chinese pine (Pinus tabuliformis), we identified a CONSTANS-LIKE gene, PtCOL5, the expression of which was strongly induced during cones development and it functioned as a repressor of flowering. PtNF-YC4, which interacted with PtCOL5, was highly correlated with PtCOL5 during growth and development, has been demonstrated. Moreover, PtNF-YC4 and PtCOL5 can bind to PtTFL2 promoter, and their interaction can enhance PtTFL2 expression. Interestingly, we found PtNF-YC4 and PtCOL5 were involved in gibberellin signaling and their interaction was inhibited by PtDELLA protein, thus affecting PtTFL2 expression. Collectively, PtCOL5-PtNF-YC4 was involved in reproductive cone development and gibberellin signaling in Chinese pine. Our findings uncovered reproductive cone development and signal transduction mechanism of COL-NF-Y in gymnosperms.

sRNA Analysis Evidenced the Involvement of Different Plant Viruses in the Activation of RNA Silencing-Related Genes and the Defensive Response Against Plum pox virus of 'GF305' Peach Grafted with 'Garrigues' Almond

Plum pox virus (PPV) causes sharka disease in Prunus trees. Peach (P. persica) trees are severely affected by PPV, and no definitive source of genetic resistance has been identified. However, previous results showed that PPV-resistant 'Garrigues' almond (P. dulcis) was able to transfer its resistance to 'GF305' peach through grafting, reducing symptoms and viral load in PPV-infected plants. A recent study tried to identify genes responsible for this effect by studying messenger RNA expression through RNA sequencing in peach and almond plants, before and after grafting and before and after PPV infection. In this work, we used the same peach and almond samples but focused the high-throughput analyses on small RNA (sRNA) expression. We studied massive sequencing data and found an interesting pattern of sRNA overexpression linked to antiviral defense genes that suggested activation of these genes followed by downregulation to basal levels. We also discovered that 'Garrigues' almond plants were infected by different plant viruses that were transferred to peach plants. The large amounts of viral sRNA found in grafted peaches indicated a strong RNA silencing antiviral response and led us to postulate that these plant viruses could be collaborating in the observed "Garrigues effect."

Integrative mRNA and Long Noncoding RNA Analysis Reveals the Regulatory Network of Floral Bud Induction in Longan (Dimocarpus longan Lour.)

Longan (Dimocarpus longan Lour.) is a tropical/subtropical fruit tree of significant economic importance. Floral induction is an essential process for longan flowering and plays decisive effects on the longan yield. Due to the instability of flowering, it is necessary to understand the molecular mechanisms of floral induction in longan. In this study, mRNA and long noncoding RNA (lncRNA) transcriptome sequencing were performed using the apical buds of fruiting branches as materials. A total of 7,221 differential expressions of mRNAs (DEmRNAs) and 3,238 differential expressions of lncRNAs (DElncRNAs) were identified, respectively. KEGG enrichment analysis of DEmRNAs highlighted the importance of starch and sucrose metabolic, circadian rhythms, and plant hormone signal transduction pathways during floral induction. Combining the analysis of weighted gene co-expression network (WGCNA) and expression pattern of DEmRNAs in the three pathways, specific transcriptional characteristics at each stage during floral induction and regulatory network involving co-expressed genes were investigated. The results showed that sucrose metabolism and auxin signal transduction may be crucial for the growth and maturity of autumn shoots in September and October (B1-B2 stage); starch and sucrose metabolic, circadian rhythms, and plant hormone signal transduction pathways participated in the regulation of floral bud physiological differentiation together in November and December (B3-B4 stage) and the crosstalk among three pathways was also found. Hub genes in the co-expression network and key DEmRNAs in three pathways were identified. The circadian rhythm genes FKF1 and GI were found to activate SOC1gene through the photoperiod core factor COL genes, and they were co-expressed with auxin, gibberellin, abscisic acid, ethylene signaling genes, and sucrose biosynthesis genes at B4 stage. A total of 12 hub-DElncRNAs had potential for positively affecting their distant target genes in three putative key pathways, predominantly in a co-transcriptional manner. A hypothetical model of regulatory pathways and key genes and lncRNAs during floral bud induction in longan was proposed finally. Our studies will provide valuable clues and information to help elucidate the potential molecular mechanisms of floral initiation in longan and woody fruit trees.

Transcriptome Profiling Reveals a Petunia Transcription Factor, PhCOL4, Contributing to Antiviral RNA Silencing

RNA silencing is a common antiviral mechanism in eukaryotic organisms. However, the transcriptional regulatory mechanism that controls the RNA silencing process remains elusive. Here, we performed high-depth transcriptome analysis on petunia (Petunia hybrida) leaves infected with tobacco rattle virus (TRV) strain PPK20. A total of 7,402 differentially expressed genes (DEGs) were identified. Of them, some RNA silencing-related transcripts, such as RNA-dependent RNA polymerases (RDRs), Dicer-like RNase III enzymes (DCLs), and Argonautes (AGOs), were induced by viral attack. Furthermore, we performed TRV-based virus-induced gene silencing (VIGS) assay on 39 DEGs encoding putative transcription factors (TFs), using green fluorescent protein (GFP) and phytoene desaturase (PhPDS) as reporters. Results showed that the down-regulation of PhbHLH41, PhbHLH93, PhZPT4-3, PhCOL4, PhHSF-B3A, PhNAC90, and PhWRKY75 led to enhanced TRV accumulation and inhibited PhPDS-silenced photobleaching phenotype. In contrast, silencing of PhERF22 repressed virus accumulation and promoted photobleaching development. Thus, these TFs were identified as potential positive and negative regulators of antiviral RNA silencing, respectively. One positive regulator PhCOL4, belonging to the B-box zinc finger family, was selected for further functional characterization. Silencing and transient overexpression of PhCOL4 resulted in decreased and increased expression of several RNA silencing-related genes. DNA affinity purification sequencing analysis revealed that PhCOL4 targeted PhRDR6 and PhAGO4. Dual luciferase and yeast one-hybrid assays determined the binding of PhCOL4 to the PhRDR6 and PhAGO4 promoters. Our findings suggest that TRV-GFP-PhPDS-based VIGS could be helpful to identify transcriptional regulators of antiviral RNA silencing.

Transcriptome landscape of early inflorescence developmental stages identifies key flowering time regulators in chickpea

Transcriptome landscape during early inflorescence developmental stages identified candidate flowering time regulators including Early Flowering 3a. Further genomics approaches validated the role of this gene in flowering time regulation. The early stages of inflorescence development in plants are as crucial as the later floral developmental stages. Several traits, such as inflorescence architecture and flower developmental timings, are determined during those early stages. In chickpea, diverse forms of inflorescence architectures regarding meristem determinacy and the number of flowers per node are observed within the germplasm. Transcriptome analysis in four desi chickpea accessions with such unique inflorescence characteristics identifies the underlying shared regulatory events leading to inflorescence development. The vegetative to reproductive stage transition brings about major changes in the transcriptome landscape. The inflorescence development progression associated genes identified through co-expression network analysis includes both protein-coding genes and long non-coding RNAs (lncRNAs). Few lncRNAs identified in our study positively regulate flowering-related mRNA stability by acting competitively with miRNAs. Bulk segregrant analysis and association mapping narrowed down an InDel marker regulating flowering time in chickpea. Deletion of 11 bp in first exon of a negative flowering time regulator, Early Flowering 3a gene, leads to early flowering phenotype in chickpea. Understanding the key players involved in vegetative to reproductive stage transition and floral meristem development will be useful in manipulating flowering time and inflorescence architecture in chickpea and other legumes.

Isolation and Functional Characterization of Two CONSTANS-like 16 (MiCOL16) Genes from Mango

CONSTANS (CO) is an important regulator of photoperiodic flowering and functions at a key position in the flowering regulatory network. Here, two CO homologs, MiCOL16A and MiCOL16B, were isolated from “SiJiMi” mango to elucidate the mechanisms controlling mango flowering. The MiCOL16A and MiCOL16B genes were highly expressed in the leaves and expressed at low levels in the buds and flowers. The expression levels of MiCOL16A and MiCOL16B increased during the flowering induction period but decreased during the flower organ development and flowering periods. The MiCOL16A gene was expressed in accordance with the circadian rhythm, and MiCOL16B expression was affected by diurnal variation, albeit not regularly. Both the MiCOL16A and MiCOL16B proteins were localized in the nucleus of cells and exerted transcriptional activity through their MR domains in yeast. Overexpression of both the MiCOL16A and MiCOL16B genes significantly repressed flowering in Arabidopsis under short-day (SD) and long-day (LD) conditions because they repressed the expression of AtFT and AtSOC1. This research also revealed that overexpression of MiCOL16A and MiCOL16B improved the salt and drought tolerance of Arabidopsis, conferring longer roots and higher survival rates to overexpression lines under drought and salt stress. Together, our results demonstrated that MiCOL16A and MiCOL16B not only regulate flowering but also play a role in the abiotic stress response in mango.

BBX17 Interacts with CO and Negatively Regulates Flowering Time in Arabidopsis thaliana

Floral transition, the change from vegetative growth to reproductive development, is dramatic in flowering plants. Here, we show that one subgroup III member of the B-box (BBX) family, BBX17, is a repressor of floral transition under long-day conditions. BBX17 contains a B-box domain and a CCT domain. Although the phenotype of the BBX17 loss-of-function plants was comparable to that of wild-type plants, BBX17-overexpression plants displayed a delayed-flowering phenotype under long-day conditions. The delayed-flowering phenotype was not the result of an altered CONSTANS (CO) expression level but rather the repression of the FLOWERING LOCUS T (FT) expression level. BBX17 physically associated with CO and repressed its ability to control FT expression. Furthermore, the BBX17 protein degraded in the dark, but irradiating seedlings with white, blue, red or far-red light stabilized the BBX17 level. We also proved that the degradation of BBX17 was via 26S proteasome and requires COP1. Thus, BBX17 acts as a key factor in the CO-FT regulatory system to control Arabidopsis thaliana flowering.

Comparative Transcriptome Analysis Reveals Sex-Biased Expression of Hormone-Related Genes at an Early Stage of Sex Differentiation in Red Bayberry (Morella rubra)

The molecular mechanism of sex development and differentiation in the economically important dioecious fruit tree, red bayberry (Morella rubra), was revealed using next-generation transcriptome sequencing (NGS), and comparative analyses were used to identify differentially expressed genes (DEGs) in female and male flower buds. A total of 7,029 of these DEGs were identified at two early development stages. KEGG pathway enrichment analysis revealed that plant hormone signal transduction was significantly overrepresented, and 91 genes related to hormones were identified. An analysis of 7,029 DEGs revealed 161 hormone-related genes, with the 42 related to auxin and 26 related to ethylene being the most highly represented. A total of 62 genes were significantly up-regulated in females and 29 were in males, with 18 of them specifically expressed in females and 10 in males. A total of 415 transcription factors were identified, with 129 genes up-regulated in females and 53 in males. Moreover, 38 had female-specific expression and 18 had male-specific expression. Using weighted gene co-expression network analysis (WGCNA), two modules were found to be associated with sexual type. In the module coded light-green, there were five genes related to hormones, one to flower development and ten transcription factors with four genes specifically expressed in the males and four in females. The hub gene in the light-green module is MR0TCONS_00017483.1 (ACO), which is involved in ethylene biosynthesis and had male-specific expression. Among the transcription factors, three of the four male-specific expressed genes involved in flavonoid biosynthesis, including the MYB gene MR1TCONS_00020658.1 and two BHLH genes, MR6G001563.1 and MR8G020751.1, played important roles in male floral differentiation. In the dark-cyan module, six hormone-related genes, five transcription factors and three flower development genes were identified with the hub gene MR1G019545.1 (ETR1), which participates in the ethylene signaling pathway, and MR4G023618.1, which encodes the C3H zinc finger transcription factor. These results indicate that ethylene is the key hormone that interacts with other hormones and transcription factors to regulate sex differentiation in the red bayberry, which also provides new insights into the mechanism of sex determination and differentiation in the red bayberry.

Genome-Wide Identification and Expression Profiling of CONSTANS-Like Genes in Pepper (Capsicum annuum): Gaining an Insight to Their Phylogenetic Evolution and Stress-Specific Roles

CONSTANS-like (COL) genes play important regulatory roles in multiple growth and development processes of plants but have rarely been studied in Capsicum annuum. This study explored the evolutionary relationship and expression patterns of COL genes from C. annuum. A total of 10 COL genes were identified in the genome of the cultivated pepper Zunla-1 and were named CaCOL01-10. These genes were unequally distributed among five chromosomes and could be divided into three groups based on differences in gene structure characteristics. During evolutionary history, duplications and retentions were divergent among different groups of COL genes. Tandem duplication caused amplification of group I genes. Genetic distance among COL genes was the largest in group III, suggesting that group III genes undergo more relaxed selection pressure compared with the other groups. Expression patterns of CaCOLs in tissues were significantly different, with CaCOL08 exhibiting the highest expression in stem and leaf. Some COL orthologous genes showed markedly different expression patterns in pepper compared with tomato, such as COL_1 orthologs, which may be involved in fruit development in pepper. In addition, CaCOLs participated in the regulation of abiotic stresses to varying degrees. Five CaCOL genes were induced by cold, and CaCOL02 and CaCOL03 were specifically upregulated by cold and downregulated by heat. This study provides a theoretical basis for the in-depth understanding of the functions of COL genes in pepper and their molecular mechanisms involved in growth and development and responses to abiotic stresses.

Identification of Flowering Regulatory Networks and Hub Genes Expressed in the Leaves of Elymus sibiricus L. Using Comparative Transcriptome Analysis

Flowering is a significant stage from vegetative growth to reproductive growth in higher plants, which impacts the biomass and seed yield. To reveal the flowering time variations and identify the flowering regulatory networks and hub genes in Elymus sibiricus, we measured the booting, heading, and flowering times of 66 E. sibiricus accessions. The booting, heading, and flowering times varied from 136 to 188, 142 to 194, and 148 to 201 days, respectively. The difference in flowering time between the earliest- and the last-flowering accessions was 53 days. Furthermore, transcriptome analyses were performed at the three developmental stages of six accessions with contrasting flowering times. A total of 3,526 differentially expressed genes (DEGs) were predicted and 72 candidate genes were identified, including transcription factors, known flowering genes, and plant hormone-related genes. Among them, four candidate genes (LATE, GA2OX6, FAR3, and MFT1) were significantly upregulated in late-flowering accessions. LIMYB, PEX19, GWD3, BOR7, PMEI28, LRR, and AIRP2 were identified as hub genes in the turquoise and blue modules which were related to the development time of flowering by weighted gene co-expression network analysis (WGCNA). A single-nucleotide polymorphism (SNP) of LIMYB found by multiple sequence alignment may cause late flowering. The expression pattern of flowering candidate genes was verified in eight flowering promoters (CRY, COL, FPF1, Hd3, GID1, FLK, VIN3, and FPA) and four flowering suppressors (CCA1, ELF3, Ghd7, and COL4) under drought and salt stress by qRT-PCR. The results suggested that drought and salt stress activated the flowering regulation pathways to some extent. The findings of the present study lay a foundation for the functional verification of flowering genes and breeding of new varieties of early- and late-flowering E. sibiricus.

Conservation and Divergence of the CONSTANS-Like (COL) Genes Related to Flowering and Circadian Rhythm in Brassica napus

The CONSTANS-LIKE (COL) genes are important signaling component in the photoperiod pathway and flowering regulation pathway. However, people still know little about their role in Brassica napus. To achieve a better understanding of the members of the BnaCOL gene family, reveal their evolutionary relationship and related functions involved in photoperiod regulation, we systematically analyzed the BnaCOL family members in B. napus genome. A total of 33 BnaCOL genes distributed unevenly on 16 chromosomes were identified in B. napus and could be classified into three subfamilies. The same subfamilies have relatively conservative gene structures, three-dimensional protein structures and promoter motifs such as light-responsive cis-elements. The collinearity analysis detected 37 pairs of repetitive genes in B. napus genome. A 67.7% of the BnaCOL genes were lost after B. napus genome polyploidization. In addition, the BnaCOL genes showed different tissue-specific expression patterns. A 81.8% of the BnaCOL genes were mainly expressed in leaves, indicating that they may play a conservative role in leaves. Subsequently, we tested the circadian expression profiles of nine homologous genes that regulate flowering in Arabidopsis. Most BnaCOL genes exhibit several types of circadian rhythms, indicating that these BnaCOL genes are involved in the photoperiod pathway. As such, our research has laid the foundation for understanding the exact role of the BnaCOL family in the growth and development of rapeseed, especially in flowering.

Genome-wide identification, genomic organization, and expression profiling of the CONSTANS-like (COL) gene family in petunia under multiple stresses

Background

CONSTANS-like (CO-like, COL) are putative zinc-finger transcription factors known to play vital role in various plant biological processes such as control of flowering time, regulation of plant growth and development and responses to stresses. However, no systematic analysis of COL family gene regarding the plant development and stress response has been previously performed in any solanaceous crop. In the present study, a comprehensive genome-wide analysis of COL family genes in petunia has been conducted to figure out their roles in development of organs and stress response.

Results

A total of 33 COL genes, 15 PaCOL genes in P. axillaris and 18 PiCOL genes in P. inflata, were identified in petunia. Subsequently, a genome-wide systematic analysis was performed in 15 PaCOL genes. Considering the domain composition and sequence similarity the 15 PaCOL and 18 PiCOL genes were phylogenetically classified into three groups those are conserved among the flowering plants. Moreover, all of the 15 PaCOL proteins were localized in nucleus. Furthermore, differential expression patterns of PaCOL genes were observed at different developmental stages of petunia. Additionally, transcript expression of 15 PaCOL genes under various abiotic and phytohormone treatments showed their response against stresses. Moreover, several cis-elements related to stress, light-responsive, hormone signaling were also detected in different PaCOL genes.

Conclusion

The phylogenetic clustering, organ specific expression pattern and stress responsive expression profile of conserved petunia COL genes indicating their involvement in plant growth and development and stress response mechanism. This work provide a significant foundation for understanding the biological roles of petunia COL genes in plant growth, development and in stress response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08019-w.

Two B-box domain proteins, BBX28 and BBX29, regulate flowering time at low ambient temperature in Arabidopsis

This paper demonstrates that BBX28 and BBX29 proteins in Arabidopsis promote flowering in association with the CO-FT regulatory module at low ambient temperature under LD conditions. Flowering plants integrate internal developmental signals with external environmental stimuli for precise flowering time control. The expression of BBX29 is up-regulated by low temperature treatment, but the biological function of BBX29 in low temperature response is unknown. In the current study, we examined the biological role of BBX29 and its close-related protein BBX28 in flowering time control under long-day conditions. Although neither BBX28 single mutant nor BBX29 single mutant has a flowering-associated phenotype, the bbx28 bbx29 double mutant plants have an obvious delayed flowering phenotype grown at low ambient temperature (16°C) compared to the wild-type (WT) plants. The expression of FT and TSF was lower in bbx28 bbx29 double mutant plants than in wild-type plants at 16°C. Both BBX28 and BBX29 interact with CONSTANS (CO), an important flowering integrator that directly binds to the FLOWERING LOCUS T (FT) promoter. In the effector-reporter assays, transcriptional activation activity of CO on the FT promoter was reduced in bbx28 bbx29 double mutant plants compared to that in WT plants. Taken together, our results reveal that BBX28 and BBX29 are promoters of flowering in Arabidopsis, especially at low ambient temperature.

A CONSTANS-LIKE gene of Nelumbo nucifera could promote potato tuberization

CONSTANS-LIKE 5 of Nelumbo nucifera is capable of promoting potato tuberization through CONSTANS-FLOWERING LOCUS T and gibberellin signaling pathways with a probable association with lotus rhizome enlargement. Lotus (Nelumbo nucifera) is an aquatic plant that is affiliated to the Nelumbonaceace family. It is widely used as an ornamental, vegetable, and medicinal herb with its rhizome being a popular vegetable. To explore the molecular mechanism underlying its rhizome enlargement, we conducted a systematic analysis on the CONSTANS-LIKE (COL) gene family, with the results, indicating that this gene plays a role in regulating potato tuber expansion. These analyses included phylogenetic relationships, gene structure, and expressional patterns of lotus COL family genes. Based on these analyses, NnCOL5 was selected for further study on its potential function in lotus rhizome formation. NnCOL5 was shown to be located in the nucleus, and its expression was positively associated with the enlargement of lotus rhizome. Besides, the overexpression of NnCOL5 in potato led to increased tuber weight and starch content under short-day conditions without changing the number of tubers. Further analysis suggested that the observed tuber changes might be mediated by affecting the expression of genes in CO-FT and GA signaling pathways. These results provide valuable insight in understanding the functions of COL gene as well as the enlargement of lotus rhizome.

ThCOL2 Improves the Salt Stress Tolerance of Tamarix hispida

The CONSTANS-LIKE (COL) transcription factor has been reported to play important roles in regulating plant flowering and the response to abiotic stress. To clone and screen COL genes with excellent salt tolerance from the woody halophyte Tamarix hispida, 8 ThCOL genes were identified in this study. The expression patterns of these genes under different abiotic stresses (high salt, osmotic, and heavy metal) and abscisic acid (ABA) treatment were detected using quantitative real-time PCR (qRT-PCR). The expression levels of 8 ThCOL genes changed significantly after exposure to one or more stresses, indicating that these genes were all stress-responsive genes and may be involved in the stress resistance response of T. hispida. In particular, the expression level of ThCOL2 changed significantly at most time points in the roots and leaves of T. hispida under salt stress and after ABA treatments, which may play an important role in the response process of salt stress through a mechanism dependent on the ABA pathway. The recombinant vectors pROKII-ThCOL2 and pFGC5941-ThCOL2 were constructed for the transient transformation of T. hispida, and the transient infection of T. hispida with the pROKII empty vector was used as the control to further verify whether the ThCOL2 gene was involved in the regulation of the salt tolerance response of T. hispida. Overexpression of the ThCOL2 gene in plants under 150 mM NaCl stress increased the ability of transgenic T. hispida cells to remove reactive oxygen species (ROS) by regulating the activity of protective enzymes and promoting a decrease in the accumulation of O^2- and H₂O₂, thereby reducing cell damage or cell death and enhancing salt tolerance. The ThCOL2 gene may be a candidate gene associated with excellent salt tolerance. Furthermore, the expression levels of some genes related to the ABA pathway were analyzed using qRT-PCR. The results showed that the expressions of ThNCED1 and ThNCED4 were significantly higher, and the expressions of ThNCED3, ThZEP, and ThAAO3 were not significantly altered in OE compared with CON under normal conditions. But after 24 h of salt stress, the expressions of all five studied genes all were lower than the normal condition. In the future, the downstream genes directly regulated by the ThCOL2 transcription factor will be searched and identified to analyze the salt tolerance regulatory network of ThCOL2.

Identification and characterization of the CONSTANS (CO)/CONSTANS-like (COL) genes related to photoperiodic signaling and flowering in tomato

CO is an important regulator of photoperiodic response and flowering. However, the biological functions of CO and COL genes in tomato (Solanum lycopersicum) remain elusive. Here we identified 13 members in CO/COL family from the tomato genome. They were divided into three groups, and each group had specific characteristics in gene structures and protein domains. The SlCO/SlCOL genes showed different tissue-specific expression patterns and circadian rhythms, indicating their functional diversity in tomato. Moreover, among 13 members, the expression of SlCOL, SlCOL4a, and SlCOL4b was negatively correlated with flowering time variation in ten tomato lines. Through interaction network prediction, we found three FLOWERING LOCUS T (FT) orthologs, SINGLE FLOWER TRUSS (SFT), FT-like (FTL), and FT-like 1 (FTL1), which functioned as candidate interactors of SlCOL, SlCOL4a, and SlCOL4b. Further expression analyses suggested that SFT coincided with the three SlCOL genes in ten tomato lines with varied flowering time. These findings implied that SlCOL, SlCOL4a, and SlCOL4b are potential flowering inducers in tomato, and SFT may act as their downstream target. Thus, our study built a foundation for understanding the precise roles of SlCO/SlCOL family in plant growth and development of tomato, especially in flowering.

B-box proteins: Pivotal players in light-mediated development in plants

Light signals mediate a number of physiological and developmental processes in plants, such as flowering, photomorphogenesis, and pigment accumulation. Emerging evidence has revealed that a group of B-box proteins (BBXs) function as central players in these light-mediated developmental processes. B-box proteins are a class of zinc-coordinated transcription factors or regulators that not only directly mediate the transcription of target genes but also interact with various other factors to create a complex regulatory network involved in the precise control of plant growth and development. This review summarizes and highlights the recent findings concerning the critical regulatory functions of BBXs in photoperiodic flowering, light signal transduction and light-induced pigment accumulation and their molecular modes of action at the transcriptional and post-translational levels in plants.

Let it bloom: cross-talk between light and flowering signaling in Arabidopsis

The terrestrial environment is complex, with many parameters fluctuating on daily and seasonal basis. Plants, in particular, have developed complex sensory and signaling networks to extract and integrate information about their surroundings in order to maximize their fitness and mitigate some of the detrimental effects of their sessile lifestyles. Light and temperature each provide crucial insights on the surrounding environment and, in combination, allow plants to appropriately develop, grow and adapt. Cross-talk between light and temperature signaling cascades allows plants to time key developmental decisions to ensure they are 'in sync' with their environment. In this review, we discuss the major players that regulate light and temperature signaling, and the cross-talk between them, in reference to a crucial developmental decision faced by plants: to bloom or not to bloom?

The Response of COL and FT Homologues to Photoperiodic Regulation in Carrot (Daucus carota L.)

Carrot (Daucus carota L.) is a biennial plant requiring vernalization to induce flowering, but long days can promote its premature bolting and flowering. The basic genetic network controlling the flowering time has been constructed for carrot, but there is limited information on the molecular mechanisms underlying the photoperiodic flowering response. The published carrot genome could provide an effective tool for systematically retrieving the key integrator genes of GIGANTEA (GI), CONSTANS-LIKE (COL), FLOWERING LOCUS T (FT), and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) homologues in the photoperiod pathway. In this study, the bolting time of wild species “Songzi” (Ws) could be regulated by different photoperiods, but the orange cultivar “Amsterdam forcing” (Af) displayed no bolting phenomenon. According to the carrot genome and previous de novo transcriptome, 1 DcGI, 15 DcCOLs, 2 DcFTs, and 3 DcSOC1s were identified in the photoperiod pathway. The circadian rhythm peaks of DcGI, DcCOL2, DcCOL5a, and DcCOL13b could be delayed under long days (LDs). The peak value of DcCOL2 in Af (12.9) was significantly higher than that in Ws (6.8) under short day (SD) conditions, and was reduced under LD conditions (5.0). The peak values of DcCOL5a in Ws were constantly higher than those in Af under the photoperiod treatments. The expression levels of DcFT1 in Ws (463.0) were significantly upregulated under LD conditions compared with those in Af (1.4). These responses of DcCOL2, DcCOL5a, and DcFT1 might be related to the different bolting responses of Ws and Af. This study could provide valuable insights into understanding the key integrator genes in the carrot photoperiod pathway.

Identification and characterization of the CONSTANS-like gene family and its expression profiling under light treatment in Populus

The CONSTANS-like (COL) genes play an important role in the photoperiodic flowering pathway. Poplar is a perennial woody plant with a long juvenile phase, but the molecular characterization of COL genes in Populus is limited. In this study, 14 COL genes were identified in the Populus genome. Phylogenetic analysis indicated the PtCOL proteins were divided into three subgroups, and the members of each subgroup had similar gene structure and motif composition. Chromosome distribution analysis showed that 14 PtCOL genes were distributed on 10 chromosomes. Multiple sequence alignment indicated that these proteins contained a highly conserved B-box1 and a conserved CCT domain, but the B-box2 structure was divided into three different types. Promoter analysis found that there were several light-responsive cis-elements in the PtCOL genes. Furthermore, tissue-specific expression showed that all nine PtCOL genes were widely expressed in various tissues and organs of Populus, and were preferentially expressed in the leaves. Additionally, the transcription level of PtCOL exhibited a diurnal oscillation pattern in different light conditions. This study not only provided comprehensive information for further analysis of the function of the PtCOL gene family, but also revealed the biological roles of PtCOL genes in the photoperiod-dependent flowering process of Populus.

Genome-Wide DNA Methylation Analysis of Soybean Curled-Cotyledons Mutant and Functional Evaluation of a Homeodomain-Leucine Zipper (HD-Zip) I Gene GmHDZ20

DNA methylation is a major, conserved epigenetic modification that influences many biological processes. Cotyledons are specialized tissues that provide nutrition for seedlings at the early developmental stage. To investigate the patterns of genomic DNA methylation of germinated cotyledons in soybean (Glycine max) and its effect on cotyledon development, we performed a genome-wide comparative analysis of DNA methylation between the soybean curled-cotyledons (cco) mutant, which has abnormal cotyledons, and its corresponding wild type (WT) by whole-genome bisulfite sequencing. The cco mutant was methylated at more sites but at a slightly lower level overall than the WT on the whole-genome level. A total of 46 CG-, 92 CHG-, and 9723 CHH- (H = A, C, or T) differentially methylated genes (DMGs) were identified in cotyledons. Notably, hypomethylated CHH-DMGs were enriched in the gene ontology term "sequence-specific DNA binding transcription factor activity." We selected a DMG encoding a homeodomain-leucine zipper (HD-Zip) I subgroup transcription factor (GmHDZ20) for further functional characterization. GmHDZ20 localized to the nucleus and was highly expressed in leaf and cotyledon tissues. Constitutive expression of GmHDZ20 in Arabidopsis thaliana led to serrated rosette leaves, shorter siliques, and reduced seed number per silique. A yeast two-hybrid assay revealed that GmHDZ20 physically interacted with three proteins associated with multiple aspects of plant growth. Collectively, our results provide a comprehensive study of soybean DNA methylation in normal and aberrant cotyledons, which will be useful for the identification of specific DMGs that participate in cotyledon development, and also provide a foundation for future in-depth functional study of GmHDZ20 in soybean.