CmBBX8 accelerates flowering by targeting CmFTL1 directly in summer chrysanthemum

Transcript-Wide Identification and Characterization of the BBX Gene Family in Trichosanthes kirilowii and Its Potential Roles in Development and Abiotic Stress

The B-box (BBX) protein has an impact on flowering physiology, photomorphogenesis, shade effects, and responses to both biotic and abiotic stresses. Although recent research described the BBX gene family in numerous plants, knowledge of the BBX gene in Trichosanthes kirilowii was sparse. In this study, we identified a total of 25 TkBBX genes, and phylogenetic analysis showed that these genes were divided into five subfamilies. Analyses of gene structure and motifs for each group found relative conservation. Ka/Ks values showed that most TkBBX genes have undergone negative selection. qRT-PCR analyses revealed that TkBBX1, TkBB4, TkBBX5, TkBBX7, TkBBX15, TkBBX16, TkBBX17, TkBBX19, and TkBBX21 genes respond to salt and drought treatment. Furthermore, we cloned TkBBX7 and TkBBX17 genes and performed a subcellular localization experiment, which revealed that these two genes were both located in the nucleus. Transgenic yeast experiments demonstrated that TkBBX7 and TkBBX17 enhanced yeast tolerance to both salt and drought stresses. These findings provide a theoretical foundation for further investigation on the functions of TkBBX genes in Trichosanthes kirilowii.

Genetic dissection of flowering time and fine mapping of qFT.A02-1 in rapeseed (Brassica napus L.)

KEY MESSAGE

qFT.A02-1, a major quantitative trait locus controlling flowering time in Brassica napus, was mapped to a 104.8-kb region on chromosome A02, and BnaA02G0156900ZS is the candidate gene in response for flowering time. Flowering time is a key agronomic trait that determines the adaptability of crops to the environment and thus affects yields. The mechanism underlying flowering time is still far from clear in Brassica napus. In this study, a recombinant inbred line population composed of 215 lines was constructed and 35 flowering time QTLs were identified. One major QTL, qFT.A02-1 (explaining 16.40-17.80% of phenotypic variation), was detected in two environments, which was confirmed by QTL-seq. A residual heterozygous line containing qFT.A02-1 for flowering time was further constructed, and qFT.A02-1 was subsequently fine-mapped to a 104.8-kb interval, wherein a total of 11 genes were predicted. Candidate gene functional annotation implied that BnaA02G0156900ZS, a homologous gene of FLOWERING LOCUS T in B. napus, was likely the candidate gene for qFT.A02-1. HiFi sequencing of the two parents was subsequently conducted, and a 1,079-bp insertion in the promoter of BnaA02. FT was confirmed. The allelic variation analysis in a diversity of accessions identified another 6 SNPs existing in the non-coding region of BnaA02. FT and the 1,079-bp insertion in promoter region are closely associated with the flowering time in B. napus. Haplotype analysis indicated that the flowering time of Hap02 is significantly earlier than Hap01 and Hap04, and Hap05 is significantly earlier than Hap04. Yield-related trait analysis revealed that there are no significant differences in yield-related traits between the two near-isogenic lines based on the target locus. These results may advance our understanding of the mechanism underlying flowering time in B. napus.

The B-box protein CmBBX8 recruits chromatin modifiers CmFDM2/CmSWI3B to induce flowering in summer chrysanthemum

The transition from vegetative to reproductive growth is essential for the flowering process of plants. In summer chrysanthemum, CmBBX8 exploits prominence function in floral transition by activating the expression of CmFTL1. However, how CmBBX8 induces CmFTL1 during the photoperiod inductive cycles remains unknown. Here, we show that CmBBX8 interacts with the SGS3-like protein CmFDM2, and the CmFDM2 overexpression strains presented early flowering, while knockdown strains delayed flowering. Additionally, CmFDM2 could bind to the CmFTL1 promoter and activate the expression of CmFTL1, and associate with chromatin remodeling factor CmSWI3B, and CmBBX8 induces flowering dependent on CmFDM2 and CmSWI3B. CmFDM2 also partially depends on CmSWI3B. The CmSWI3B knockdown strains exhibited a significant late flowering phenotype. Interestingly, CmBBX8 also interacts with CmSWI3B. Moreover, the level of H3K27me3 at the CmFTL1 locus was reduced when CmBBX8 and CmFDM2/CmSWI3B occupied the locus to promote chrysanthemum flowering during the photoperiod inductive cycles, which was accompanied by the increasing level of CmFTL1 transcripts. Thus, our work provides novel insights into the gradually increasing level of CmFTL1 for the floral transition through CmBBX8 recruiting chromatin modifiers CmFDM2/CmSWI3B.

Overexpression of StBBX14 Enhances Cold Tolerance in Potato

Potato (Solanum tuberosum L.) is an important food crop, but low temperature affects the potato growth and yield. In this study, the expression level of StBBX14 was significantly increased over 1 h and then gradually decreased under cold stress. The subcellular localization of the StBBX14 protein took place in the nucleus. The OE-StBBX14 transgenic lines showed less leaf damage and significantly lower electrolyte leakage compared with the WT under cold stress, indicating that the overexpression of StBBX14 in the potato enhanced the cold resistance. A transcriptome analysis showed that a total of 2449 and 6274 differentially expressed genes were identified in WT-1 h and WT-12 h, respectively, when compared with WT-0h. A Gene Ontology enrichment analysis revealed that photosynthesis, cell wall, thylakoid, transcription regulator activity, oxidoreductase activity and glucosyltransferase activity were significantly enriched in OE-StBBX14 and WT. A total of 14 distinct modules were generated by a WGCNA analysis based on all differentially expressed genes (DEGs). Four major modules with cold-related genes were isolated. RT-qPCR analysis showed that the expression patterns of eight DEGs were consistent between the qPCR and RNA-seq. These findings illustrate that the StBBX14 played an important role in cold stress in potato and provided a data basis for the genetic improvement of cold resistance traits of potato.

ChBBX6 and ChBBX18 are positive regulators of anthocyanins biosynthesis and carotenoids degradation in Cerasus humilis

B-box zinc-finger transcription factor (BBX) plays important regulatory roles in plant secondary metabolism. Here, we identified 21 BBXs that could be further categorized into five subfamilies from Cerasus humilis. Two segmentally duplicated Subfamily IV members, ChBBX6 and ChBBX18, were found to share high homology with reported anthocyanin-related BBXs and express highly in fruits with high anthocyanins but low carotenoids contents. Their transient overexpression in apple and C. humilis fruits both led to significantly increased anthocyanins accumulation and significantly upregulated expression of anthocyanins-related genes. However, their overexpression resulted in decreased carotenoids accumulation and greatly upregulated the expression of carotenoids-related genes especially degradation-related genes. Additionally, their overexpression both greatly improved the ABA content in C. humilis fruits. Through yeast one-hybrid and dual-luciferase reporter assays, we found that both ChBBX6 and ChBBX18 could bind to and activate the promoters of chalcone synthase (ChCHS), flavanone 3-hydroxylase (ChF3H), and 9-cis-epoxycarotenoid dioxygenase 5 (ChNCED5). Our study demonstrates that ChBBX6 and ChBBX18 are positive regulators of anthocyanins biosynthesis and carotenoids degradation and can provide basis for understanding the roles of BBX genes in C. humilis.

A truncated B-box zinc finger transcription factor confers drought sensitivity in modern cultivated tomatoes

Enhancing drought tolerance in crops and understanding the underlying mechanisms have been subject of intense research. The precise function and molecular mechanisms of B-box zinc finger proteins (BBX) remain elusive. Here, we report a natural allele of BBX18 (BBX18TT) that encodes a C-terminal truncated protein. While most wild tomato germplasms contain the BBX18CC allele and show more drought tolerant, modern cultivated tomatoes mostly carry BBX18TT allele and are more drought sensitive. Knockout of BBX18 leads to improved drought tolerance in transgenic plants of cultivated tomato. Ascorbate peroxidase 1 (APX1) is identified as a BBX18-interacting protein that acts as a positive regulator of drought resistance in tomato. Chromatin immunoprecipitation sequencing analyses reveal that BBX18 binds to a unique cis-acting element of the APX1 promoter and represses its gene expression. This study provides insights into the molecular mechanism underlying drought resistance mediated by the BBX18-APX1 module in plants.

Identification of SDG gene family members and exploration of flowering related genes in different cultivars of chrysanthemums and their wild ancestors

The SET domain genes (SDGs) are significant contributors to various aspects of plant growth and development, mainly includes flowering, pollen development, root growth, regulation of the biological clock and branching patterns. To clarify the biological functions of the chrysanthemum SDG family, the SDG family members of four chrysanthemum cultivars and three related wild species were identified; their physical and chemical properties, protein domains and conserved motifs were predicted and analyzed. The results showed that 59, 67, 67, 102, 106, 114, and 123 SDGs were identified from Chrysanthemum nankingense, Chrysanthemum lavandulifolium, Chrysanthemum seticuspe, Chrysanthemum × morifolium cv. 'Hechengxinghuo', 'Zhongshanzigui', 'Quanxiangshuichang' and 'Jinbeidahong', respectively. The SDGs were divided into 5-7 subfamilies by cluster analysis; different conserved motifs were observed in particular families. The SDGs of C. lavandulifolium and C. seticuspe were distributed unevenly on 9 chromosomes. SDG promoters of different species include growth and development, photo-response, stress response and hormone responsive elements, among them, the cis-acting elements related to MeJA response had the largest proportion. The expression of chrysanthemum SDG genes was observed for most variable selected genes which has close association with important Arabidopsis thaliana genes related to flowering regulation. The qPCR results showed that the expression trend of SDG genes varied in different tissues at different growth stages with high expression in the flowering period. The ClSDG29 showed higher expression in the flower and bud tissues, which indicate that ClSDG29 might be associated with flowering regulation in chrysanthemum. In summary, the results of this study can provide a basis for subsequent research on chrysanthemum flowering time regulation.

Genome-Wide Identification of B-Box Family Genes and Their Potential Roles in Seed Development under Shading Conditions in Rapeseed

B-box (BBX) proteins, a subfamily of zinc-finger transcription factors, are involved in various environmental signaling pathways. In this study, we conducted a comprehensive analysis of BBX family members in Brassica crops. The 482 BBX proteins were divided into five groups based on gene structure, conserved domains, and phylogenetic analysis. An analysis of nonsynonymous substitutions and (Ka)/synonymous substitutions (Ks) revealed that most BBX genes have undergone purifying selection during evolution. An analysis of transcriptome data from rapeseed (Brassica napus) organs suggested that BnaBBX3d might be involved in the development of floral tissue-specific RNA-seq expression. We identified numerous light-responsive elements in the promoter regions of BnaBBX genes, which were suggestive of participation in light signaling pathways. Transcriptomic analysis under shade treatment revealed 77 BnaBBX genes with significant changes in expression before and after shading treatment. Of these, BnaBBX22e showed distinct expression patterns in yellow- vs. black-seeded materials in response to shading. UPLC-HESI-MS/MS analysis revealed that shading influences the accumulation of 54 metabolites, with light response BnaBBX22f expression correlating with the accumulation of the flavonoid metabolites M46 and M51. Additionally, BnaBBX22e and BnaBBX22f interact with BnaA10.HY5. These results suggest that BnaBBXs might function in light-induced pigment accumulation. Overall, our findings elucidate the characteristics of BBX proteins in six Brassica species and reveal a possible connection between light and seed coat color, laying the foundation for further exploring the roles of BnaBBX genes in seed development.

B-BOX proteins:Multi-layered roles of molecular cogs in light-mediated growth and development in plants

B-box containing proteins (BBXs) are a class of zinc-ligating transcription factors or regulators that play essential roles in various physiological and developmental processes in plants. They not only directly associate with target genes to regulate their transcription, but also interact with other transcription factors to mediate target genes' expression, thus forming a complex transcriptional network ensuring plants' adaptation to dynamically changing light environments. This review summarizes and highlights the molecular and biochemical properties of BBXs, as well as recent advances with a focus on their critical regulatory functions in photomorphogenesis (de-etiolation), shade avoidance, photoperiodic-mediated flowering, and secondary metabolite biosynthesis and accumulation in plants.

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.

Involvement of the tomato BBX16 and BBX17 microProteins in reproductive development

BBXs are B-Box zinc finger proteins that can act as transcription factors and regulators of protein complexes. Several BBX proteins play important roles in plant development. Two Arabidopsis thaliana microProteins belonging to the BBX family, named miP1a and miP1b, homotypically interact with and modulate the activity of other BBX proteins, including CONSTANS, which transcriptionally activates the florigen, FLOWERING LOCUS T. Arabidopsis plants overexpressing miP1a and miP1b showed delayed flowering. In tomato, the closest homologs of miP1a and miP1b are the microProteins SlBBX16 and SlBBX17. This study was aimed at investigating whether the constitutive expression of SlBBX16/17 in Arabidopsis and tomato impacted reproductive development. The heterologous expression of the two tomato microProteins in Arabidopsis caused a delay in the flowering transition; however, the effect was weaker than that observed when the native miP1a/b were overexpressed. In tomato, overexpression of SlBBX17 prolonged the flowering period; this effect was accompanied by downregulation of the flowering inhibitors Self Pruning (SP) and SP5G. SlBBX16 and SlBBX17 can hetero-oligomerize with TCMP-2, a cystine-knot peptide involved in flowering pattern regulation and early fruit development in tomato. The increased expression of both microProteins also caused alterations in tomato fruit development: we observed in the case of SlBBX17 a decrease in the number and size of ripe fruits as compared to WT plants, while for SlBBX16, a delay in fruit production up to the breaker stage. These effects were associated with changes in the expression of GA-responsive genes.

GhRCD1 promotes cotton tolerance to cadmium by regulating the GhbHLH12-GhMYB44-GhHMA1 transcriptional cascade

Heavy metal pollution poses a significant risk to human health and wreaks havoc on agricultural productivity. Phytoremediation, a plant-based, environmentally benign, and cost-effective method, is employed to remove heavy metals from contaminated soil, particularly in agricultural or heavy metal-sensitive lands. However, the phytoremediation capacity of various plant species and germplasm resources display significant genetic diversity, and the mechanisms underlying these differences remain hitherto obscure. Given its potential benefits, genetic improvement of plants is essential for enhancing their uptake of heavy metals, tolerance to harmful levels, as well as overall growth and development in contaminated soil. In this study, we uncover a molecular cascade that regulates cadmium (Cd2+) tolerance in cotton, involving GhRCD1, GhbHLH12, GhMYB44, and GhHMA1. We identified a Cd2+-sensitive cotton T-DNA insertion mutant with disrupted GhRCD1 expression. Genetic knockout of GhRCD1 by CRISPR/Cas9 technology resulted in reduced Cd2+ tolerance in cotton seedlings, while GhRCD1 overexpression enhanced Cd2+ tolerance. Through molecular interaction studies, we demonstrated that, in response to Cd2+ presence, GhRCD1 directly interacts with GhbHLH12. This interaction activates GhMYB44, which subsequently activates a heavy metal transporter, GhHMA1, by directly binding to a G-box cis-element in its promoter. These findings provide critical insights into a novel GhRCD1-GhbHLH12-GhMYB44-GhHMA1 regulatory module responsible for Cd2+ tolerance in cotton. Furthermore, our study paves the way for the development of elite Cd2+-tolerant cultivars by elucidating the molecular mechanisms governing the genetic control of Cd2+ tolerance in cotton.

Natural variation in the promoter of FLOWERING LOCUS T-LIKE 2 in pumpkin (Cucurbita moschata Duch.) is associated with flowering time under short-day conditions

Late flowering is a serious bottleneck in pumpkin (Cucurbita moschata Duch.) agriculture production. Although key genes governing flowering time have been reported in many species, the regulatory network of flowering in pumpkin remains largely obscure, thereby impeding the resolution of industry-wide challenges associated with delayed fruit ripening in pumpkin cultivation. Here, we report an early flowering pumpkin germplasm accession (LXX-4). Using LXX-4 and a late flowering germplasm accession (HYM-9), we constructed an F2 segregation population. A significant difference in FLOWERING LOCUS T-LIKE 2 (FTL2) expression level was identified to be the causal factor of the flowering time trait discrepancy in LXX-4 and HYM-9. Moreover, we have shown that a 21 bp InDel in the FTL2 promoter was the key reason for the waxing and waning of its transcript level. The 21 bp deletion excluded a repressor-AGL19 and recruited activators-BBX7, WRKY40 and SVP to the FTL2 promoter in LXX-4. Together, our data add a useful element to our knowledge which could be used to simplify breeding efforts for early-maturing pumpkin.

NnSnRK1-NnATG1-mediated autophagic cell death governs flower bud abortion in shaded lotus

Many plants can terminate their flowering process in response to unfavourable environments, but the mechanisms underlying this response are poorly understood. In this study, we observed that the lotus flower buds were susceptible to abortion under shaded conditions. The primary cause of abortion was excessive autophagic cell death (ACD) in flower buds. Blockade of autophagic flux in lotus flower buds consistently resulted in low levels of ACD and improved flowering ability under shaded conditions. Further evidence highlights the importance of the NnSnRK1-NnATG1 signalling axis in inducing ACD in lotus flower buds and culminating in their timely abortion. Under shaded conditions, elevated levels of NnSnRK1 activated NnATG1, which subsequently led to the formation of numerous autophagosome structures in lotus flower bud cells. Excessive autophagy levels led to the bulk degradation of cellular material, which triggered ACD and the abortion of flower buds. NnSnRK1 does not act directly on NnATG1. Other components, including TOR (target of rapamycin), PI3K (phosphatidylinositol 3-kinase) and three previously unidentified genes, appeared to be pivotal for the interaction between NnSnRK1 and NnATG1. This study reveals the role of autophagy in regulating the abortion of lotus flower buds, which could improve reproductive success and act as an energy-efficient measure in plants.

Multi-locus genome-wide association study and genomic prediction for flowering time in chrysanthemum

MAIN CONCLUSION

Multi-locus GWAS detected several known and candidate genes responsible for flowering time in chrysanthemum. The associations could greatly increase the predictive ability of genome selection that accelerates the possible application of GS in chrysanthemum breeding. Timely flowering is critical for successful reproduction and determines the economic value for ornamental plants. To investigate the genetic architecture of flowering time in chrysanthemum, a multi-locus genome-wide association study (GWAS) was performed using a collection of 200 accessions and 330,710 single-nucleotide polymorphisms (SNPs) via 3VmrMLM method. Five flowering time traits including budding (FBD), visible colouring (VC), early opening (EO), full-bloom (OF) and senescing (SF) stages, plus five derived conditional traits were recorded in two environments. Extensive phenotypic variations were observed for these flowering time traits with coefficients of variation ranging from 6.42 to 38.27%, and their broad-sense heritability ranged from 71.47 to 96.78%. GWAS revealed 88 stable quantitative trait nucleotides (QTNs) and 93 QTN-by-environment interactions (QEIs) associated with flowering time traits, accounting for 0.50-8.01% and 0.30-10.42% of the phenotypic variation, respectively. Amongst the genes around these stable QTNs and QEIs, 21 and 10 were homologous to known flowering genes in Arabidopsis; 20 and 11 candidate genes were mined by combining the functional annotation and transcriptomics data, respectively, such as MYB55, FRIGIDA-like, WRKY75 and ANT. Furthermore, genomic selection (GS) was assessed using three models and seven unique marker datasets. We found the prediction accuracy (PA) using significant SNPs identified by GWAS under SVM model exhibited the best performance with PA ranging from 0.90 to 0.95. Our findings provide new insights into the dynamic genetic architecture of flowering time and the identified significant SNPs and candidate genes will accelerate the future molecular improvement of chrysanthemum.

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

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

Genetic Engineering and Genome Editing Advances to Enhance Floral Attributes in Ornamental Plants: An Update

The ornamental horticulture industry is a highly dynamic and rapidly changing market. Constant development of novel cultivars with elite traits is essential to sustain competitiveness. Conventional breeding has been used to develop cultivars, which is often laborious. Biotechnological strategies such as genetic engineering have been crucial in manipulating and improving various beneficial traits that are technically not possible through cross-breeding. One such trait is the highly desired blue-colored flower in roses and chrysanthemums, which can be achieved through transgenic technology. Advances in genome sequencing platforms have enhanced the opportunities to access the whole genome sequence in various ornamentals, facilitating the dissection of the molecular genetics and regulatory controls of different traits. The recent advent of genome editing tools, including CRISPR/Cas9, has revolutionized plant breeding. CRISPR/Cas9-based gene editing offers efficient and highly precise trait modification, contributing to various beneficial advancements. Although genome editing in ornamentals is currently in its infancy, the recent increase in the availability of ornamental genome sequences provides a platform to extend the frontiers of future genome editing in ornamentals. Hence, this review depicts the implication of various commercially valuable ornamental attributes, and details the research attempts and achievements in enhancing floral attributes using genetic engineering and genome editing in ornamental plants.

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.

Flowering repressor CmSVP recruits the TOPLESS corepressor to control flowering in chrysanthemum

Plant flowering time is induced by environmental and endogenous signals perceived by the plant. The MCM1-AGAMOUSDEFICIENS-Serum Response Factor-box (MADS-box) protein SHORT VEGETATIVE PHASE (SVP) is a pivotal repressor that negatively regulates the floral transition during the vegetative phase; however, the transcriptional regulatory mechanism remains poorly understood. Here, we report that CmSVP, a chrysanthemum (Chrysanthemum morifolium Ramat.) homolog of SVP, can repress the expression of a key flowering gene, a chrysanthemum FLOWERING LOCUS T-like gene (CmFTL3), by binding its promoter CArG element to delay flowering in the ambient temperature pathway in chrysanthemum. Protein-protein interaction assays identified an interaction between CmSVP and CmTPL1-2, a chrysanthemum homologue of TOPLESS (TPL) that plays critical roles as transcriptional corepressor in many aspects of plant life. Genetic analyses revealed the CmSVP-CmTPL1-2 transcriptional complex is a prerequisite for CmSVP to act as a floral repressor. Furthermore, overexpression of CmSVP rescued the phenotype of the svp-31 mutant in Arabidopsis (Arabidopsis thaliana), overexpression of AtSVP or CmSVP in the Arabidopsis dominant-negative mutation tpl-1 led to ineffective late flowering, and AtSVP interacted with AtTPL, confirming the conserved function of SVP in chrysanthemum and Arabidopsis. We have validated a conserved machinery wherein SVP partially relies on TPL to inhibit flowering via a thermosensory pathway.

Designing of future ornamental crops: a biotechnological driven perspective

With a basis in human appreciation of beauty and aesthetic values, the new era of ornamental crops is based on implementing innovative technologies and transforming symbols into tangible assets. Recent advances in plant biotechnology have attracted considerable scientific and industrial interest, particularly in terms of modifying desired plant traits and developing future ornamental crops. By utilizing omics approaches, genomic data, genetic engineering, and gene editing tools, scientists have successively explored the underlying molecular mechanism and potential gene(s) behind trait regulation such as floral induction, plant architecture, stress resistance, plasticity, adaptation, and phytoremediation in ornamental crop species. These signs of progress lay a theoretical and practical foundation for designing and enhancing the efficiency of ornamental plants for a wide range of applications. In this review, we briefly summarized the existing literature and advances in biotechnological approaches for the improvement of vital traits in ornamental plants. The future ornamental plants, such as light-emitting plants, biotic/abiotic stress detectors, and pollution abatement, and the introduction of new ornamental varieties via domestication of wild species are also discussed.

Arabidopsis BBX14 negatively regulates nitrogen starvation- and dark-induced leaf senescence

Senescence is a highly regulated process driven by developmental age and environmental factors. Although leaf senescence is accelerated by nitrogen (N) deficiency, the underlying physiological and molecular mechanisms are largely unknown. Here, we reveal that BBX14, a previously uncharacterized BBX-type transcription factor in Arabidopsis, is crucial for N starvation-induced leaf senescence. We find that inhibiting BBX14 by artificial miRNA (amiRNA) accelerates senescence during N starvation and in darkness, while BBX14 overexpression (BBX14-OX) delays it, identifying BBX14 as a negative regulator of N starvation- and dark-induced senescence. During N starvation, nitrate and amino acids like glutamic acid, glutamine, aspartic acid, and asparagine were highly retained in BBX14-OX leaves compared to the wild type. Transcriptome analysis showed a large number of senescence-associated genes (SAGs) to be differentially expressed between BBX14-OX and wild-type plants, including ETHYLENE INSENSITIVE3 (EIN3) which regulates N signaling and leaf senescence. Chromatin immunoprecipitation (ChIP) showed that BBX14 directly regulates EIN3 transcription. Furthermore, we revealed the upstream transcriptional cascade of BBX14. By yeast one-hybrid screen and ChIP, we found that MYB44, a stress-responsive MYB transcription factor, directly binds to the promoter of BBX14 and activates its expression. In addition, Phytochrome Interacting Factor 4 (PIF4) binds to the promoter of BBX14 to repress BBX14 transcription. Thus, BBX14 functions as a negative regulator of N starvation-induced senescence through EIN3 and is directly regulated by PIF4 and MYB44.

The Cm14-3-3μ protein and CCT transcription factor CmNRRa delay flowering in chrysanthemum

Floral transition from vegetative to reproductive growth is pivotal in the plant life cycle. NUTRITION RESPONSE AND ROOT GROWTH (OsNRRa) as a CONSTANS, CONSTANS-LIKE, TOC1 (CCT) domain protein delays flowering in rice and an orthologous gene CmNRRa inhibits flowering in chrysanthemum; however, the mechanism remains unknown. In this study, using yeast two-hybrid screening, we identified the 14-3-3 family member Cm14-3-3µ as a CmNRRa-interacting protein. Biochemical assays using a combination of bimolecular fluorescence complementation (BiFC), pull-down, and Co-immunoprecipitation (Co-IP) were performed to confirm the physical interaction between CmNRRa and Cm14-3-3µ in chrysanthemum. In addition, expression analysis showed that CmNRRa, but not Cm14-3-3µ, responded to the diurnal rhythm, whereas both genes were highly expressed in the leaves. Moreover, the function in flowering time regulation of Cm14-3-3µ is similar to that of CmNRRa. Furthermore, CmNRRa repressed chrysanthemum FLOWERING LOCUS T-like 3 (CmFTL3) and APETALA 1 (AP1)/FRUITFULL (FUL)-like gene (CmAFL1), but induced TERMINAL FLOWER1 (CmTFL1) directly by binding to their promoters. Cm14-3-3µ enhanced the ability of CmNRRa to regulate the expression of these genes. These findings suggest that there is a synergistic relationship between CmNRRa and Cm14-3-3µ in flowering repression in chrysanthemum.

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.

Molecular module of CmMYB15-like-Cm4CL2 regulating lignin biosynthesis of chrysanthemum (Chrysanthemum morifolium) in response to aphid (Macrosiphoniella sanborni) feeding

Lignin is a major component of plant cell walls and a conserved basic defense mechanism in higher plants deposited in response to aphid infection. However, the molecular mechanisms of lignin biosynthesis in response to aphid infection and the effect of lignin on aphid feeding behavior remain unclear. We report that 4-Coumarate:coenzyme A ligase 2 (Cm4CL2), a gene encoding a key enzyme in the lignin biosynthesis pathway, is induced by aphid feeding, resulting in lignin deposition and reduced aphid attack. Upstream regulator analysis showed that the expression of Cm4CL2 in response to aphid feeding was directly upregulated by CmMYB15-like, an SG2-type R2R3-MYB transcription factor. CmMYB15-like binds directly to the AC cis-element in the promoter region of Cm4CL2. Genetic validation demonstrated that CmMYB15-like was induced by aphid infection and contributed to lignin deposition and cell wall thickening, which consequently enhanced aphid resistance in a Cm4CL2-dependent manner. This study is the first to show that the CmMYB15-like-Cm4CL2 module regulates lignin biosynthesis in response to aphid feeding.

An ethylene-responsive transcription factor and a B-box protein coordinate vegetative growth and photoperiodic flowering in chrysanthemum

Plants employ several endogenous and exogenous signals to guarantee timely floral transitions with floral integrators. To avoid premature flowering, flowering plants must control the balance between vegetative and floral development. As a Group II member of BBX family, CmBBX8 promotes flowering by directly activating CmFTL1 in summer-flowering chrysanthemum. However, the mechanisms underlying this floral transition is yet to be elucidated. Here, we report that the chrysanthemum ERF3 homologue, CmERF3, physically interacts with CmBBX8 through yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC), pull-down, and luciferase complementation (LCI) assays. We found that CmERF3 was highly expressed at the vegetative stage and rarely expressed in the reproductive phase, indicating that CmERF3 may play a critical role in maintaining vegetative growth to prevent premature flowering. Rhythm analysis revealed that CmERF3 had a different response to rhythm compared to CmBBX8. Knockdown of CmERF3 facilitated floral initiation, whereas overexpression of CmERF3 delayed floral transition. We further found that CmERF3 repressed the transactivation activity of CmBBX8 on the downstream CmFTL1 gene. Collectively, our results indicate that the CmERF3-CmBBX8 transcriptional complex is a crucial module that balances the vegetative growth and reproductive development of chrysanthemum.

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.

Hybrid weakness and continuous flowering caused by compound expression of FTLs in Chrysanthemum morifolium × Leucanthemum paludosum intergeneric hybridization

Hybridization is an important evolutionary mechanism ubiquitous to plants. Previous studies have shown that hybrid polyploidization of cultivated chrysanthemum, 'Zhongshanzigui', and Leucanthemum paludosum exhibit spring-flowering traits. This study explores the function of the LpFTLs gene via the phenotype of A. thaliana after heterologous transformation of the LpFTLs gene, and analyzes the mechanism ofthe continuous flowering phenotype and heterosis of hybrid offspring. The results suggest that the flowering phenotype of hybrid offspring in spring may be related to the expression of the LpFTLs gene. Ectopic expression of Leucanthemum paludosumLpFTLs in Arabidopsis thaliana resulted in earlier flowering, indicating that the LpFTLs gene also affects the flowering time in L. paludosum. Compound expression of FTLs in C. morifolium × L. paludosum intergeneric hybridization directly leads to serious heterosis in the hybrid offspring. Moreover, continuous flowering appears to be accompanied by hybrid weakness under the balance of vegetative and reproductive growth. Therefore, in future studies on chrysanthemum breeding, a suitable balance point must be established to ensure the target flowering time under normal growth.

The TIFY family protein CmJAZ1-like negatively regulates petal size via interaction with the bHLH transcription factor CmBPE2 in Chrysanthemum morifolium

Petals are the second floral whorl of angiosperms, exhibiting astonishing diversity in their size between and within species. This variation is essential for protecting their inner reproductive organs and attracting pollinators for fertilization. However, currently, the genetic and developmental control of petal size remains unexplored. Chrysanthemum (Chrysanthemum morifolium) belongs to the Asteraceae family, the largest group of angiosperms, and the extraordinary diversity of petal size in chrysanthemums makes it an ideal model for exploring the regulation mechanism of petal size. Here, we reveal that overexpression of a JAZ repressor CmJAZ1-like exhibits decreased petal size compared to that of the wild-type as a result of repressed cell expansion. Through further in-depth exploration, we confirm an interaction pair between CmJAZ1-like and the bHLH transcription factor CmBPE2. The inhibition of CmBPE2 expression negatively regulates petal size by downregulating the expression of genes involved in cell expansion. Furthermore, CmJAZ1-like significantly reduced the activation ability of CmBPE2 on its target gene CmEXPA7 by directly interacting with it, thus participating in the regulation of petal size development in chrysanthemum. Our results will provide insights into the molecular mechanisms of petal size regulation in flowering plants.

CRISPR/Cas9-Mediated Targeted Mutagenesis of BnaCOL9 Advances the Flowering Time of Brassica napus L

Rapeseed (Brassica napus L.) is one of the most important oil crops in the world. The planting area and output of rapeseed are affected by the flowering time, which is a critical agronomic feature. COL9 controls growth and development in many different plant species as a member of the zinc finger transcription factor family. However, BnaCOL9 in rapeseed has not been documented. The aim of this study was to apply CRISPR/Cas9 technology to create an early-flowering germplasm resource to provide useful material for improving the early-maturing breeding of rapeseed. We identified four COL9 homologs in rapeseed that were distributed on chromosomes A05, C05, A03, and C03. We successfully created quadruple BnaCOL9 mutations in rapeseed using the CRISPR/Cas9 platform. The quadruple mutants of BnaCOL9 flowered earlier than the wild-type. On the other hand, the flowering time of the BnaCOL9 overexpression lines was delayed. An analysis of the expression patterns revealed that these genes were substantially expressed in the leaves and flowers. A subcellular localization experiment demonstrated that BnaCOL9 was in the nucleus. Furthermore, we discovered that two key flowering-related genes, BnaCO and BnaFT, were highly elevated in the BnaCOL9 mutants, but dramatically downregulated in the BnaCOL9 overexpression lines. Our findings demonstrate that BnaCOL9 is a significant flowering inhibitor in rapeseed and may be employed as a crucial gene for early-maturing breeding.

AcBBX5, a B-box transcription factor from pineapple, regulates flowering time and floral organ development in plants

Flowering is an important factor to ensure the success of plant reproduction, and reasonable flowering time is crucial to the crop yield. BBX transcription factors can regulate several growth and development processes. However, there is little research on whether BBX is involved in flower formation and floral organ development of pineapple. In this study, AcBBX5, a BBX family gene with two conserved B-box domains, was identified from pineapple. Subcellular localization analysis showed that AcBBX5 was located in the nucleus. Transactivation analysis indicated that AcBBX5 had no significant toxic effects on the yeast system and presented transcriptional activation activity in yeast. Overexpression of AcBBX5 delayed flowering time and enlarged flower morphology in Arabidopsis. Meanwhile, the expression levels of AtFT, AtSOC1, AtFUL and AtSEP3 were decreased, and the transcription levels of AtFLC and AtSVP were increased in AcBBX5-overexpressing Arabidopsis, which might lead to delayed flowering of transgenic plants. Furthermore, transcriptome data and QRT-PCR results showed that AcBBX5 was expressed in all floral organs, with the high expression levels in stamens, ovaries and petals. Yeast one-hybrid and dual luciferase assay results showed that AcBBX5 bound to AcFT promoter and inhibited AcFT gene expression. In conclusion, AcBBX5 was involved in flower bud differentiation and floral organ development, which provides an important reference for studying the functions of BBX and the molecular regulation of flower.

Identification of Differentially Expressed Genes Related to Floral Bud Differentiation and Flowering Time in Three Populations of Lycoris radiata

The transition from vegetative to reproductive growth is important for controlling the flowering of Lycoris radiata. However, the genetic control of this complex developmental process remains unclear. In this study, 18 shoot apical meristem (SAM) samples were collected from early-, mid- and late-flowering populations during floral bud differentiation. The histological analysis of paraffin sections showed that the floral bud differentiation could be divided into six stages; the differentiation time of the early group was earlier than that of the middle and late groups, and the late group was the latest. In different populations, some important differential genes affecting the flowering time were identified by transcriptome profiles of floral bud differentiation samples. Weighted gene co-expression network analysis (WGCNA) was performed to enrich the gene co-expression modules of diverse flowering time populations (FT) and floral bud differentiation stages (ST). In the MEyellow module, five core hub genes were identified, including CO14, GI, SPL8, SPL9, and SPL15. The correlation network of hub genes showed that they interact with SPLs, AP2, hormone response factors (auxin, gibberellin, ethylene, and abscisic acid), and several transcription factors (MADS-box transcription factor, bHLH, MYB, and NAC3). It suggests the important role of these genes and the complex molecular mechanism of floral bud differentiation and flowering time in L. radiata. These results can preliminarily explain the molecular mechanism of floral bud differentiation and provide new candidate genes for the flowering regulation of Lycoris.

Integrated next-generation sequencing and comparative transcriptomic analysis of leaves provides novel insights into the ethylene pathway of Chrysanthemum morifolium in response to a Chinese isolate of chrysanthemum virus B

Background

Chrysanthemum virus B (CVB), a key member of the genus Carlavirus, family Betaflexiviridae, causes severe viral diseases in chrysanthemum (Chrysanthemum morifolium) plants worldwide. However, information on the mechanisms underlying the response of chrysanthemum plants to CVB is scant.

Methods

Here, an integrated next-generation sequencing and comparative transcriptomic analysis of chrysanthemum leaves was conducted to explore the molecular response mechanisms of plants to a Chinese isolate of CVB (CVB-CN) at the molecular level.

Results

In total, 4934 significant differentially expressed genes (SDEGs) were identified to respond to CVB-CN, of which 4097 were upregulated and 837 were downregulated. Gene ontology and functional classification showed that the majority of upregulated SDEGs were categorized into gene cohorts involved in plant hormone signal transduction, phenylpropanoid and flavonoid biosynthesis, and ribosome metabolism. Enrichment analysis demonstrated that ethylene pathway-related genes were significantly upregulated following CVB-CN infection, indicating a strong promotion of ethylene biosynthesis and signaling. Furthermore, disruption of the ethylene pathway in Nicotiana benthamiana, a model plant, using virus-induced gene silencing technology rendered them more susceptible to cysteine-rich protein of CVB-CN induced hypersensitive response, suggesting a crucial role of this pathway in response to CVB-CN infection.

Conclusion

This study provides evidence that ethylene pathway has an essential role of plant in response to CVB and offers valuable insights into the defense mechanisms of chrysanthemum against Carlavirus.

Towards the Improvement of Ornamental Attributes in Chrysanthemum: Recent Progress in Biotechnological Advances

Incessant development and introduction of novel cultivars with improved floral attributes are vital in the dynamic ornamental industry. Chrysanthemum (Chrysanthemum morifolium) is a highly favored ornamental plant, ranking second globally in the cut flower trade, after rose. Development of new chrysanthemum cultivars with improved and innovative modifications in ornamental attributes, including floral color, shape, plant architecture, flowering time, enhanced shelf life, and biotic and abiotic stress tolerance, is a major goal in chrysanthemum breeding. Despite being an economically important ornamental plant, the application of conventional and molecular breeding approaches to various key traits of chrysanthemum is hindered owing to its genomic complexity, heterozygosity, and limited gene pool availability. Although classical breeding of chrysanthemum has resulted in the development of several hundreds of cultivars with various morphological variations, the genetic and transcriptional control of various important ornamental traits remains unclear. The coveted blue colored flowers of chrysanthemums cannot be achieved through conventional breeding and mutation breeding due to technical limitations. However, blue-hued flower has been developed by genetic engineering, and transgenic molecular breeding has been successfully employed, leading to substantial progress in improving various traits. The recent availability of whole-genome sequences of chrysanthemum offers a platform to extensively employ MAS to identify a large number of markers for QTL mapping, and GWAS to dissect the genetic control of complex traits. The combination of NGS, multi-omic platforms, and genome editing technologies has provided a tremendous scope to decipher the molecular and regulatory mechanisms. However, the application and integration of these technologies remain inadequate for chrysanthemum. This review, therefore, details the significance of floral attributes, describes the efforts of recent advancements, and highlights the possibilities for future application towards the improvement of crucial ornamental traits in the globally popular chrysanthemum plant.

RETRACTED: Genome-wide analysis of the B-box gene family in the sweetpotato wild ancestor Ipomoea trifida and determination of the function of IbBBX28 in the regulation of flowering time of Arabidopsis

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of of the Editors-in-Chief. A large part of the article is highly similar to the paper previously published by Wenqian Hou, Lei Ren, Yang Zhang, Haoyun Sun, Tianye Shi, Yulan Gu, Aimin Wang, Daifu Ma, Zongyun Li and Lei Zhang in Scientia Horticulturae 288 (2021) 110374 https://doi.org/10.1016/j.scienta.2021.110374. In particular, a large part of the two articles shows a study on the same gene family in the same plant, with similar methodological approaches, resulting in a series of highly similar figures. One of the conditions of submission of a paper for publication is that authors declare explicitly that their work is original and has not appeared in a publication elsewhere. Re-use of any data should be appropriately cited. As such this article represents a severe abuse of the scientific publishing system. The scientific community takes a very strong view on this matter and apologies are offered to readers of the journal that this was not detected during the submission process.

Functional diversification and molecular mechanisms of FLOWERING LOCUS T/TERMINAL FLOWER 1 family genes in horticultural plants

Flowering is an important process in higher plants and is regulated by a variety of factors, including light, temperature, and phytohormones. Flowering restriction has a considerable impact on the commodity value and production cost of many horticultural crops. In Arabidopsis, the FT/TFL1 gene family has been shown to integrate signals from various flowering pathways and to play a key role in the transition from flower production to seed development. Studies in several plant species of the FT/TFL1 gene family have revealed it harbors functional diversity in the regulation of flowering. Here, we review the functional evolution of the FT/TFL1 gene family in horticulture plants and its unique regulatory mechanisms; in addition, the FT/TFL1 family of genes as an important potential breeding target is explored.

Two B-Box Proteins, MaBBX20 and MaBBX51, Coordinate Light-Induced Anthocyanin Biosynthesis in Grape Hyacinth

Floral colour is an important agronomic trait that influences the commercial value of ornamental plants. Anthocyanins are a class of flavonoids and confer diverse colours, and elucidating the molecular mechanisms that regulate their pigmentation could facilitate artificial manipulation of flower colour in ornamental plants. Here, we investigated the regulatory mechanism of light-induced anthocyanin biosynthesis during flower colouration in grape hyacinth (Muscari spp.). We studied the function of two B-box proteins, MaBBX20 and MaBBX51. The qPCR revealed that MaBBX20 and MaBBX51 were associated with light-induced anthocyanin biosynthesis. Both MaBBX20 and MaBBX51 are transcript factors and are specifically localised in the nucleus. Besides, overexpression of MaBBX20 in tobacco slightly increased the anthocyanin content of the petals, but reduced in MaBBX51 overexpression lines. The yeast one-hybrid assays indicated that MaBBX20 and MaBBX51 did not directly bind to the MaMybA or MaDFR promoters, but MaHY5 did. The BiFC assay revealed that MaBBX20 and MaBBX51 physically interact with MaHY5. A dual luciferase assay further confirmed that the MaBBX20-MaHY5 complex can strongly activate the MaMybA and MaDFR transcription in tobacco. Moreover, MaBBX51 hampered MaBBX20-MaHY5 complex formation and repressed MaMybA and MaDFR transcription by physically interacting with MaHY5 and MaBBX20. Overall, the results suggest that MaBBX20 positively regulates light-induced anthocyanin biosynthesis in grape hyacinth, whereas MaBBX51 is a negative regulator.

An ethylene-responsive transcription factor and a flowering locus KH domain homologue jointly modulate photoperiodic flowering in chrysanthemum

The timely transition from vegetative to reproductive development is coordinated through the quantitative regulation of floral pathway genes in response to physiological and environmental cues. The function of ethylene-responsive element-binding protein (ERF) transcription factors in the regulation of flowering in chrysanthemum (Chrysanthemum morifolium Ramat.) is not well understood. Here, chrysanthemum overexpressing CmERF110 flowered earlier than the wild-type plants, while those in which CmERF110 was suppressed flowered later. RNA-seq results revealed that several genes involved in the circadian rhythm were transcribed differently in CmERF110 transgenic plants from that of the wild-type plants. The rhythm peak of the circadian clock genes in transgenic plants was delayed. Yeast two-hybrid screening of CmERF110 interactors identified a chrysanthemum FLOWERING LOCUS KH DOMAIN (FLK) homologue CmFLK, which was further confirmed with both in vitro and in vivo assays. KEGG pathway enrichment also revealed that CmFLK is involved in the regulation of circadian rhythm-related genes. CmFLK transgenic plants showed a change in flowering time and delayed rhythm peak of the circadian rhythm genes. Taken together, the present data not only suggest that CmERF110 interacts with CmFLK to promote floral transition by tuning the circadian clock, but also provides evidence for the evolutionary conservation of the components in the autonomous pathway in chrysanthemum.

Regulation of lignin biosynthesis by an atypical bHLH protein CmHLB in Chrysanthemum

Stem mechanical strength is one of the most important agronomic traits that affects the resistance of plants against insects and lodging, and plays an essential role in the quality and yield of plants. Several transcription factors regulate mechanical strength in crops. However, mechanisms of stem strength formation and regulation remain largely unexplored, especially in ornamental plants. In this study, we identified an atypical bHLH transcription factor CmHLB (HLH PROTEIN INVOLVED IN LIGNIN BIOSYNTHESIS) in chrysanthemum, belonging to a small bHLH sub-family - the PACLOBUTRAZOL RESISTANCE (PRE) family. Overexpression of CmHLB in chrysanthemum significantly increased mechanical strength of the stem, cell wall thickness, and lignin content, compared with the wild type. In contrast, CmHLB RNA interference lines exhibited the opposite phenotypes. RNA-seq analysis indicated that CmHLB promoted the expression of genes involved in lignin biosynthesis. Furthermore, we demonstrated that CmHLB interacted with Chrysanthemum KNOTTED ARABIDOPSIS THALIANA7 (CmKNAT7) through the KNOX2 domain, which has a conserved function, i.e. it negatively regulates secondary cell wall formation of fibres and lignin biosynthesis. Collectively, our results reveal a novel role for CmHLB in regulating lignin biosynthesis by interacting with CmKNAT7 and affecting stem mechanical strength in Chrysanthemum.

Genome-Wide Identification of the B-Box Gene Family and Expression Analysis Suggests Their Potential Role in Photoperiod-Mediated β-Carotene Accumulation in the Endocarp of Cucumber (Cucumis sativus L.) Fruit

Carotenoids are indispensable to plants and essential for human nutrition and health. Carotenoid contents are strongly influenced by light through light-responsive genes such as B-Box (BBX) genes. BBX proteins, a class of zinc-finger transcription factors, mediate many light-signaling pathways, leading to the biosynthesis of important metabolites in plants. However, the identification of the BBX gene family and expression analysis in response to photoperiod-mediated carotenoid accumulation in cucumber remains unexplored. We performed a genome-wide study and determined the expression of cucumber BBX genes (hereafter referred to as CsaBBXs genes) in the endocarp of Xishuangbanna cucumber fruit (a special type of cucumber accumulating a high level of β-carotene in the endocarp) using an RNA-seq analysis of plants previously subjected to two photoperiodic conditions. Here, 26 BBX family genes were identified in the cucumber genome and named serially CsaBBX1 through CsaBBX26. We characterized CsaBBX genes in terms of their phylogenetic relationships, exon-intron structures, cis-acting elements, and syntenic relationships with Arabidopsis thaliana (L.) Heynh. RNA-seq analysis revealed a varied expression of CsaBBX genes under photoperiod treatment. The analysis of CsaBBXs genes revealed a strong positive correlation between CsaBBX17 and carotenoid biosynthetic pathway genes (phytoene synthase, ζ-carotene desaturase, lycopene ε-cyclase, β-carotene hydroxylase-1), thus suggesting its involvement in β-carotene biosynthesis. Additionally, nine CsaBBX genes (CsaBBX 4, 5, 7, 9, 11, 13, 15, 17 and 22) showed a significant positive correlation with β-carotene content. The selected CsaBBX genes were verified by qRT-PCR and confirmed the validity of RNA-seq data. The results of this study established the genome-wide analysis of the cucumber BBX family and provide a framework for understanding their biological role in carotenoid accumulation and photoperiodic responses. Further investigations of CsaBBX genes are vital since they are promising candidate genes for the functional analysis of carotenoid biosynthesis and can provide genetic tools for the molecular breeding of carotenoids in plants.

Isolation and functional analysis of two CONSTANS-like 1 genes from mango

The CONSTANS-LIKE1 (COL1) gene plays an important role in the regulation of photoperiodic flowering in plants. In this study, two COL1 homolog genes, MiCOL1A and MiCOL1B, were isolated from mango (Mangifera indica L.). The open reading frames of MiCOL1A and MiCOL1B are 852 and 822 bp in length and encode 284 and 274 amino acids, respectively. The MiCOL1A and MiCOL1B proteins contain only one CCT domain and belong to the CO/COL group IV protein family. MiCOL1A and MiCOL1B were expressed both in vegetative and reproductive organs but with expression level differences. MiCOL1A was highly expressed in juvenile and adult leaves, but MiCOL1B was highly expressed in flowers. Seasonal expression analysis showed that MiCOL1A and MiCOL1B have similar expression patterns and higher expression levels during flower induction and flower organ differentiation periods. However, MiCOL1A and MiCOL1B exhibited unstable patterns in circadian expression analysis. MiCOL1A and MiCOL1B were localized in the nucleus and had transcriptional activation activity in yeast. Overexpression of MiCOL1A and MiCOL1B resulted in significantly delayed flowering time in Arabidopsis. Furthermore, we also found that overexpression of MiCOL1A and MiCOL1B enhanced drought tolerance in transgenic Arabidopsis. The results demonstrated that MiCOL1A and MiCOL1B are not only involved in flowering regulation but also play a role in the stress response of plants.

The BBX gene CmBBX22 negatively regulates drought stress tolerance in chrysanthemum

BBX transcription factors play vital roles in plant growth, development, and stress responses. Although BBX proteins have been studied in great detail in the model plant Arabidopsis, their roles in crop plants such as chrysanthemum are still largely uninvestigated. Here, we cloned CmBBX22 and further determined the function of CmBBX22 in response to drought treatment. Subcellular localization and transactivation assay analyses revealed that CmBBX22 was localized in the nucleus and possessed transactivation activity. Overexpression of CmBBX22 in chrysanthemum was found to reduce plant drought tolerance, whereas expression of the chimeric repressor CmBBX22-SRDX was found to promote a higher drought tolerance than that shown by wild-type plants, indicating that CmBBX22 negatively regulates drought tolerance in chrysanthemum. Transcriptome analysis and physiological measurements indicated the potential involvement of the CmBBX22-mediated ABA response, stomatal conductance, and antioxidant responses in the negative regulation of drought tolerance in chrysanthemum. Based on the findings of this study, we were thus able to establish the mechanisms whereby the transcriptional activator CmBBX22 negatively regulates drought tolerance in chrysanthemum via the regulation of the abscisic acid response, stomatal conductance, and antioxidant responses.

A novel transcription factor CmMYB012 inhibits flavone and anthocyanin biosynthesis in response to high temperatures in chrysanthemum

Flavones are among the major colorless pigments synthesized through branches of the flavonoid pathway in plants. However, due to the absence of a gene encoding flavone synthase (FNS) in the model plant Arabidopsis thaliana species, the regulatory mechanism of FNS-catalyzed flavone biosynthesis has rarely been studied in plants. Here, it was found that flavones play a predominant role in the elimination of excess reactive oxygen species (ROS) at high temperatures in colorless plant organs. A novel atypical subgroup 7 (SG7) R2R3-MYB transcription factor, CmMYB012, was found to be induced in response to prolonged high temperatures and to inhibit flavone biosynthesis by directly regulating CmFNS. Moreover, CmMYB012 was also found to inhibit anthocyanin biosynthesis by suppressing the expression of CmCHS, CmDFR, CmANS, and CmUFGT. CmMYB012 overexpression exerted a negative influence on plant fitness and pink flower color formation, while CmMYB012 suppression had the opposite effect in response to high temperatures. Our findings provide new insights into the mechanisms by which high temperatures regulate the metabolism of flavones and anthocyanins to affect plant fitness and flower color formation.

An Evolutionary Analysis of B-Box Transcription Factors in Strawberry Reveals the Role of FaBBx28c1 in the Regulation of Flowering Time

Flowering connects vegetative and generative developmental phases and plays a significant role in strawberry production. The mechanisms that regulate strawberry flowering time are unclear. B-box transcription factors (BBXs) play important roles in the flowering time regulation of plants. Nevertheless, BBXs in octoploid cultivated strawberry (Fragaria ananassa) and their functions in flowering time regulation have not been identified. Here, we identified 51 FaBBXs from cultivated strawberry and 16 FvBBXs from diploid wild strawberry (Fragaria vesca), which were classified into five groups according to phylogenetic analysis. Further evolutionary analysis showed that whole-genome duplication or segmental duplication is a crucial factor that leads to the expansion of the BBX gene family in two strawberry species. Moreover, some loss and acquisition events of FaBBX genes were identified in the genome of cultivated strawberry that could have affected traits of agronomic interest, such as fruit quality. The promoters of FaBBX genes showed an enrichment in light-responsive, cis-regulatory elements, with 16 of these genes showing changes in their transcriptional activity in response to blue light treatment. On the other hand, FaBBX28c1, whose transcriptional activity is reduced in response to blue light, showed a delay in flowering time in Arabidopsis transgenic lines, suggesting its role in the regulation of flowering time in cultivated strawberry. Our results provide new evolutionary insight into the BBX gene family in cultivated strawberry and clues regarding their function in flowering time regulation in plants.

The BBX gene family in Moso bamboo (Phyllostachys edulis): identification, characterization and expression profiles

BACKGROUND

The BBX (B-box) family are zinc finger protein (ZFP) transcription factors that play an essential role in plant growth, development and response to abiotic stresses. Although BBX genes have been characterized in many model organisms, genome-wide identification of the BBX family genes have not yet been reported in Moso bamboo (Phyllostachys edulis), and the biological functions of this family remain unknown.

RESULT

In the present study, we identified 27 BBX genes in the genome of Moso bamboo, and analysis of their conserved motifs and multiple sequence alignments revealed that they all shared highly similar structures. Additionally, phylogenetic and homology analyses indicated that PeBBX genes were divided into three clusters, with whole-genome duplication (WGD) events having facilitated the expansion of this gene family. Light-responsive and stress-related cis-elements were identified by analyzing cis-elements in the promoters of all PeBBX genes. Short time-series expression miner (STEM) analysis revealed that the PeBBX genes had spatiotemporal-specific expression patterns and were likely involved in the growth and development of bamboo shoots. We further explored the downstream target genes of PeBBXs, and GO/KEGG enrichment analysis predicted multiple functions of BBX target genes, including those encoding enzymes involved in plant photosynthesis, pyruvate metabolism and glycolysis/gluconeogenesis.

CONCLUSIONS

In conclusion, we analyzed the PeBBX genes at multiple different levels, which will contribute to further studies of the BBX family and provide valuable information for the functional validation of this family.

CmRCD1 represses flowering by directly interacting with CmBBX8 in summer chrysanthemum

The CmBBX8-CmFTL1 regulatory module is a key determinant in the transition from vegetative growth to reproductive development in summer-flowering chrysanthemum. However, the detailed regulatory mechanism of CmBBX8-mediated flowering remains elusive. In this study, we revealed that RADICAL-INDUCED CELL DEATH 1 (CmRCD1) physically associated with CmBBX8 through bimolecular fluorescence complementation (BiFC), pulldown and Coimmunoprecipitation (CoIP) assays. Furthermore, the RCD1-SRO1-TAF4 (RST) domain of CmRCD1 and the B-box of CmBBX8 mediated their interaction. In addition, Luciferase (LUC) assays and electrophoretic mobility shift assay (EMSAs) showed that CmRCD1 repressed the transcriptional activity of CmBBX8 and interfered with its binding to the CmFTL1 promoter, thereby leading to delayed flowering in the summer chrysanthemum 'Yuuka'. These results provide insight into the molecular framework of CmRCD1-CmBBX8-mediated flowering in chrysanthemum.

Beyond Arabidopsis: BBX Regulators in Crop Plants

B-box proteins represent diverse zinc finger transcription factors and regulators forming large families in various plants. A unique domain structure defines them—besides the highly conserved B-box domains, some B-box (BBX) proteins also possess CCT domain and VP motif. Based on the presence of these specific domains, they are mostly classified into five structural groups. The particular members widely differ in structure and fulfill distinct functions in regulating plant growth and development, including seedling photomorphogenesis, the anthocyanins biosynthesis, photoperiodic regulation of flowering, and hormonal pathways. Several BBX proteins are additionally involved in biotic and abiotic stress response. Overexpression of some BBX genes stimulates various stress-related genes and enhanced tolerance to different stresses. Moreover, there is evidence of interplay between B-box and the circadian clock mechanism. This review highlights the role of BBX proteins as a part of a broad regulatory network in crop plants, considering their participation in development, physiology, defense, and environmental constraints. A description is also provided of how various BBX regulators involved in stress tolerance were applied in genetic engineering to obtain stress tolerance in transgenic crops.

Genes and genome editing tools for breeding desirable phenotypes in ornamentals

We review the main genes underlying commercial traits in cut flower species and critically discuss the possibility to apply genome editing approaches to produce novel variation and phenotypes. Promoting flowering and flower longevity as well as creating novelty in flower structure, colour range and fragrances are major objectives of ornamental plant breeding. The novel genome editing techniques add new possibilities to study gene function and breed new varieties. The implementation of such techniques, however, relies on detailed information about structure and function of genomes and genes. Moreover, improved protocols for efficient delivery of editing reagents are required. Recent results of the application of genome editing techniques to elite ornamental crops are discussed in this review. Enabling technologies and genomic resources are reviewed in relation to the implementation of such approaches. Availability of the main gene sequences, underlying commercial traits and in vitro transformation protocols are provided for the world's best-selling cut flowers, namely rose, lily, chrysanthemum, lisianthus, tulip, gerbera, freesia, alstroemeria, carnation and hydrangea. Results obtained so far are described and their implications for the improvement of flowering, flower architecture, colour, scent and shelf-life are discussed.

B-box transcription factor 28 regulates flowering by interacting with constans

B-box transcription factors (BBXs) are important regulators of flowering, photomorphogenesis, shade-avoidance, abiotic and biotic stresses and plant hormonal pathways. In Arabidopsis, 32 BBX proteins have been identified and classified into five groups based on their structural domains. Little is known about the fifth group members (BBX26–BBX32) and the detailed molecular mechanisms relevant to their functions. Here we identified B-box transcription factor 28 (BBX28) that interacts with Constans (CO), a transcriptional activator of Flowering Locus T (FT). Overexpressing BBX28 leads to late flowering with dramatically decreased FT transcription, and bbx28 deficient mutant displays a weak early flowering phenotype under long days (LD), indicating that BBX28 plays a negative and redundant role in flowering under LD. Additionally, the interaction between BBX28 and CO decreases the recruitment of CO to FT locus without affecting the transcriptional activation activity of CO. Moreover, the N-terminal cysteines, especially those within the B-box domain, are indispensable for the heterodimerization between BBX28 and CO and activation of CO on FT transcription. Genetic evidences show that the later flowering caused by BBX28 overexpression is compromised by CO ectopic expression. Collectively, these results supported that BBX28 functions with CO and FT to negatively regulate Arabidopsis flowering, in which the N-terminal conserved cysteines of BBX28 might play a central role.