The R2R3 transcription factor HlMYB8 and its role in flavonoid biosynthesis in hop (Humulus lupulus L.)

Spotlight on Secondary Metabolites Produced by an Early-Flowering Apulian Artichoke Ecotype Sanitized from Virus Infection by Meristem-Tip-Culture and Thermotherapy

Globe artichoke (Cynara cardunculus L. subsp. scolymus) is an important crop of the Mediterranean basin characterized by many properties, like hepatoprotective, anticarcinogenic, antioxidant, antibacterial, and beneficial to human health. The high bioactive compounds (BACs) content, as polyphenols, has attracted the research interest in artichoke extracts. We analysed the changes in polyphenol transcriptome profile between sanitized (S) virus-free and non-sanitized (NS) artichoke plants, focusing on genes involved in phenylpropanoid metabolic pathway and flavonoid biosynthesis. A total of 2458 upregulated and 2154 downregulated differentially expressed genes (DEGs) were functionally characterized. Among them, 31 and 35 KEGG orthology entries characterized by upregulated and downregulated DEGs, respectively, were involved in the biosynthesis of other secondary metabolites. A downregulation of PAL, C4H, 4CL, HST/HQT, C3'H, CCoAMT, CCR1, and F5H, was observed in S artichoke compared to NS one, whereas the CSE, CHS, and CHI genes were upregulated in S samples. Transcriptome results were compared to the polyphenols accumulation in S and NS artichoke leaves. A higher content of total polyphenols was observed in older leaves of NS samples, compared to extracts obtained from young leaves or from S plants, and this result was associated with the presence of viral infections in NS plants. In all the conditions tested, the most represented compound was chlorogenic acid, followed by luteolin-7-O-glucoside. The different composition of each extract was evaluated by a polyphenol dose-response treatment on the rodent hepatoma FaO cell line to the accumulation of reactive oxygen species (ROS). A significant reduction in ROS content ranging between -40% and -48% was observed when 10-20 mg/L of polyphenols from NS or S plants were used, characterized by a specific profile of compounds. To reduce MetOH residues in polyphenol extracts, a supercritical fluid CO2 extraction was evaluated to propose a sustainable green extraction.

Transcriptomic and metabolomic analyses reveal the flavor of bitterness in the tip shoots of Bambusa oldhamii Munro

The young bamboo shoot of Bambusa oldhamii (green bamboo) has a good taste and is rich in nutrition and widely used in traditional Chinese cuisines. But the shoots flavor of Bambusa oldhamii changed from deliciously sweet to a little bitter when the shoots grew from underground to aboveground. In this paper, we explored the bitterness chemicals of the green bamboo shoot when growing from underground to aboveground using transcriptome and metabolome techniques. Finally, several bitter chemicals were mined out counting for the flavor transformation, such as Solanidine, Amygdalin, Salicin, Arbutin, and others. The transcription factor family of AP2/ERF plays the main role in key bitter chemical regulation via correlation analysis. Moreover, the pathway of Biosynthesis of phenylpropanoids might be the key pathway in the formation of the bitter chemicals in green bamboo shoot development.

CRISPR/Cas9-mediated mutagenesis of the mediator complex subunits MED5a and MED5b genes impaired secondary metabolite accumulation in hop (Humulus lupulus)

Hop (Humulus lupulus L.) is an important commercial crop known for the biosynthesis of valuable specialized secondary metabolites in glandular trichomes (lupulin glands), which are used for the brewing industry. To achieve burgeoning market demands is the essentiality of comprehensive understanding of the mechanisms of biosynthesis of secondary metabolites in hop. Over the past year, several studies using structural biology and functional genomics approaches have shown that Mediator (MED) serves as an integrative hub for RNAP II-mediated transcriptional regulation of various physiological and cellular processes, including involvement of MED5a and MED5b in hyperaccumulation of phenylpropanoid in A. thaliana. In the present work, an unprecedented attempt was made to generate Hlmed5a/med5b double loci mutant lines in hop using a CRISPR/Cas9-based genome editing system. The Hlmed5a/med5b double loci mutant lines showed reduced expression of structural genes of the flavonoid, humulone, and terpenoid biosynthetic pathways, which was more pronounced in the lupulin gland compared to leaf tissue and was consistent with their reduced accumulation. Phenotypic and anatomical observations revealed that Hlmed5a/med5b double loci mutant line exhibited robust growth, earlier flowering, earlier cone maturity, reduced cone size, variations in floral structure patterns, and distorted lupulin glands without any remarkable changes in leaf morphology, intensity of leaf color, and chlorophyll content. Comparative transcriptome analysis of leaf and lupulin gland tissues indicates that the expression of enzymatic genes related to secondary metabolite biosynthesis, phytohormone biosynthesis, floral organs, flowering time, and trichome development, including other genes related to starch and sucrose metabolism and defense mechanisms, were differentially modulated in the Hlmed5a/med5b lines. The combined results from functional and transcriptomic analyses illuminates the pivotal function of HlMED5a and HlMED5b in homeostasis of secondary meatbolites accumulation in hop.

Comparative transcriptome analysis reveals the involvement of an MYB transcriptional activator, SmMYB108, in anther dehiscence in eggplant

Male sterility is a highly attractive agronomic trait as it effectively prevents self-fertilization and facilitates the production of high-quality hybrid seeds in plants. Timely release of mature pollen following anther dehiscence is essential for stamen development in flowering plants. Although several theories have been proposed regarding this, the specific mechanism of anther development in eggplant remains elusive. In this study, we selected an R2R3-MYB transcription factor gene, SmMYB108, that encodes a protein localized primarily in the nucleus by comparing the transcriptomics of different floral bud developmental stages of the eggplant fertile line, F142. Quantitative reverse transcription polymerase chain reaction revealed that SmMYB108 was preferentially expressed in flowers, and its expression increased significantly on the day of flowering. Overexpression of SmMYB108 in tobacco caused anther dehiscence. In addition, we found that SmMYB108 primarily functions as a transcriptional activator via C-terminal activation (amino acid 262-317). Yeast one-hybrid and dual-luciferase reporter assays revealed that genes (SmMYB21, SmARF6, and SmARF8) related to anther development targeted the SmMYB108 promoter. Overall, our results provide insights into the molecular mechanisms involved in the regulation of anther development by SmMYB108.

MdERF114 enhances the resistance of apple roots to Fusarium solani by regulating the transcription of MdPRX63

As the main fungal etiologic agent of apple (Malus domestica) replant disease (ARD), Fusarium solani seriously damages apple roots. Ethylene response factors (ERFs) play an important role in plant resistance to biotic stress. Here, we show that MdERF114 is expressed during F. solani infections and positively regulates the resistance of apple roots to F. solani. Yeast one-hybrid, dual-luciferase, electrophoretic mobility shift assays and determinations of lignin content indicated that MdERF114 directly binds the GCC-box of the MdPEROXIDASE63 (MdPRX63) promoter and activates its expression, resulting in lignin deposition in apple roots and increased resistance to F. solani. We identified a WRKY family transcription factor, MdWRKY75, that binds to the W-box of the MdERF114 promoter. Overexpression of MdWRKY75 enhanced resistance of apple roots to F. solani. MdMYB8 interacted with MdERF114 to enhance resistance to F. solani by promoting the binding of MdERF114 to the MdPRX63 promoter. In summary, our findings reveal that the MdWRKY75-MdERF114-MdMYB8-MdPRX63 module is required for apple resistance to F. solani and the application of this mechanism by Agrobacterium rhizogenes-mediated root transformation provides a promising strategy to prevent ARD.

The Regulatory Roles of MYC TFs in Plant Stamen Development

The stamen, as the male reproductive organ of flowering plants, plays a critical role in completing the life cycle of plants. MYC transcription factors are members of the bHLH IIIE subgroup and participate in a number of plant biological processes. In recent decades, a number of studies have confirmed that MYC transcription factors actively participate in the regulation of stamen development and have a critical impact on plant fertility. In this review, we summarized how MYC transcription factors play a role in regulating secondary thickening of the anther endothecium, the development and degradation of the tapetum, stomatal differentiation, and the dehydration of the anther epidermis. With regard to anther physiological metabolism, MYC transcription factors control dehydrin synthesis, ion and water transport, and carbohydrate metabolism to influence pollen viability. Additionally, MYCs participate in the JA signal transduction pathway, where they directly or indirectly control the development of stamens through the ET-JA, GA-JA, and ABA-JA pathways. By identifying the functions of MYCs during plant stamen development, it will help us to obtain a more comprehensive understanding not only on the molecular functions of this TF family but also the mechanisms underlying stamen development.

Identification of the passion fruit (Passiflora edulis Sims) MYB family in fruit development and abiotic stress, and functional analysis of PeMYB87 in abiotic stresses

Environmental stresses are ubiquitous in agricultural cultivation, and they affect the healthy growth and development of edible tissues in passion fruit. The study of resistance mechanisms is important in understanding the adaptation and resistance of plants to environmental stresses. In this work, two differently resistant passion fruit varieties were selected, using the expression characteristics of the transcription factor MYB, to explore the resistance mechanism of the MYB gene under various environmental stresses. A total of 174 MYB family members were identified using high-quality passion fruit genomes: 98 2R-MYB, 5 3R-MYB, and 71 1R-MYB (MYB-relate). Their family information was systematically analyzed, including subcellular localization, physicochemical properties, phylogeny at the genomic level, promoter function, encoded proteins, and reciprocal regulation. In this study, bioinformatics and transcriptome sequencing were used to identify members of the PeMYB genes in passion fruit whole-genome data, and biological techniques, such as qPCR, gene clone, and transient transformation of yeast, were used to determine the function of the passion fruit MYB genes in abiotic stress tolerance. Transcriptomic data were obtained for differential expression characteristics of two resistant and susceptible varieties, three expression patterns during pulp development, and four induced expression patterns under abiotic stress conditions. We further focused on the resistance mechanism of PeMYB87 in environmental stress, and we selected 10 representative PeMYB genes for quantitative expression verification. Most of the genes were differentially induced by four abiotic stresses, among which PeMYB87 responded significantly to high-temperature-induced expression and overexpression of the PeMYB87 gene in the yeast system. The transgenic PeMYB87 in yeast showed different degrees of stress resistance under exposure to cold, high temperatures, drought, and salt stresses. These findings lay the foundation for further analysis of the biological functions of PeMYBs involved in stress resistance in passion fruit.

Novel Camellia sinensis O-Methyltransferase Regulated by CsMADSL1 Specifically Methylates EGCG in Cultivar "GZMe4"

Epigallocatechin-3-O-(4-O-methyl)gallate (EGCG4″Me) in Camellia sinensis possesses numerous beneficial biological activities. However, the germplasm rich in EGCG4″Me and the O-methyltransferase responsible for EGCG4″Me biosynthesis are poorly understood. Herein, the content of EGCG3″Me and EGCG4″Me in the shoots of 13 cultivars was analyzed to demonstrate that EGCG4″Me is characteristically accumulated in the "GZMe4" cultivar but not in the other 12 cultivars. A novel O-methyltransferase (CsOMTL1) was identified from "GZMe4" using RNA-Seq and correlation analysis. Using the recombinant enzyme, EGCG4″Me was synthesized in vitro. Overexpression of CsOMTL1 via Agrobacterium-mediated genetic transformation caused constitutive accumulation of EGCG4″Me in C. sinensis callus. Moreover, the transcription factor CsMADSL1 localized in the nucleus activated the transcription of CsOMTL1 and specifically interacted with its promoter. Hence, our study identified a novel O-methyltransferase that characteristically catalyzes the synthesis of EGCG4″Me and a positive regulator of EGCG4″Me synthesis in "GZMe4", which might provide a strategy for the breeding of a tea cultivar rich in EGCG4″Me.

The multifaceted roles of R2R3 transcription factor HlMYB7 in the regulation of flavonoid and bitter acids biosynthesis, development and biotic stress tolerance in hop (Humulus lupulus L.)

Hop (Humulus lupulus) biosynthesizes the highly economically valuable secondary metabolites, which include flavonoids, bitter acids, polyphenols and essential oils. These compounds have important pharmacological properties and are widely implicated in the brewing industry owing to bittering flavor, floral aroma and preservative activity. Our previous studies documented that ternary MYB-bHLH-WD40 (MBW) and binary WRKY1-WD40 (WW) protein complexes transcriptionally regulate the accumulation of bitter acid (BA) and prenylflavonoids (PF). In the present study, we investigated the regulatory functions of the R2R3-MYB repressor HlMYB7 transcription factor, which contains a conserved N-terminal domain along with the repressive motif EAR, in regulating the PF- and BA-biosynthetic pathway and their accumulation in hop. Constitutive expression of HlMYB7 resulted in transcriptional repression of structural genes involved in the terminal steps of biosynthesis of PF and BA, as well as stunted growth, delayed flowering, and reduced tolerance to viroid infection in hop. Furthermore, yeast two-hybrid and transient reporter assays revealed that HlMYB7 targets both PF and BA pathway genes and suppresses MBW and WW protein complexes. Heterologous expression of HlMYB7 leads to down-regulation of structural genes of flavonoid pathway in Arabidopsis thaliana, including a decrease in anthocyanin content in Nicotiana tabacum. The combined results from functional and transcriptomic analyses highlight the important role of HlMYB7 in fine-tuning and balancing the accumulation of secondary metabolites at the transcriptional level, thus offer a plausible target for metabolic engineering in hop.

R2R3-MYB gene family: Genome-wide identification provides insight to improve the content of proanthocyanidins in Trifolium repens

The R2R3-MYB family is one of largest transcription factor families in plants playing significant roles in regulating anthocyanin and proanthocyanidin biosynthesis. Proanthocyanidins are one of major objectives to improve the quality of white clover (Trifolium repens L.), which have a beneficial effect on ruminant to prevent the lethal pasture bloat. A total of 133 TrR2R3-MYB genes were identified and distributed on all 16 chromosomes based on the whole genome information of white clover. Also, by exploring the gene structure, motifs and duplication events of TrR2R3-MYBs, as well as the evolutionary relationship with TrR2R3-MYB genes of other species, 10 TrR2R3-MYB genes with the potential to regulate the anthocyanins and proanthocyanidins biosynthesis were screened. These TrR2R3-MYB genes responded significantly to low temperature in white clover. In addition, they have different expression patterns in leaves, petioles and inflorescences of white clover. Importantly, TrMYB116 and TrMYB118 may positively regulate anthocyanin accumulation and low temperature response in white clover. TrMYB118 may also be associated with anthocyanin pigmentation pattern in Purple leaves. This study provides a basis for verifying the function of TrR2R3-MYB and breeding white clover cultivars with high proanthocyanidins.

Metabolome and Transcriptome Profiling Unveil the Mechanisms of Polyphenol Synthesis in the Developing Endopleura of Walnut (Juglans regia L.)

Walnut (Juglans regia L.) is an important woody nut tree species, and its endopleura (the inner coating of a seed) is rich in many polyphenols. Thus far, the pathways and essential genes involved in polyphenol biosynthesis in developing walnut endopleura remain largely unclear. We compared metabolite differences between endopleura and embryo in mature walnuts, and analyzed the changes of metabolites in endopleura at 35, 63, 91, 119, and 147 days after pollination (DAP). A total of 760 metabolites were detected in the metabolome, and the polyphenol contents in endopleura were higher than those in embryos. A total of 15 types of procyanidins, 10 types of kaempferol glycosides, and 21 types of quercetin glycosides that accumulated during endopleura development were identified. The analysis of the phenylpropane metabolic pathway showed that phenylalanine was gradually transformed into proanthocyanidins and other secondary metabolites with the development of endopleura. A total of 49 unigenes related to polyphenol synthesis were identified by transcriptome analysis of endopleura. The expression patterns of PAL, C4H, 4CL, CHS, CHI, F3H, LDOX, and ANR were similar, and their expression levels were highest in endopleura at maturity. Transcriptome and metabolome analysis showed that endopleura rapidly synthesized and accumulated polyphenols during maturation. Moreover, the transcription factor MYB111 played an important role in synthesizing polyphenols in endopleura, and its expression pattern was positively correlated with the accumulation pattern of quercetin, kaempferol, and proanthocyanidins. MYB111 was co-expressed with NAP, NAC, ATR1, and other genes related to cell senescence and abiotic stress response. Our study analyzed the composition and molecular synthesis mechanism of polyphenols in walnut endopleura, and provided new perspectives and insights regarding the nutritional research of walnut nuts.

Identification of a WRKY transcriptional activator from Camellia sinensis that regulates methylated EGCG biosynthesis

Naturally occurring methylated catechins, especially methylated EGCG in tea leaves are known to have many health benefits. Although the genes involved in methylated EGCG biosynthesis have been studied extensively, the transcriptional factors controlling methylated EGCG biosynthesis are still poorly understood. In the present study, a WRKY domain-containing protein termed CsWRKY57like was identified, which belongs to group IIc of the WRKY family, and contains one conserved WRKY motif. CsWRKY57like was found to localize in the nucleus, function as a transcriptional activator, and its expression positively correlated with methylated EGCG level. In addition, CsWRKY57like activated the transcription of three genes related to methylated EGCG biosynthesis, including CCoAOMT, CsLAR, and CsDFR by specifically interacting with their promoters via binding to the cis-acting element (C/T)TGAC(T/C). Further assays revealed that CsWRKY57like physically interacts with CsVQ4, and participates in the metabolic regulation of O-methylated catechin biosynthesis. Collectively, we conclude that CsWRKY57like may positively impact the biosynthesis of methylated EGCG in the tea plant, which comprehensively enriches the regulatory network of WRKY TFs associated with methylated EGCG and provide a potential strategy for the breeding of specific tea plant cultivars with high methylated EGCG .

Developmental regulation of lupulin gland-associated genes in aromatic and bitter hops (Humulus lupulus L.)

BACKGROUND

Hop (Humulus lupulus L.) bitter acids are valuable metabolites for the brewing industry. They are biosynthesized and accumulate in glandular trichomes of the female inflorescence (hop cone). The content of alpha bitter acids, such as humulones, in hop cones can differentiate aromatic from bitter hop cultivars. These contents are subject to genetic and environmental control but significantly correlate with the number and size of glandular trichomes (lupulin glands).

RESULTS

We evaluated the expression levels of 37 genes involved in bitter acid biosynthesis and morphological and developmental differentiation of glandular trichomes to identify key regulatory factors involved in bitter acid content differences. For bitter acid biosynthesis genes, upregulation of humulone synthase genes, which are important for the biosynthesis of alpha bitter acids in lupulin glands, could explain the higher accumulation of alpha bitter acids in bitter hops. Several transcription factors, including HlETC1, HlMYB61 and HlMYB5 from the MYB family, as well as HlGLABRA2, HlCYCB2-4, HlZFP8 and HlYABBY1, were also more highly expressed in the bitter hop cultivars; therefore, these factors may be important for the higher density of lupulin glands also seen in the bitter hop cultivars.

CONCLUSIONS

Gene expression analyses enabled us to investigate the differences between aromatic and bitter hops. This study confirmed that the bitter acid content in glandular trichomes (lupulin glands) is dependent on the last step of alpha bitter acid biosynthesis and glandular trichome density.

The Influence of Hop Latent Viroid (HLVd) Infection on Gene Expression and Secondary Metabolite Contents in Hop (Humulus lupulus L.) Glandular Trichomes

Viroids are small infectious pathogens, composed of a short single-stranded circular RNA. Hop (Humulus lupulus L.) plants are hosts to four viroids from the family Pospiviroidae. Hop latent viroid (HLVd) is spread worldwide in all hop-growing regions without any visible symptoms on infected hop plants. In this study, we evaluated the influence of HLVd infection on the content and the composition of secondary metabolites in maturated hop cones, together with gene expression analyses of involved biosynthesis and regulation genes for Saaz, Sládek, Premiant and Agnus cultivars. We confirmed that the contents of alpha bitter acids were significantly reduced in the range from 8.8% to 34% by viroid infection. New, we found that viroid infection significantly reduced the contents of xanthohumol in the range from 3.9% to 23.5%. In essential oils of Saaz cultivar, the contents of monoterpenes, terpene epoxides and terpene alcohols were increased, but the contents of sesquiterpenes and terpene ketones were decreased. Secondary metabolites changes were supported by gene expression analyses, except essential oils. Last-step biosynthesis enzyme genes, namely humulone synthase 1 (HS1) and 2 (HS2) for alpha bitter acids and O-methytransferase 1 (OMT1) for xanthohumol, were down-regulated by viroid infection. We found that the expression of ribosomal protein L5 (RPL5) RPL5 and the splicing of transcription factor IIIA-7ZF were affected by viroid infection and a disbalance in proteosynthesis can influence transcriptions of biosynthesis and regulatory genes involved in of secondary metabolites biosynthesis. We suppose that RPL5/TFIIIA-7ZF regulatory cascade can be involved in HLVd replication as for other viroids of the family Pospiviroidae.

Comparative transcriptome analysis reveals the molecular mechanism of salt tolerance in Apocynum venetum

Apocynum venetum is a traditional Chinese medicinal herb with tolerance to various abiotic stresses, especially, salinity. However, only a few studies have investigated the salt-tolerant mechanism of this non-halophyte under salt stress at phenotypic and physiological levels. To explore the molecular mechanism of salinity tolerance in A. venetum, the global transcriptome profiles of seedling leaves under different salt-stress durations, using 200 mM NaCl, were analyzed. De novo assembly of approximately 715 million high-quality reads and approximately 105.61 Gb sequence data was performed. In total, 2822 differentially expressed genes (DEGs) were identified. DEGs were significantly enriched in flavonoid metabolism-related pathways such as "flavonoid biosynthesis" and "phenylpropanoid biosynthesis". Most of these DEGs were downregulated under salt stress. However, genes encoding the non-selective cation channels and antioxidants were upregulated under salt stress, whereas most cell wall-related DEGs were downregulated. Consequently, the concentration of flavonoids decreased, whereas that of Na⁺ increased with exposure time. Thus, we hypothesized that the accumulation of Na⁺ in the leaves, which resulted in reduced flavonoid concentration under salt stress, directly led to a decrease in the salt tolerance of A. venetum. This was verified by overexpressing four flavonoid synthesis pathway genes in Arabidopsis. The transgenic plants showed higher salt tolerance than the wild-type plants due to the accumulation of total flavonoids. These physiological and transcriptome analyses of A. venetum revealed major molecular underpinnings contributing to the responses of A. venetum to salt stress, thereby improving our understanding of the molecular mechanisms underlying salt tolerance in A. venetum and plants in general. The findings serve as a basis for functional studies on and engineering strategies for plant salinity tolerance.

Establishment of CRISPR/Cas9 mediated targeted mutagenesis in hop (Humulus lupulus)

The CRISPR/Cas9-based targeted genome editing has emerged as a versatile technique, widely employed in plant genome engineering, both to decipher gene function and as an alternative to classical breeding technique for traits improvement in plants. However, to date, no such platform has been developed for hop (Humulus lupulus L.), which is an economically important crop producing valuable secondary metabolites utilized in the brewing and pharmaceutical industries. Here, we present the first report on the successful establishment of efficient CRISPR/Cas9-based genome editing using the visible endogenous marker gene phytoene desaturase (PDS) involved in carotenoid biosynthesis to demonstrate successful genome editing in hop. Agrobacterium tumefaciens-mediated transformation of in vitro generated internodal explants was used for the stable integration of constructs expressing plant codon-optimized Cas9 and a pair of co-expressed guide RNAs to target the distinct genomic sites of the PDS gene of hop. Analysis of RNA-guided genome-editing events, including mutant lines screening and homozygosity assessment using the T7 endonuclease assay showed that 33.3% of transformed plants were successfully edited at the target site, displaying albino and mosaic regenerants. Intriguingly, the detected mutations were ranges of deletions (16 bp to 39 bp) which led to disruption of the exon-intron boundary, few base substitutions, and a 1 bp insertion at 3 bp upstream of the PAM region of the target site. The decrease in chlorophyll a/b, and carotenoid content in the mutant lines further confirmed the functional disruption of the HlPDS gene. Taken together, our results demonstrate that the CRISPR/Cas9 system can precisely edit the targeted genome sequences, which may revolutionize our way to overcome some of the obstacles that have plagued the traits improvement in hop.

The spatio-temporal biosynthesis of floral flavonols is controlled by differential phylogenetic MYB regulators in Freesia hybrida

Floral flavonols play specific pivotal roles in pollinator attraction, pollen germination and fertility, in addition to other functions in vegetative organs. For many plants, the process of flavonol biosynthesis in late flower development stages and in mature flower tissues is poorly understood, in contrast to early flower development stages. It is thought that this process may be regulated independently of subgroup 7 R2R3 MYB (SG7 MYB) transcription factors. In this study, two FLS genes were shown to be expressed synchronously with the flower development-specific and tissue-specific biosynthesis of flavonols in Freesia hybrida. FhFLS1 contributed to flavonol biosynthesis in early flower buds, toruses and calyxes, and was regulated by four well-known SG7 MYB proteins, designated as FhMYBFs, with at least partial regulatory redundancy. FhFLS2 accounted for flavonols in late developed flowers and in the petals, stamens and pistils, and was targeted directly by non SG7 MYB protein FhMYB21L2. In parallel, AtMYB21 and AtMYB24 also activated AtFLS1, a gene highly expressed in Arabidopsis anthers and pollen, indicating the conserved regulatory roles of MYB21 against FLS genes in these two evolutionarily divergent angiosperm plants. Our results reveal a novel regulatory and synthetic mechanism underlying flavonol biosynthesis in floral organs and tissues which may be exploited to investigate supplementary roles of flavonols in flowers.

Dissection of Dynamic Transcriptome Landscape of Leaf, Bract, and Lupulin Gland in Hop (Humulus lupulus L.)

The hop plant (Humulus lupulus L.) produces several valuable secondary metabolites, such as prenylflavonoid, bitter acids, and essential oils. These compounds are biosynthesized in glandular trichomes (lupulin glands) endowed with pharmacological properties and widely implicated in the beer brewing industry. The present study is an attempt to generate exhaustive information of transcriptome dynamics and gene regulatory mechanisms involved in biosynthesis and regulation of these compounds, developmental changes including trichome development at three development stages, namely leaf, bract, and mature lupulin glands. Using high-throughput RNA-Seq technology, a total of 61.13, 50.01, and 20.18 Mb clean reads in the leaf, bract, and lupulin gland libraries, respectively, were obtained and assembled into 43,550 unigenes. The putative functions were assigned to 30,996 transcripts (71.17%) based on basic local alignment search tool similarity searches against public sequence databases, including GO, KEGG, NR, and COG families, which indicated that genes are principally involved in fundamental cellular and molecular functions, and biosynthesis of secondary metabolites. The expression levels of all unigenes were analyzed in leaf, bract, and lupulin glands tissues of hop. The expression profile of transcript encoding enzymes of BCAA metabolism, MEP, and shikimate pathway was most up-regulated in lupulin glands compared with leaves and bracts. Similarly, the expression levels of the transcription factors and structural genes that directly encode enzymes involved in xanthohumol, bitter acids, and terpenoids biosynthesis pathway were found to be significantly enhanced in lupulin glands, suggesting that production of these metabolites increases after the leaf development. In addition, numerous genes involved in primary metabolism, lipid metabolism, photosynthesis, generation of precursor metabolites/energy, protein modification, transporter activity, and cell wall component biogenesis were differentially regulated in three developmental stages, suggesting their involvement in the dynamics of the lupulin gland development. The identification of differentially regulated trichome-related genes provided a new foundation for molecular research on trichome development and differentiation in hop. In conclusion, the reported results provide directions for future functional genomics studies for genetic engineering or molecular breeding for augmentation of secondary metabolite content in hop.

Genome-wide transcriptome profiling of transgenic hop (Humulus lupulus L.) constitutively overexpressing HlWRKY1 and HlWDR1 transcription factors

Background

The hop plant (Humulus lupulus L.) is a valuable source of several secondary metabolites, such as flavonoids, bitter acids, and essential oils. These compounds are widely implicated in the beer brewing industry and are having potential biomedical applications. Several independent breeding programs around the world have been initiated to develop new cultivars with enriched lupulin and secondary metabolite contents but met with limited success due to several constraints. In the present work, a pioneering attempt has been made to overexpress master regulator binary transcription factor complex formed by HlWRKY1 and HlWDR1 using a plant expression vector to enhance the level of prenylflavonoid and bitter acid content in the hop. Subsequently, we performed transcriptional profiling using high-throughput RNA-Seq technology in leaves of resultant transformants and wild-type hop to gain in-depth information about the genome-wide functional changes induced by HlWRKY1 and HlWDR1 overexpression.

Results

The transgenic WW-lines exhibited an elevated expression of structural and regulatory genes involved in prenylflavonoid and bitter acid biosynthesis pathways. In addition, the comparative transcriptome analysis revealed a total of 522 transcripts involved in 30 pathways, including lipids and amino acids biosynthesis, primary carbon metabolism, phytohormone signaling and stress responses were differentially expressed in WW-transformants. It was apparent from the whole transcriptome sequencing that modulation of primary carbon metabolism and other pathways by HlWRKY1 and HlWDR1 overexpression resulted in enhanced substrate flux towards secondary metabolites pathway. The detailed analyses suggested that none of the pathways or genes, which have a detrimental effect on physiology, growth and development processes, were induced on a genome-wide scale in WW-transgenic lines.

Conclusions

Taken together, our results suggest that HlWRKY1 and HlWDR1 simultaneous overexpression positively regulates the prenylflavonoid and bitter acid biosynthesis pathways in the hop and thus these transgenes are presented as prospective candidates for achieving enhanced secondary metabolite content in the hop.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5125-8) contains supplementary material, which is available to authorized users.