Regulation of plant secondary metabolism and associated specialized cell development by MYBs and bHLHs

QTL mapping and candidate gene identification for fodder quality traits in Pearl millet

BACKGROUND

Pearl millet is an excellent forage crop with significant potential for forage production. Its fodder is rich in protein, calcium, phosphorus and other essential minerals while being low in undesirable components such as hydrocyanic acid and oxalic acid. Globally, the shortage of high-quality fodder poses challenges for maintaining animal health and productivity, ultimately impacting dairy farmers. Therefore, improving pearl millet for fodder traits should be a priority to meet the global demand for nutritious livestock feed.

RESULTS

Significant variability was observed for all forage quality related traits at both locations. A linkage map was constructed using 755 single-nucleotide polymorphisms (SNPs) markers, spanning a total length of 3080.44 cM. A total 8, 6 and 10 QTLs were identified for Ludhiana, Abohar and across the locations, respectively, for fodder quality. A common genomic interval with flanking markers S6_234379851- S6_64109715 was associated with IVOMD, CP and ME at all locations, with 10-34% phenotypic variance. Further, expression analysis identified BHLH 148, Resistance to phytophthora, Laccase 15, cytochrome P450, PLIM2c, GRF11, NEDD AXR1, NAC 92 and TF 089 as differentially expressed candidate genes in the leaf tissues of parental lines. A phylogenetic tree constructed using these genes revealed two clades identified with six paralogous events. Additionally, a phylogenetic tree of eight cereal species showed that the majority of shared similarity with the Pgl genes, suggestinga recent speciation event among them. Common genes, including cytochrome P450, PLIM2c, NEDD AXR1 and NAC domains were identified between QTL regions and expression analysis.

CONCLUSION

The differentially expressed genes incorporating the regulatory elements governing the lignin pathway have direct or indirect effects on fodder digestibility and quality. Exploiting these factors can contribute to the direct improvement of fodder quality. The identified QTLs and candidate genes from this study could facilitate the development of gene based markers for fodder improvement.

Genetic mechanisms, biological function, and biotechnological advance in sorghum tannins research

Sorghum (Sorghum bicolor) holds a unique position in the human diet and serves as a stable food source in many developing countries especially in African and south Asian regions. Tannins, the primary secondary metabolites in sorghum, are pivotal in determining its characteristic bitter taste. Beyond their influence on flavor, tannins play a vital role in sorghum's resistance to biotic and abiotic stresses and serve as key indicators of grain quality. The concentration of tannins significantly affects the potential for diverse applications of sorghum. This review provides a comprehensive analysis of sorghum tannins, focusing on their genetic basis, biological activities, and biosynthesis mechanisms. It highlights the relationship between tannin levels and grain color and delves into the underlying biogenetic pathways. Furthermore, the potential of functional genomics and biotechnological approaches in precisely controlling tannin levels for sorghum breeding is discussed. This study aims to offer valuable insights and perspectives for advancing both the scientific understanding and practical applications of sorghum tannins.

SlMYB72 and SlMYB75 antagonistically regulate trichome formation via the MYB-bHLH-WD40 complex in tomato

Trichomes are specialized epidermal outgrowths serving as protective barriers for plants against various stresses such as herbivore attacks. MYB-bHLH-WD40 complex is of great significance for unicellular trichome formation in Arabidopsis, whereas its role in the formation of multicellular trichomes in tomatoes remains largely unknown. In the present study, we identified that the R2R3-type MYB transcription factor SlMYB72 promotes the formation of type II, V, and VI trichomes by inhibiting the expression of SlCycB2, a repressor of trichome initiation. SlMYB75 is a negative regulator of trichome formation and positively regulates SlCycB2 expression. Interaction analyses showed that SlMYB72 and SlMYB75 can form MYB-bHLH-WD40 complexes with SlbHLH150 and SlTTG1, respectively, through mutual interactions. The dual-luciferase assay demonstrated that the regulatory functions of SlMYB72 and SlMYB75 in SlCycB2 expression can be enhanced by their corresponding MYB-bHLH-WD40 complexes. Interestingly, yeast-three-hybrid assay indicated that SlMYB75 competes with SlMYB72 for SlbHLH150 and SlTTG1, and counterbalances the down-regulation of SlCycB2 expression controlled by SlMYB72 alone, which is further confirmed by genetic hybrid experiments. These results reveal that SlMYB72 and SlMYB75 antagonistically regulate trichome formation and SlCycB2 expression through MYB-bHLH-WD40 complexes. These findings provide a novel perspective and theoretical basis for the formation of multicellular trichomes in tomatoes and the development of highly resistant plants.

Update on the structure and regulated biosynthesis of the apoplastic polymers cutin and suberin

The plant lipid polymers cutin and suberin play a critical role in many aspects of plant growth, development, and physiology. The mechanisms of cutin and suberin biosynthesis are relatively well understood thanks to just over 2 decades of work with primarily Arabidopsis (Arabidopsis thaliana) mutants. Recent advances in our understanding of cutin and suberin structure have arisen through the application of novel chemistries targeted at quantitative comprehension of intermolecular linkages, isolating intact suberins and cutins, and the application of advanced analytical techniques. The advent of high-throughput transcription factor binding assays and next-generation sequencing has facilitated the discovery of numerous cutin and suberin-regulating transcription factors and their gene promoter targets. Herein we provide an overview of aspects of cutin and suberin structure, biosynthesis, and transcriptional regulation of their synthesis highlighting recent developments in our understanding of these facets of cutin and suberin biology. We further identify outstanding questions in these respective areas and provide perspectives on how to advance the field to address these questions.

Genome-wide identification of the bHLH gene family and the mechanism regulation of anthocyanin biosynthesis by ChEGL1 in Cerasus humilis

Cerasus humilis is a fruit tree with enormous potential economic value, and its fruit is rich in various bioactive substances. The basic helix loop helix (bHLH) gene family plays an important role in the biosynthesis of plant anthocyanins. However, there was no research on the ChbHLH gene family in C. humilis. In this study, 114 ChbHLH genes were identified from the C. humilis genome and divided into 17 subgroups. Then, evolutionary relationships, conserved motifs, gene structures, and cis-acting elements were analyzed. By predicting the interaction network between ChbHLH proteins and ChMYB1, it was found that ChbHLH44 (here named as ChEGL1) was located at the core of the interaction network. Further experiments revealed that ChEGL1 and ChMYB1 could interact with each other both in vivo and in vitro. In addition, ChEGL1 significantly increased the anthocyanin content in transgenic tomato plants. This study provides a comprehensive understanding of the ChbHLH gene family and supports further enrichment of the regulation mechanism of anthocyanin biosynthesis in C. humilis fruit.

PSC1, a basic/helix-loop-helix transcription factor controlling the purplish-red testa trait in peanut

Seed color is a key agronomic trait in crops such as peanut, where it is a vital indicator of both nutritional and commercial value. In recent years, peanuts with darker seed coats have gained market attention due to their high anthocyanin content. Here, we used bulk segregant analysis to identify the gene associated with the purplish-red coat trait and identified a novel gene encoding a basic/helix-loop-helix transcription factor, PURPLE RED SEED COAT1 (PSC1), which regulates the accumulation of anthocyanins in the seed coat. Specifically, we found that a 35-bp insertion in the PSC1 promoter increased the abundance of PSC1 mRNA. Transcriptomic and metabolomic analyses indicated that the purplish-red color of the seed coat was the result of decreased expression of anthocyanidin reductase (ANR), leading to increased accumulation of delphinidin, cyanidin, and pelargonidin derivatives. Further analysis revealed that PSC1 interacts with AhMYB7 to form a complex that specifically binds to the ANR promoter to suppress its expression, resulting in increased anthocyanin accumulation. Moreover, overexpression of PSC1 increased anthocyanin content in Arabidopsis thaliana and peanut callus. Our study reveals a new gene that controls seed coat color by regulating anthocyanin metabolism and provides a valuable genetic resource for breeding peanuts with a purplish-red seed coat.

Biological Characteristics of a Novel Bibenzyl Synthase (DoBS1) Gene from Dendrobium officinale Catalyzing Dihydroresveratrol Synthesis

Bibenzyl compounds are one of the most important bioactive components of natural medicine. However, Dendrobium officinale as a traditional herbal medicine is rich in bibenzyl compounds and performs functions such as acting as an antioxidant, inhibiting cancer cell growth, and assisting in neuro-protection. The biosynthesis of bibenzyl products is regulated by bibenzyl synthase (BBS). In this study, we have cloned the cDNA gene of the bibenzyl synthase (DoBS1) from D. officinale using PCR with degenerate primers, and we have identified a novel type III polyketide synthase (PKS) gene by phylogenetic analyses. In a series of perfect experiments, DoBS1 was expressed in Escherichia coli, purified and some catalytic properties of the recombinant protein were investigated. The molecular weight of the recombinant protein was verified to be approximately 42.7 kDa. An enzyme activity analysis indicated that the recombinant DoBS1-HisTag protein was capable of using 4-coumaryol-CoA and 3 malonyl-CoA as substrates for dihydroresveratrol (DHR) in vitro. The Vmax and Km of the recombinant protein for DHR were 3.57 ± 0.23 nmol·min-1·mg-1 and 0.30 ± 0.08 mmol, respectively. The present study provides further insights into the catalytic mechanism of the active site in the biosynthetic pathway for the catalytic production of dihydroresveratrol by bibenzylase in D. officinale. The results can be used to optimize a novel biosynthetic pathway for the industrial synthesis of DHR.

Protein kinase MeSnRK2.3 positively regulates starch biosynthesis by interacting with the transcription factor MebHLH68 in cassava

Starch biosynthesis involves numerous enzymes and is a crucial metabolic activity in plant storage organs. Sucrose non-fermenting related protein kinase 2 (SnRK2) is an abscisic acid (ABA)-dependent kinase and a significant regulatory enzyme in the ABA signaling pathway. However, whether SnRK2 kinases regulate starch biosynthesis is unclear. In this study, we identified that MeSnRK2.3, encoding an ABA-dependent kinase, was highly expressed in the storage roots of cassava (Manihot esculenta) and was induced by ABA. Overexpression of MeSnRK2.3 in cassava significantly increased the starch content in the storage roots and promoted plant growth. MeSnRK2.3 was further found to interact with the cassava basic helix-loop-helix 68 (MebHLH68) transcription factor in vivo and in vitro. MebHLH68 directly bound to the promoters of sucrose synthase 1 (MeSUS1), granule-bound starch synthase I a (MeGBSSIa), and starch-branching enzyme 2.4 (MeSBE2.4), thereby up-regulating their transcriptional activities. Additionally, MebHLH68 negatively regulated the transcriptional activity of sucrose phosphate synthase B (MeSPSB). Moreover, MebHLH68 phosphorylated by MeSnRK2.3 up-regulated the transcription activity of MeSBE2.4. These findings demonstrated that the MeSnRK2.3-MebHLH68 module connects the ABA signaling pathway and starch biosynthesis in cassava, thereby providing direct evidence of ABA-mediated participation in the sucrose metabolism and starch biosynthesis pathways.

Genetic regulation and manipulation of nicotine biosynthesis in tobacco: strategies to eliminate addictive alkaloids

Tobacco (Nicotiana tabacum L.) is a widely cultivated crop of the genus Nicotiana. Due to the highly addictive nature of tobacco products, tobacco smoking remains the leading cause of preventable death and disease. There is therefore a critical need to develop tobacco varieties with reduced or non-addictive nicotine levels. Nicotine and related pyridine alkaloids biosynthesized in the roots of tobacco plants are transported to the leaves, where they are stored in vacuoles as a defense against predators. Jasmonate, a defense-related plant hormone, plays a crucial signaling role in activating transcriptional regulators that coordinate the expression of downstream metabolic and transport genes involved in nicotine production. In recent years, substantial progress has been made in molecular and genomics research, revealing many metabolic and regulatory genes involved in nicotine biosynthesis. These advances have enabled us to develop tobacco plants with low or ultra-low nicotine levels through various methodologies, such as mutational breeding, genetic engineering, and genome editing. We review the recent progress on genetic manipulation of nicotine production in tobacco, which serves as an excellent example of plant metabolic engineering with profound social implications.

Re-discovering Prunus fruit varieties as antiangiogenic agents by metabolomic and bioinformatic approach

In this work, a comparative chemical-biological study of nine plum varieties (Prunus domestica L. and Prunus salicina Lindl.) with two commercial ones was carried out to improve their cultivation and use in the agri-food chain. The chemical quali-quantitative fingerprint by HR-Orbitrap/ESI-MS showed similar profiles, being 'Rossa Casa Velasco' the richest in phenols and anthocyanins. All the extracts were investigated for their in vitro antioxidant as well as antiangiogenic activity by two in vivo models, chick chorioallantoic membrane and zebrafish embryos. Among investigated varieties 'Scarrafona', 'Rusticano', 'Marisa', 'Rossa Casa Velasco', 'Verdone', and 'Sangue di Drago' showed the best antiangiogenic activities (30-50 % inhibition). Finally, the chemical/biological datasets processed with a bioinformatic approach revealed that a large group of flavonoids, procyanidins, and anthocyanins significantly correlated with all the three antioxidant tests (DPPH, FRAP, and ABTS), while quinic acid and icariside F2 resulted positively correlated with CAM at both 100 and 200 μg/egg.

Enhancement of specialized metabolites using CRISPR/Cas gene editing technology in medicinal plants

Plants are the richest source of specialized metabolites. The specialized metabolites offer a variety of physiological benefits and many adaptive evolutionary advantages and frequently linked to plant defense mechanisms. Medicinal plants are a vital source of nutrition and active pharmaceutical agents. The production of valuable specialized metabolites and bioactive compounds has increased with the improvement of transgenic techniques like gene silencing and gene overexpression. These techniques are beneficial for decreasing production costs and increasing nutritional value. Utilizing biotechnological applications to enhance specialized metabolites in medicinal plants needs characterization and identification of genes within an elucidated pathway. The breakthrough and advancement of CRISPR/Cas-based gene editing in improving the production of specific metabolites in medicinal plants have gained significant importance in contemporary times. This article imparts a comprehensive recapitulation of the latest advancements made in the implementation of CRISPR-gene editing techniques for the purpose of augmenting specific metabolites in medicinal plants. We also provide further insights and perspectives for improving metabolic engineering scenarios in medicinal plants.

Improved production of the antidiabetic metabolite montbretin A in Nicotiana benthamiana: discovery, characterization, and use of Crocosmia shikimate shunt genes

The plant-specialized metabolite montbretin A (MbA) is being developed as a new treatment option for type-2 diabetes, which is among the ten leading causes of premature death and disability worldwide. MbA is a complex acylated flavonoid glycoside produced in small amounts in below-ground organs of the perennial plant Montbretia (Crocosmia × crocosmiiflora). The lack of a scalable production system limits the development and potential application of MbA as a pharmaceutical or nutraceutical. Previous efforts to reconstruct montbretin biosynthesis in Nicotiana benthamiana (Nb) resulted in low yields of MbA and higher levels of montbretin B (MbB) and montbretin C (MbC). MbA, MbB, and MbC are nearly identical metabolites differing only in their acyl moieties, derived from caffeoyl-CoA, coumaroyl-CoA, and feruloyl-CoA, respectively. In contrast to MbA, MbB and MbC are not pharmaceutically active. To utilize the montbretia caffeoyl-CoA biosynthesis for improved MbA engineering in Nb, we cloned and characterized enzymes of the shikimate shunt of the general phenylpropanoid pathway, specifically hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyl transferase (CcHCT), p-coumaroylshikimate 3'-hydroxylase (CcC3'H), and caffeoylshikimate esterase (CcCSE). Gene expression patterns suggest that CcCSE enables the predominant formation of MbA, relative to MbB and MbC, in montbretia. This observation is supported by results from in vitro characterization of CcCSE and reconstruction of the shikimate shunt in yeast. Using CcHCT together with montbretin biosynthetic genes in multigene constructs resulted in a 30-fold increase of MbA in Nb. This work advances our understanding of the phenylpropanoid pathway and features a critical step towards improved MbA production in bioengineered Nb.

The leaf idioblastome of the medicinal plant Catharanthus roseus is associated with stress resistance and alkaloid metabolism

Catharanthus roseus leaves produce a range of monoterpenoid indole alkaloids (MIAs) that include low levels of the anticancer drugs vinblastine and vincristine. The MIA pathway displays a complex architecture spanning different subcellular and cell type localizations, and is under complex regulation. As a result, the development of strategies to increase the levels of the anticancer MIAs has remained elusive. The pathway involves mesophyll specialized idioblasts where the late unsolved biosynthetic steps are thought to occur. Here, protoplasts of C. roseus leaf idioblasts were isolated by fluorescence-activated cell sorting, and their differential alkaloid and transcriptomic profiles were characterized. This involved the assembly of an improved C. roseus transcriptome from short- and long-read data, IDIO+. It was observed that C. roseus mesophyll idioblasts possess a distinctive transcriptomic profile associated with protection against biotic and abiotic stresses, and indicative that this cell type is a carbon sink, in contrast to surrounding mesophyll cells. Moreover, it is shown that idioblasts are a hotspot of alkaloid accumulation, suggesting that their transcriptome may hold the key to the in-depth understanding of the MIA pathway and the success of strategies leading to higher levels of the anticancer drugs.

The R2R3-MYB Transcriptional Repressor TgMYB4 Negatively Regulates Anthocyanin Biosynthesis in Tulips (Tulipa gesneriana L.)

Anthocyanins play a paramount role in color variation and significantly contribute to the economic value of ornamental plants. The conserved activation complex MYB-bHLH-WD40 (MBW; MYB: v-myb avian myeloblastosis viral oncogene homolog; bHLH: basic helix-loop-helix protein; WD40:WD-repeat protein) involved in anthocyanin biosynthesis has been thoroughly researched, but there have been limited investigations into the function of repressor factors. In this study, we characterized TgMYB4, an R2R3-MYB transcriptional repressor which is highly expressed during petal coloration in red petal cultivars. TgMYB4-overexpressing tobaccos exhibited white or light pink petals with less anthocyanin accumulation compared to control plants. TgMYB4 was found to inhibit the transcription of ANTHOCYANIDIN SYNTHASE (TfANS1) and DIHYDRO-FLAVONOL-4-REDUCTASE (AtDFR), although it did not bind to their promoters. Moreover, the TgMYB4 protein was able to compete with the MYB activator to bind to the :bHLHprotein, thereby suppressing the function of the activator MBW complex. These findings demonstrate that TgMYB4 plays a suppressive role in the regulation of anthocyanin synthesis during flower pigmentation.

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

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

Evolutionary studies of the bHLH transcription factors belonging to MBW complex: their role in seed development

BACKGROUND AND AIMS

The MBW complex consist of proteins belonging to three major families (MYB, bHLH and WDR) involved in various processes throughout plant development: epidermal cell development, mucilage secretory cells and flavonoid biosynthesis. Recently, it has been reported that TT8, encoding a bHLH transcription factor, is involved in the biosynthesis of flavonoids in the seed coat and it also plays a role in bypassing the postzygotic barrier resulting from an unbalance in genetic loads of the parental lines. Here, we focus on the functional evolution, in seed development, of the bHLH proteins that are part of the MBW complex, complemented with a literature review.

METHODS

Phylogenetic analyses performed across seed plants and expression analyses in the reproductive tissues of four selected angiosperms (Arabidopsis thaliana, Brassica napus, Capsella rubella and Solanum lycopersicum) allow us to hypothesize on the evolution of its functions.

KEY RESULTS

TT8 expression in the innermost layer of the seed coat is conserved in the selected angiosperms. However, except for Arabidopsis, TT8 is also expressed in ovules, carpels and fruits. The homologues belonging to the sister clade of TT8, EGL3/GL3, involved in trichome development, are expressed in the outermost layer of the seed coat, suggesting potential roles in mucilage.

CONCLUSIONS

The ancestral function of these genes appears to be flavonoid biosynthesis, and the conservation of TT8 expression patterns in the innermost layer of the seed coat in angiosperms suggests that their function in postzygotic barriers might also be conserved. Moreover, the literature review and the results of the present study suggest a sophisticated association, linking the mechanisms of action of these genes to the cross-communication activity between the different tissues of the seed. Thus, it provides avenues to study the mechanisms of action of TT8 in the postzygotic triploid block, which is crucial because it impacts seed development in unbalanced crosses.

Induced production of specialized steroids by transcriptional reprogramming in Petunia hybrida

Plants produce specialized metabolites with defensive properties that are often synthesized through the coordinated regulation of metabolic genes by transcription factors in various biological contexts. In this study, we investigated the regulatory function of the transcription factor PhERF1 from petunia (Petunia hybrida), which belongs to a small group of ETHYLENE RESPONSE FACTOR (ERF) family members that regulate the biosynthesis of bioactive alkaloids and terpenoids in various plant lineages. We examined the effects of transiently overexpressing PhERF1 in petunia leaves on the transcriptome and metabolome, demonstrating the production of a class of specialized steroids, petuniolides, and petuniasterones in these leaves. We also observed the activation of many metabolic genes, including those involved in sterol biosynthesis, as well as clustered genes that encode new metabolic enzymes, such as cytochrome P450 oxidoreductases, 2-oxoglutarate-dependent dioxygenases, and BAHD acyltransferases. Furthermore, we determined that PhERF1 transcriptionally induces downstream metabolic genes by recognizing specific cis-regulatory elements in their promoters. This study highlights the potential of evolutionarily conserved transcriptional regulators to induce the production of specialized products through transcriptional reprogramming.

AaWRKY6 contributes to artemisinin accumulation during growth in Artemisia annua

Artemisinin, which is extracted from the plant Artemisia annua L., is a crucial drug for curing malaria and has potential applications for treating cancer, diabetes, pulmonary tuberculosis, and other conditions. Demand for artemisinin is therefore high, and enhancing its yield is important. Artemisinin dynamics change during the growth cycle of A. annua; however, the regulatory networks underlying these changes are poorly understood. Here, we collected A. annua leaves at different growth stages and identified target genes from transcriptome data. We determined that WRKY6 binds to the promoters of the artemisinin biosynthesis gene artemisinic aldehyde Δ11(13) reductase (DBR2). In agreement, overexpression of WRKY6 in A. annua resulted in higher expression levels of genes in the artemisinin biosynthesis pathway and greater artemisinin contents than in the wild type. When expression of WRKY6 was down-regulated, artemisinin biosynthesis pathway genes were also down-regulated and the content of artemisinin was lower. WRKY6 mediates the transcriptional activation of artemisinin biosynthesis by binding to the promoter of DBR2, making it a key regulator for modulating the dynamics of artemisinin changes during the A. annua growth cycle.

Enhancing Centelloside Production in Centella asiatica Hairy Root Lines through Metabolic Engineering of Triterpene Biosynthetic Pathway Early Genes

Centella asiatica is a medicinal plant with a rich tradition of use for its therapeutic properties. Among its bioactive compounds are centellosides, a group of triterpenoid secondary metabolites whose potent pharmacological activities have attracted significant attention. Metabolic engineering has emerged as a powerful biotechnological tool to enhance the production of target compounds. In this study, we explored the effects of overexpressing the squalene synthase (SQS) gene and transcription factor TSAR2 on various aspects of C. asiatica hairy root lines: the expression level of centelloside biosynthetic genes, morphological traits, as well as squalene, phytosterol, and centelloside content. Three distinct categories of transformed lines were obtained: LS, harboring At-SQS; LT, overexpressing TSAR2; and LST, simultaneously carrying both transgenes. These lines displayed noticeable alterations in morphological traits, including changes in branching rate and biomass production. Furthermore, we observed that the expression of T-DNA genes, particularly aux2 and rolC genes, significantly modulated the expression of pivotal genes involved in centelloside biosynthesis. Notably, the LS lines boasted an elevated centelloside content but concurrently displayed reduced phytosterol content, a finding that underscores the intriguing antagonistic relationship between phytosterol and triterpene pathways. Additionally, the inverse correlation between the centelloside content and morphological growth values observed in LS lines was countered by the action of TSAR2 in the LST and LT lines. This difference could be attributed to the simultaneous increase in the phytosterol content in the TSAR2-expressing lines, as these compounds are closely linked to root development. Overall, these discoveries offer valuable information for the biotechnological application of C. asiatica hairy roots and their potential to increase centelloside production.

Identification of two terpenoids that accumulate in Chinese water chestnut in response to fresh-cut processing

As a form of vegetable in China, freshly cut corms of Chinese water chestnuts (Eleocharis dulcis) are well received by consumers. Few studies have investigated the metabolites present in fresh-cut E. dulcis, particularly during the storage stage. Two compounds, triterpenoids and apocarotenoids, were identified in fresh-cut E. dulcis during the late storage period using thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and nuclear magnetic resonance (NMR) spectroscopy. The content of these two compounds gradually increased in the surface tissue of fresh-cut E. dulcis during storage. Moreover, the transcript levels of 10 genes involved in terpenoid backbone biosynthesis and five genes involved in carotenoid precursor biosynthesis were evaluated via quantitative real-time PCR (qRT-PCR). Expression of the rate-limiting enzyme-coding genes CwDXS and CwHMGS was significantly induced by wounding. CwMYC and CwbHLH18, which belong to bHLH transcription factors (TFs) IIIe and VIa subgroup, were isolated from E. dulcis corm. Phylogenetic analysis showed that CwMYC and CwbHLH18 grouped with other terpenoid-regulated bHLHs, and their transcript levels were strongly induced after fresh-cut processing. These results suggested that the biosynthesis of terpenoids and apocarotenoids in fresh-cut E. dulcis strongly depended on the transcriptional regulation of structural genes involved in the methylerythritol 4-phosphate (MEP) and mevalonate (MVA) pathways. However, the complex secondary metabolism of fresh-cut E. dulcis during late storage requires further investigation.

Transcriptome-wide expression analysis of MYB gene family leads to functional characterization of flavonoid biosynthesis in fruit coloration of Ziziphus Mill

The Ziziphus mauritiana Lam. and Z. jujuba Mill. are the two most economically important members of the genus Ziziphus. The fruit color of Z. mauritiana remains green throughout fruit development in the majority of commercial cultivars, whereas its close relative, Z. jujuba Mill. turns from green to red in all cultivars. However, the lack of transcriptomic and genomic information confines our understanding of the molecular mechanisms underlying fruit coloration in Z. mauritiana (Ber). In the present study, we performed the transcriptome-wide analysis of MYB transcription factors (TFs) genes in Z. mauritiana and Z. jujuba, and identified 56 ZmMYB and 60 ZjMYB TFs in Z. mauritiana and Z. jujuba, respectively. Through transcriptomic expression analysis, four similar MYB genes (ZmMYB/ZjMYB13, ZmMYB/ZjMYB44, ZmMYB/ZjMYB50, and ZmMYB/ZjMYB56) from Z. mauritiana and Z. jujuba were selected as candidate key genes regulating flavonoid biosynthesis. Among these genes, the ZjMYB44 gene was transiently highly expressed in fruit, and flavonoid content accumulation also increased, indicating that this gene can influence flavonoid content during the period of fruit coloration in Z. jujuba. The current study adds to our understanding of the classification of genes, motif structure, and predicted functions of the MYB TFs, as well as identifying MYBs that regulate flavonoid biosynthesis in Ziziphus (Z. mauritiana and Z. jujuba). Based on this information, we concluded that MYB44 is involved in the flavonoids biosynthesis pathway during the fruit coloring of Ziziphus. Our research results provide an important understanding of the molecular mechanism of flavonoid biosynthesis resulting in fruit coloration and laying a foundation for further genetic improvement of fruit color in Ziziphus.

Long-term cold, freezing and drought: overlapping and specific regulatory mechanisms and signal transduction in tea plant (Camellia sinensis (L.) Kuntze)

Introduction

Low temperatures and drought are two main environmental constraints reducing the yield and geographical distribution of horticultural crops worldwide. Understanding the genetic crosstalk between stress responses has potential importance for crop improvement.

Methods

In this study, Illumina RNA-seq and Pac-Bio genome resequencing were used to annotate genes and analyze transcriptome dynamics in tea plants under long-term cold, freezing, and drought.

Results

The highest number of differentially expressed genes (DEGs) was identified under long-term cold (7,896) and freezing (7,915), with 3,532 and 3,780 upregulated genes, respectively. The lowest number of DEGs was observed under 3-day drought (47) and 9-day drought (220), with five and 112 genes upregulated, respectively. The recovery after the cold had 6.5 times greater DEG numbers as compared to the drought recovery. Only 17.9% of cold-induced genes were upregulated by drought. In total, 1,492 transcription factor genes related to 57 families were identified. However, only 20 transcription factor genes were commonly upregulated by cold, freezing, and drought. Among the 232 common upregulated DEGs, most were related to signal transduction, cell wall remodeling, and lipid metabolism. Co-expression analysis and network reconstruction showed 19 genes with the highest co-expression connectivity: seven genes are related to cell wall remodeling (GATL7, UXS4, PRP-F1, 4CL, UEL-1, UDP-Arap, and TBL32), four genes are related to calcium-signaling (PXL1, Strap, CRT, and CIPK6), three genes are related to photo-perception (GIL1, CHUP1, and DnaJ11), two genes are related to hormone signaling (TTL3 and GID1C-like), two genes are involved in ROS signaling (ERO1 and CXE11), and one gene is related to the phenylpropanoid pathway (GALT6).

Discussion

Based on our results, several important overlapping mechanisms of long-term stress responses include cell wall remodeling through lignin biosynthesis, o-acetylation of polysaccharides, pectin biosynthesis and branching, and xyloglucan and arabinogalactan biosynthesis. This study provides new insight into long-term stress responses in woody crops, and a set of new target candidate genes were identified for molecular breeding aimed at tolerance to abiotic stresses.

The Influence of Methyl Jasmonate on Expression Patterns of Rosmarinic Acid Biosynthesis Genes, and Phenolic Compounds in Different Species of Salvia subg. Perovskia Kar L

Salvia yangii B.T. Drew and Salvia abrotanoides Kar are two important fragrant and medicinal plants that belong to the subgenus Perovskia. These plants have therapeutic benefits due to their high rosmarinic acid (RA) content. However, the molecular mechanisms behind RA generation in two species of Salvia plants are still poorly understood. As a first report, the objectives of the present research were to determine the effects of methyl jasmonate (MeJA) on the rosmarinic acid (RA), total flavonoid and phenolic contents (TFC and TPC), and changes in the expression of key genes involved in their biosynthesis (phenylalanine ammonia lyase (PAL), 4-coumarate-CoA ligase (4CL), and rosmarinic acid synthase (RAS)). The results of High-performance liquid chromatography (HPLC) analysis indicated that MeJA significantly increased RA content in S. yungii and S. abrotanoides species (to 82 and 67 mg/g DW, respectively) by 1.66- and 1.54-fold compared with untreated plants. After 24 h, leaves of Salvia yangii and Salvia abrotanoides species treated with 150 M MeJA had the greatest TPC and TFC (80 and 42 mg TAE/g DW, and 28.11 and 15.14 mg QUE/g DW, respectively), which was in line with the patterns of gene expression investigated. Our findings showed that MeJA dosages considerably enhanced the RA, TPC, and TFC contents in both species compared with the control treatment. Since increased numbers of transcripts for PAL, 4CL, and RAS were also detected, the effects of MeJA are probably caused by the activation of genes involved in the phenylpropanoid pathway.

Comparative analysis of the MYB gene family in seven Ipomoea species

The MYB transcription factors regulate plant growth, development, and defense responses. However, information about the MYB gene family in Ipomoea species is rare. Herein, we performed a comprehensive genome-wide comparative analysis of this gene family among seven Ipomoea species, sweet potato (I. batatas), I. trifida, I. triloba, I. nil, I. purpurea, I. cairica, and I. aquatic, and identified 296, 430, 411, 291, 226, 281, and 277 MYB genes, respectively. The identified MYB genes were classified into five types: 1R-MYB (MYB-related), 2R-MYB (R2R3-MYB), 3R-MYB (R1R2R3-MYB), 4R-MYB, and 5R-MYB, and the MYB-related or R2R3-MYB type was the most abundant MYB genes in the seven species. The Ipomoea MYB genes were classed into distinct subgroups based on the phylogenetic topology and the classification of the MYB superfamily in Arabidopsis. Analysis of gene structure and protein motifs revealed that members within the same phylogenetic group presented similar exon/intron and motif organization. The identified MYB genes were unevenly mapped on the chromosomes of each Ipomoea species. Duplication analysis indicated that segmental and tandem duplications contribute to expanding the Ipomoea MYB genes. Non-synonymous substitution (Ka) to synonymous substitution (Ks) [Ka/Ks] analysis showed that the duplicated Ipomoea MYB genes are mainly under purifying selection. Numerous cis-regulatory elements related to stress responses were detected in the MYB promoters. Six sweet potato transcriptome datasets referring to abiotic and biotic stresses were analyzed, and MYB different expression genes' (DEGs') responses to stress treatments were detected. Moreover, 10 sweet potato MYB DEGs were selected for qRT-PCR analysis. The results revealed that four responded to biotic stress (stem nematodes and Ceratocystis fimbriata pathogen infection) and six responded to the biotic stress (cold, drought, and salt). The results may provide new insights into the evolution of MYB genes in the Ipomoea genome and contribute to the future molecular breeding of sweet potatoes.

Mechanisms underlying the formation of complex color patterns on Nigella orientalis (Ranunculaceae) petals

Complex color patterns on petals are widespread in flowering plants, yet the mechanisms underlying their formation remain largely unclear. Here, by conducting detailed morphological, anatomical, biochemical, optical, transcriptomic, and functional studies, we investigated the cellular bases, chromogenic substances, reflectance spectra, developmental processes, and underlying mechanisms of complex color pattern formation on Nigella orientalis petals. We found that the complexity of the N. orientalis petals in color pattern is reflected at multiple levels, with the amount and arrangement of different pigmented cells being the key. We also found that biosynthesis of the chromogenic substances of different colors is sequential, so that one color/pattern is superimposed on another. Expression and functional studies further revealed that a pair of R2R3-MYB genes function cooperatively to specify the formation of the eyebrow-like horizontal stripe and the Mohawk haircut-like splatters. Specifically, while NiorMYB113-1 functions to draw a large splatter region, NiorMYB113-2 functions to suppress the production of anthocyanins from the region where a gap will form, thereby forming the highly specialized pattern. Our results provide a detailed portrait for the spatiotemporal dynamics of the coloration of N. orientalis petals and help better understand the mechanisms underlying complex color pattern formation in plants.

Downregulation of HbFPS1 affects rubber biosynthesis of Hevea brasiliensis suffering from tapping panel dryness

Tapping panel dryness (TPD) is a century-old problem that has plagued the natural rubber production of Hevea brasiliensis. TPD may result from self-protective mechanisms of H. brasiliensis in response to stresses such as excessive hormone stimulation and mechanical wounding (bark tapping). It has been hypothesized that TPD impairs rubber biosynthesis; however, the underlying mechanisms remain poorly understood. In the present study, we firstly verified that TPD-affected rubber trees exhibited lower rubber biosynthesis activity and greater rubber molecular weight compared to healthy rubber trees. We then demonstrated that HbFPS1, a key gene of rubber biosynthesis, and its expression products were downregulated in the latex of TPD-affected rubber trees, as revealed by transcriptome sequencing and iTRAQ-based proteome analysis. We further discovered that the farnesyl diphosphate synthase HbFPS1 could be recruited to small rubber particles by HbSRPP1 through protein-protein interactions to catalyze farnesyl diphosphate (FPP) synthesis and facilitate rubber biosynthesis initiation. FPP content in the latex of TPD-affected rubber trees was significantly decreased with the downregulation of HbFPS1, ultimately resulting in abnormal development of rubber particles, decreased rubber biosynthesis activity, and increased rubber molecular weight. Upstream regulator assays indicated that a novel regulator, MYB2-like, may be an important regulator of downregulation of HbFPS1 in the latex of TPD-affected rubber trees. Our findings not only provide new directions for studying the molecular events involved in rubber biosynthesis and TPD syndrome and contribute to rubber management strategies, but also broaden our knowledge of plant isoprenoid metabolism and its regulatory networks.

Transcriptional Reactivation of Lignin Biosynthesis for the Heterologous Production of Etoposide Aglycone in Nicotiana benthamiana

Nicotiana benthamiana is a valuable plant chassis for heterologous production of medicinal plant natural products. This host is well suited for the processing of organelle-localized plant enzymes, and the conservation of the primary metabolism across the plant kingdom often provides required plant-specific precursor molecules that feed a given pathway. Despite this commonality in metabolism, limited precursor supply and/or competing host pathways can interfere with yields of heterologous products. Here, we use transient transcriptional reprogramming of endogenous N. benthamiana metabolism to drastically improve flux through the etoposide pathway derived from the medicinal plant Podophyllum spp. Specifically, coexpression of a single lignin-associated transcription factor, MYB85, with pathway genes results in unprecedented levels of heterologous product accumulation in N. benthamiana leaves: 1 mg/g dry weight (DW) of the etoposide aglycone, 35 mg/g DW (-)-deoxypodophyllotoxin, and 3.5 mg/g DW (-)-epipodophyllotoxin─up to two orders of magnitude above previously reported biosynthetic yields for the etoposide aglycone and eight times higher than what is observed for (-)-deoxypodophyllotoxin in the native medicinal plant. Unexpectedly, transient activation of lignin metabolism by transcription factor overexpression also reduces the production of undesired side products that likely result from competing N. benthamiana metabolism. Our work demonstrates that synthetic activation of lignin biosynthesis in leaf tissue is an effective strategy for optimizing the production of medicinal compounds derived from phenylpropanoid precursors in the plant chassis N. benthamiana. Furthermore, our results highlight the engineering value of MYB85, an early switch in lignin biosynthesis, for on-demand modulation of monolignol flux and support the role of MYB46 as a master regulator of lignin polymer deposition.

Conduction of a chemical structure-guided metabolic phenotype analysis method targeting phenylpropane pathway via LC-MS: Ginkgo biloba and soybean as examples

The phenylpropane pathway (PPP) is one of the most extensively investigated metabolic routes. This pathway biosynthesizes many important active ingredients such as phenylpropanoids and flavonoids that affect the flavor, taste and nutrients of food. How to elucidate the metabolic phenotype of PPP is fundamental in food research and development. In this study, we designed a structural periodical table filled with 103 metabolites produced from PPP. All of them especially the 62 structural isomers were qualified and quantified with high resolution and sensitivity via multiple reaction mode in liquid chromatography tandem triple quadrupole mass spectrometry. Ginkgo biloba and soybean were used as samples for the practical application of this method: The delicate spatial-temporal metabolic balance of PPP from ginkgo biloba has been first elucidated; It is first confirmed that the salt and draught stresses could redirect the biosynthesis trend of PPP to produce more isoflavones in soybean leaves.

Transcription-associated metabolomic profiling reveals the critical role of frost tolerance in wheat

BACKGROUND

Low temperature is a crucial stress factor of wheat (Triticum aestivum L.) and adversely impacts on plant growth and grain yield. Multi-million tons of grain production are lost annually because crops lack the resistance to survive in winter. Particularlly, winter wheat yields was severely damaged under extreme cold conditions. However, studies about the transcriptional and metabolic mechanisms underlying cold stresses in wheat are limited so far.

RESULTS

In this study, 14,466 differentially expressed genes (DEGs) were obtained between wild-type and cold-sensitive mutants, of which 5278 DEGs were acquired after cold treatment. 88 differential accumulated metabolites (DAMs) were detected, including P-coumaroyl putrescine of alkaloids, D-proline betaine of mino acids and derivativ, Chlorogenic acid of the Phenolic acids. The comprehensive analysis of metabolomics and transcriptome showed that the cold resistance of wheat was closely related to 13 metabolites and 14 key enzymes in the flavonol biosynthesis pathway. The 7 enhanced energy metabolites and 8 up-regulation key enzymes were also compactly involved in the sucrose and amino acid biosynthesis pathway. Moreover, quantitative real-time PCR (qRT-PCR) revealed that twelve key genes were differentially expressed under cold, indicating that candidate genes POD, Tacr7, UGTs, and GSTU6 which were related to cold resistance of wheat.

CONCLUSIONS

In this study, we obtained the differentially expressed genes and differential accumulated metabolites in wheat under cold stress. Using the DEGs and DAMs, we plotted regulatory pathway maps of the flavonol biosynthesis pathway, sucrose and amino acid biosynthesis pathway related to cold resistance of wheat. It was found that candidate genes POD, Tacr7, UGTs and GSTU6 are related to cold resistance of wheat. This study provided valuable molecular information and new genetic engineering clues for the further study on plant resistance to cold stress.

Analyses on Flavonoids and Transcriptome Reveals Key MYB Gene for Proanthocyanidins Regulation in Onobrychis Viciifolia

Onobrychis viciifolia (sainfoin) is one of the most high-quality legume forages, which is rich in proanthocyanidins that is beneficial for the health and production of animals. In this study, proanthocyanidins and total flavonoids in leaves of 46 different sainfoin germplasm resources were evaluated, and it showed that soluble proanthocyanidin contents varied greatly in these sainfoin germplasm resources, but total flavonoids did not show significant difference. Transcriptome sequencing with high and low proanthocyanidins sainfoin resulted in the identification of totally 52,926 unigenes in sainfoin, and they were classed into different GOC categories. Among them, 1,608 unigenes were differentially expressed in high and low proanthocyanidins sainfoin samples, including 1,160 genes that were upregulated and 448 genes that were downregulated. Analysis on gene enrichment via KEGG annotation revealed that the differentially expressed genes were mainly enriched in the phenylpropanoid biosynthetic pathway and the secondary metabolism pathway. We also analyzed the expression levels of structural genes of the proanthocyanidin/flavonoid pathway in roots, stems, and leaves in the high proanthocyanidin sainfoin via RT-qPCR and found that these genes were differentially expressed in these tissues. Among them, the expression levels of F3'5'H and ANR were higher in leaves than in roots or stems, which is consistent with proanthocyanidins content in these tissues. Among MYB genes that were differentially expressed, the expression of OvMYBPA2 was relatively high in high proanthocyanidin sainfoin. Over-expression level of OvMYBPA2 in alfalfa hairy roots resulted in decreased anthocyanin content but increased proanthocyanidin content. Our study provided transcriptome information for further functional characterization of proanthocyanidin biosynthesis-related genes in sainfoin and candidate key MYB genes for bioengineering of proanthocyanidins in plants.

Characterization of the Key Bibenzyl Synthase in Dendrobium sinense

Dendrobium sinense, an endemic medicinal herb in Hainan Island, is rich in bibenzyls. However, the key rate-limited enzyme involved in bibenzyl biosynthesis has yet to be identified in D. sinense. In this study, to explore whether there is a significant difference between the D. sinense tissues, the total contents of bibenzyls were determined in roots, pseudobulbs, and leaves. The results indicated that roots had higher bibenzyl content than pseudobulbs and leaves. Subsequently, transcriptomic sequencings were conducted to excavate the genes encoding type III polyketide synthase (PKS). A total of six D. sinense PKS (DsPKS) genes were identified according to gene function annotation. Phylogenetic analysis classified the type III DsPKS genes into three groups. Importantly, the c93636.graph_c0 was clustered into bibenzyl synthase (BBS) group, named as D. sinense BBS (DsBBS). The expression analysis by FPKM and RT-qPCR indicated that DsBBS showed the highest expression levels in roots, displaying a positive correlation with bibenzyl contents in different tissues. Thus, the recombinant DsBBS-HisTag protein was constructed and expressed to study its catalytic activity. The molecular weight of the recombinant protein was verified to be approximately 45 kDa. Enzyme activity analysis indicated that the recombinant DsBBS-HisTag protein could use 4-coumaryol-CoA and malonyl-CoA as substrates for resveratrol production in vitro. The Vmax of the recombinant protein for the resveratrol production was 0.88 ± 0.07 pmol s⁻¹ mg⁻¹. These results improve our understanding with respect to the process of bibenzyl biosynthesis in D. sinense.

Comparative transcriptome and tissue-specific expression analysis of genes reveal tissue-cultured plants as an alternative source for phenylethanoids and phenylpropanoids in Rhodiola imbricata (Edgew.)

Rhodiola imbricata (Crassulaceae) is a traditional trans-Himalayan endangered medicinal herb with immense therapeutic applications. Over the years, over-exploitation, un-managed harvesting, and lack of captive cultivation procedures persuaded threat to its wild habitat. Plant tissue culture and RNA-Seq-based molecular bioprospection of key regulatory genes aid the understanding of molecular dynamics involved in specialized metabolites (phenylethanoids and phenylpropanoids) biosynthesis and its sustainable production. Hence, comparative transcriptomic analysis was performed using leaf and root tissues from the wild and tissue-cultured plants, revealing tissue-specific production of salidroside and rosavin. The transcriptome profiling resulted in 345 million high-quality reads yielding 92,380 unique transcripts with an N₅₀ of 1260 bp. Tissue-specific gene expression analysis revealed that both phenylethanoids and phenylpropanoids biosynthesis are predominantly associated with the shikimate pathway. In addition to RNA-Seq data, the downstream biosynthesis pathways genes viz., phospho-2-dehydro-3-deoxyheptonate aldolase (DAHPS), 3-dehydroquinate synthase (DHQS), shikimate kinase (SK), chorismate mutase (CM), arogenate dehydrogenase (TYRAAT), aromatic-L-amino-acid decarboxylase (TDC), phenylalanine ammonia-lyase (PAL), 4-coumarate-CoA ligase (4-CL), cinnamoyl-CoA reductase (CCR), and cinnamyl alcohol dehydrogenase (CAD) showed higher expression pattern in wild plant tissues compared to tissue-cultured plants. The transcript fold expression determined by RT-qPCR results followed similar patterns as those observed in RNA-seq and targeted metabolite profiling data. Salidroside and rosavin content in wild plants exhibited 2.40 fold and 1.77 fold increase accumulation compared to the tissue-cultured plant. The present investigation explained the tissue and condition-specific significant differences between the expression of proposed biosynthetic pathway genes and salidroside and rosavin content. Additionally, NAC, bHLH, and ARF were the most abundant transcription factor families found in the transcriptomic analysis of R. imbricata. The generated transcriptome dataset provides a valuable gene(s)/transcription factors hub that can be used for the sustainable production of salidroside and rosavin in R. imbricata under tissue culture conditions.

Molecular Mechanisms of Plant Trichome Development

Plant trichomes, protrusions formed from specialized aboveground epidermal cells, provide protection against various biotic and abiotic stresses. Trichomes can be unicellular, bicellular or multicellular, with multiple branches or no branches at all. Unicellular trichomes are generally not secretory, whereas multicellular trichomes include both secretory and non-secretory hairs. The secretory trichomes release secondary metabolites such as artemisinin, which is valuable as an antimalarial agent. Cotton trichomes, also known as cotton fibers, are an important natural product for the textile industry. In recent years, much progress has been made in unraveling the molecular mechanisms of trichome formation in Arabidopsis thaliana, Gossypium hirsutum, Oryza sativa, Cucumis sativus, Solanum lycopersicum, Nicotiana tabacum, and Artemisia annua. Here, we review current knowledge of the molecular mechanisms underlying fate determination and initiation, elongation, and maturation of unicellular, bicellular and multicellular trichomes in several representative plants. We emphasize the regulatory roles of plant hormones, transcription factors, the cell cycle and epigenetic modifications in different stages of trichome development. Finally, we identify the obstacles and key points for future research on plant trichome development, and speculated the development relationship between the salt glands of halophytes and the trichomes of non-halophytes, which provides a reference for future studying the development of plant epidermal cells.

Identification of a Novel Metabolic Target for Bioactive Triterpenoids Biosynthesis in Ganoderma lucidum

Triterpenoids are crucial active ingredients of Ganoderma lucidum (G. lucidum) with various health benefits. Yet the low biosynthesis greatly restricts their industrial applications, novel metabolic engineering strategies are needed to further enhance Ganoderma triterpenoids production. Transcription factors play vital roles in the metabolic regulation of terpenoids, which are still scarce to study in G. lucidum. Herein, a transcription factor GlbHLH5 (GenBank No. MZ436906.1) potential for metabolic regulation of Ganoderma triterpenes was identified for the first time. MeJA could increase Ganoderma triterpenoids biosynthesis, and GlbHLH5 significantly responded to MeJA induction, suggesting GlbHLH5 is a new target for Ganoderma triterpenoids overproduction. The regulatory effect of the newly identified target was further validated by homologous gene overexpression and silence in G. lucidum. It’s demonstrated that overexpression of GlbHLH5 significantly increased triterpenoids accumulation and the key enzyme genes transcription in the biosynthetic pathway, while silencing it displayed the opposite effect, indicating GlbHLH5 could positively regulate the triterpenoids biosynthesis by activating the synergistic expression of key enzyme genes in the biosynthetic pathway. Consequently, GlbHLH5 was identified as a positive regulator and novel metabolic target for Ganoderma triterpenoids biosynthesis, it sheds new lights on the regulatory effect regulation and synthetic biology of Ganoderma triterpenoids.

A plant balancing act: Meshing new and existing metabolic pathways towards an optimized system

Specialized metabolic pathways evolve from existing pathways, creating new functionality potentially boosting fitness. However, how these pathways are integrated into a pre-existing working and well-balanced metabolic system is unclear. They could be integrated to the system as a functional appendage, or they could be fully embedded into primary metabolism by establishing new biochemical and regulatory connections. A full integration into the primary metabolic system requires substantial system re-wiring and because of this complexity, the latter is often not experimentally pursued. New studies provide evidence that some specialized metabolic pathways are fully embedded in primary metabolism with extensive new regulatory and biochemical connections. This suggests, that we should consider whether other specialized metabolic pathways could be fully integrated rather than being simple appendages. In this mini review, we survey compelling evidence supporting that some specialized metabolic pathways are fully integrated and ask if these metabolites now act as de-facto primary metabolites?

Change in Secondary Metabolites and Expression Pattern of Key Rosmarinic Acid Related Genes in Iranian Lemon Balm (Melissa officinalis L.) Ecotypes Using Methyl Jasmonate Treatments

The medicinal herb, lemon balm (Melissa officinalis L.), which is high in rosmarinic acid (RA), has well-known therapeutic value. The goals of this study were to investigate the effects of methyl jasmonate (MeJA) on RA content, total phenolic content (TPC), and total flavonoid content (TFC), as well as changes in expression of their biosynthesis-related key genes (MoPAL, Mo4CL, and MoRAS) in Iranian lemon balm ecotypes, as first reported. Our results revealed that MeJA doses significantly increase the RA content, TPC, and TFC in both ecotypes compared with the control samples. Additionally, the higher expression levels of MoPAL, Mo4CL, and MoRAS following treatment were linked to RA accumulation in all treatments for both Iranian lemon balm ecotypes. After 24 h of exposure to 150 µM MeJA concentration, HPLC analysis showed that MeJA significantly increased RA content in Esfahan and Ilam ecotypes, which was about 4.18- and 7.43-fold higher than untreated plants. Our findings suggested that MeJA has a considerable influence on RA, TPC, and TFC accumulation in MeJA-treated Iranian M. officinalis, which might be the result of gene activation from the phenylpropanoid pathway. As a result of our findings, we now have a better understanding of the molecular processes behind RA production in lemon balm plants.

Genome wide characterization of R2R3 MYB transcription factor from Apocynum venetum revealed potential stress tolerance and flavonoid biosynthesis genes

MYB transcription factors are crucial in regulating stress tolerance and expression of major genes involved in flavonoid biosynthesis. The functions of MYBs is well explored in a number of plants, yet no studies is reported in Apocynum venetum. We identified a total of 163 MYB candidates, that comprised of 101 (61.96%) R2R3, 6 3R, 1 4R and 55 1R. Syntenic analysis of A. venetum R2R3 (AvMYB) showed highest orthologous pairs with Vitis vinifera MYBs followed by Arabidopsis thaliana among the four species evaluated. Thirty segmental duplications and 6 tandem duplications were obtained among AvMYB gene pairs signifying their role in the MYB gene family expansion. Nucleotide substitution analysis (Ka/Ks) showed the AvMYBs to be under the influence of strong purifying selection. Expression analysis of selected AvMYB under low temperature and cadmium stresses resulted in the identification of AvMYB48, AvMYB97, AvMYB8,AvMYB4 as potential stress responsive genes and AvMYB10 and AvMYB11 in addition, proanthocyanidin biosynthesis regulatory genes which is consistent with their annotated homologues in Arabidopsis. Tissue specific expression profile analysis of AvMYBs further supported the qPCR analysis result. MYBs with higher transcript levels in root, stem and leaf like AvMYB4 forexample, was downregulated under the stresses and such with low transcript level such as AvMYB48 which had low transcript in the leaf was upregulated under both stresses. Transcriptome and phylogenetic analysis suggested AvMYB42 as a potential regulator of anthocyanin biosynthesis. Thus, this study provided valuable information on AvR2R3-MYB gene family with respect to stress tolerance and flavonoid biosynthesis.

Histopathology of the Plasmodiophora brassicae-Chinese Cabbage Interaction in Hosts Carrying Different Sources of Resistance

Clubroot is a serious soil-borne disease of crucifers caused by the obligate parasite Plasmodiophora brassicae. The genetic basis and histopathology of clubroot resistance in two Chinese cabbage (Brassica rapa ssp. pekinensis) inbred lines Bap055 and Bap246, challenged with pathotype 4 of P. brassicae, was evaluated. The Chinese cabbage cultivar "Juxin" served as a susceptible check. The resistance in Bap055 was found to be controlled by the CRa gene, while resistance in Bap246 fit a model of control by unknown recessive gene. Infection of the roots by P. brassicae was examined by inverted microscopy. Despite their resistance, primary and secondary infection were observed to occur in Bap055 and Bap246. Primary infection was detected at 2 days post-inoculation (DPI) in "Juxin," at 4 DPI in Bap055, and at 6 DPI in Bap246. Infection occurred most quickly on "Juxin," with 60% of the root hairs infected at 10 DPI, followed by Bap055 (31% of the root hairs infected at 12 DPI) and Bap246 (20% of the root hairs infected at 14 DPI). Secondary infection of "Juxin" was first observed at 8 DPI, while in Bap055 and Bap246, secondary infection was first observed at 10 DPI. At 14 DPI, the percentage of cortical infection in "Juxin," Bap055 and Bap246 was 93.3, 20.0, and 11.1%, respectively. Although cortical infection was more widespread in Bap055 than in Bap246, secondary infection in both of these hosts was restricted relative to the susceptible check, and the vascular system remained intact. A large number of binucleate secondary plasmodia were observed in "Juxin" and the vascular system was disrupted at 16 DPI; in Bap055 and Bap246, only a few secondary plasmodia were visible, with no binucleate secondary plasmodia. The defense mechanisms and expression of resistance appears to differ between Chinese cabbage cultivars carrying different sources of resistance.

Growing a glue factory: Open questions in laticifer development

Latex-containing cells called laticifers are present in at least 41 flowering plant families and are thought to have convergently evolved at least 12 times. These cells are known to function in defense, but little is known about the molecular genetic mechanisms of their development. The expansion of laticifers into their distinctive tube shape can occur through two distinct mechanisms, cell fusion and intrusive growth. The mechanism and extent of intrusive laticifer growth are still being investigated. Hormonal regulation by jasmonic acid and ethylene is important for both laticifer differentiation and latex biosynthesis. Current evidence suggests that laticifers can be specified independently of latex production, but extensive latex production requires specified laticifers. Laticifers are an emerging system for studying the intersection of cell identity specification and specialized metabolism.

Chemical Differentiation of Plant Latexes and Their Anti-herbivory Activity against Thrips Frankliniella occidentalis

Despite the extensive studies on latex, some fundamental questions on their chemical specialization and the factors influencing this specialization have yet to be investigated. To address this issue, latexes and their bearing tissues from diverse species were profiled by ¹HNMR and GC-MS. Additionally, the antiherbivory activity of these materials was tested against thrips (Frankliniella occidentalis Pergande, 1895). The multivariate data analysis showed a clear separation between latexes and leaves from the same species. Conversely, the chemical profiles of latexes from different species were highly similar, that is, they displayed much less metabolic species-specificity. These shared chemical profiles of latexes were reflected in their overall higher mortality index (80.4% ± 7.5) against thrips compared with their bearing tissues (55.5% ± 14.9). The metabolites correlated to the antiherbivory activity of latexes were triterpenoids and steroids. However, the activity could not be attributed to any single terpenoid. This discrepancy and the reduction of the latex activity after fractionation suggested a complementary effect of the compounds when in a mixture as represented by the latex. Additionally, aqueous fractions of several latexes were found to possess simple spectra, even with only 1 metabolite. These metabolites were determined to be organic acids that might be involved in the modulation of the rate of latex coagulation, potentially increasing the sealing and trapping effects of the latex.

Sodium Hydrosulfide Induces Resistance Against Penicillium expansum in Apples by Regulating Hydrogen Peroxide and Nitric Oxide Activation of Phenylpropanoid Metabolism

As a multifunctional signaling molecule, hydrogen sulfide (H₂S) has been reported to induce plant responses to a variety of abiotic stresses. However, there are no reports on H₂S treatment inducing resistance in apples against Penicillium expansum, a biotic factor, and its possible mechanism of action. In this study, fumigating apples with 5 mM sodium hydrosulfide (NaHS), the exogenous donor of H₂S, for 12 h reduced the diameter of lesions in fruit colonized by P. expansum. NaHS treatment markedly promoted the synthesis of endogenous H₂S, hydrogen peroxide (H₂O₂), and nitrogen oxide (NO). In vivo NaHS treatment enhanced the activities of phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, p-coumarate:coenzyme A ligase isoenzymes, caffeoyl-CoA-O-methyltransferase, caffeic acid-O-methyltransferase, ferulic acid-5-hydroxylase, cinnamyl-CoA reductase, and cinnamyl-alcohol dehydrogenase. The treatment also facilitated the production of specific phenolic acids, such as cinnamic acid, p-coumaric acid, caffeic acid, ferulic acid, and sinapic acid; total phenolic compounds; p-coumaryl alcohol; coniferyl alcohol; sinapyl alcohol; and lignin. NaHS treatment induced resistance against P. expansum in apples through H₂O₂- and NO-mediated activation of phenylpropanoid metabolism.

Unraveling the roles of plant specialized metabolites: using synthetic biology to design molecular biosensors

Plants are a rich source of specialized metabolites with a broad range of bioactivities and many applications in human daily life. Over the past decades significant progress has been made in identifying many such metabolites in different plant species and in elucidating their biosynthetic pathways. However, the biological roles of plant specialized metabolites remain elusive and proposed functions lack an identified underlying molecular mechanism. Understanding the roles of specialized metabolites frequently is hampered by their dynamic production and their specific spatiotemporal accumulation within plant tissues and organs throughout a plant's life cycle. In this review, we propose the employment of strategies from the field of Synthetic Biology to construct and optimize genetically encoded biosensors that can detect individual specialized metabolites in a standardized and high-throughput manner. This will help determine the precise localization of specialized metabolites at the tissue and single-cell levels. Such information will be useful in developing complete system-level models of specialized plant metabolism, which ultimately will demonstrate how the biosynthesis of specialized metabolites is integrated with the core processes of plant growth and development.

Repressors of anthocyanin biosynthesis

Anthocyanins play a variety of adaptive roles in both vegetative tissues and reproductive organs of plants. The broad functionality of these compounds requires sophisticated regulation of the anthocyanin biosynthesis pathway to allow proper localization, timing, and optimal intensity of pigment deposition. While it is well-established that the committed steps of anthocyanin biosynthesis are activated by a highly conserved MYB-bHLH-WDR (MBW) protein complex in virtually all flowering plants, anthocyanin repression seems to be achieved by a wide variety of protein and small RNA families that function in different tissue types and in response to different developmental, environmental, and hormonal cues. In this review, we survey recent progress in the identification of anthocyanin repressors and the characterization of their molecular mechanisms. We find that these seemingly very different repression modules act through a remarkably similar logic, the so-called 'double-negative logic'. Much of the double-negative regulation of anthocyanin production involves signal-induced degradation or sequestration of the repressors from the MBW protein complex. We discuss the functional and evolutionary advantages of this logic design compared with simple or sequential positive regulation. These advantages provide a plausible explanation as to why plants have evolved so many anthocyanin repressors.

Genome-wide analysis of MYB transcription factors and their responses to salt stress in Casuarina equisetifolia

BACKGROUND

MYB transcription factors are a kind of DNA binding protein that can specifically interact with the promoter region. Members of MYB TFs are widely involved in plant growth and development, secondary metabolism, stress response, and hormone signal transduction. However, there is no report of comprehensive bioinformatics analysis on the MYB family of Casuarina equisetifolia.

RESULTS

In this study, bioinformatics methods were used to screen out 182 MYB transcription factors from the Casuarina equisetifolia genome database, including 69 1R-MYB, 107 R2R3-MYB, 4 R1R2R3-MYB, and 2 4R-MYB. The C. equisetifolia R2R3-MYB genes were divided into 29 groups based on the phylogenetic topology and the classification of the MYB superfamily in Arabidopsis thaliana, while the remaining MYB genes (1R-MYB, R1R2R3-MYB, and 4R-MYB) was divided into 19 groups. Moreover, the conserved motif and gene structure analysis shown that the members of the CeqMYBs were divided into the same subgroups with mostly similar gene structures. In addition, many conserved amino acids in the R2 and R3 domains of CeqMYBs by WebLogo analysis, especially tryptophan residues (W), with 3 conserved W in R2 repeat and 2 conserved W in R3 repeat. Combining promoter and GO annotation analysis, speculated on the various biological functions of CeqMYBs, thus 32 MYB genes were selected to further explore its response to salt stress by using qPCR analysis technique. Most CeqMYB genes were differentially regulated following multiple salt treatments.

CONCLUSIONS

Seven genes (CeqMYB164, CeqMYB4, CeqMYB53, CeqMYB32, CeqMYB114, CeqMYB71 and CeqMYB177) were assigned to the "response to salt stress" by GO annotation. Among them, the expression level of CeqMYB4 was up-regulated under various salt treatments, indicating CeqMYB4 might participated in the response to salt stress. Our results provide important information for the biological function of C. equisetifolia, as well as offer candidate genes for further study of salt stress mechanism.

Retrograde sulfur flow from glucosinolates to cysteine in Arabidopsis thaliana

Specialized (secondary) metabolites have been largely considered bioactive “end” products synthesized from primary metabolites. We report biochemical evidence of a retrograde flow of sulfur atoms from specialized metabolites (glucosinolates) to primary metabolites (cysteine) in Arabidopsis thaliana. The reaction begins with glucosinolate breakdown by specific beta-glucosidases, which facilitates sulfur deficiency tolerance, demonstrating a physiological advantage of utilizing specialized metabolites as nutrient reservoirs. Our findings address the breadth of turnover systems in nature and enhance our understanding of how plants coordinate primary and specialized metabolism under different environmental conditions.

Specialized (secondary) metabolic pathways in plants have long been considered one-way routes of leading primary metabolite precursors to bioactive end products. Conversely, endogenous degradation of such “end” products in plant tissues has been observed following environmental stimuli, including nutrition stress. Therefore, it is of general interest whether specialized metabolites can be reintegrated into primary metabolism to recover the invested resources, especially in the case of nitrogen- or sulfur-rich compounds. Here, we demonstrate that endogenous glucosinolates (GLs), a class of sulfur-rich plant metabolites, are exploited as a sulfur source by the reallocation of sulfur atoms to primary metabolites such as cysteine in Arabidopsis thaliana. Tracer experiments using ³⁴S- or deuterium-labeled GLs depicted the catabolic processing of GL breakdown products in which sulfur is mobilized from the thioglucoside group in GL molecules, potentially accompanied by the release of the sulfate group. Moreover, we reveal that beta-glucosidases BGLU28 and BGLU30 are the major myrosinases that initiate sulfur reallocation by hydrolyzing particular GL species, conferring sulfur deficiency tolerance in A. thaliana, especially during early development. The results delineate the physiological function of GL as a sulfur reservoir, in addition to their well-known functions as defense chemicals. Overall, our findings demonstrate the bidirectional interaction between primary and specialized metabolism, which enhances our understanding of the underlying metabolic mechanisms via which plants adapt to their environments.

Comparative Metabolite and Gene Expression Analyses in Combination With Gene Characterization Revealed the Patterns of Flavonoid Accumulation During Cistus creticus subsp. creticus Fruit Development

Cistus creticus L. subsp. creticus (rockrose) is a shrub widespread in Greece and the Mediterranean basin and has been used in traditional medicine as herb tea for colds, for healing and digestive hitches, for the treatment of maladies, as perfumes, and for other purposes. Compounds from its flavonoid fraction have recently drawn attention due to antiviral action against influenza virus and HIV. Although several bioactive metabolites belonging to this group have been chemically characterized in the leaves, the genes involved in their biosynthesis in Cistus remain largely unknown. Flavonoid metabolism during C. creticus fruit development was studied by adopting comparative metabolomic and transcriptomic approaches. The present study highlights the fruit of C. creticus subsp. creticus as a rich source of flavonols, flavan-3-ols, and proanthocyanidins, all of which displayed a decreasing trend during fruit development. The majority of proanthocyanidins recorded in Cistus fruit are B-type procyanidins and prodelphinidins, while gallocatechin and catechin are the dominant flavan-3-ols. The expression patterns of biosynthetic genes and transcription factors were analyzed in flowers and throughout three fruit development stages. Flavonoid biosynthetic genes were developmentally regulated, showing a decrease in transcript levels during fruit maturation. A high degree of positive correlations between the content of targeted metabolites and the expression of biosynthetic genes indicated the transcriptional regulation of flavonoid biosynthesis during C. creticus fruit development. This is further supported by the high degree of significant positive correlations between the expression of biosynthetic genes and transcription factors. The results suggest that leucoanthocyanidin reductase predominates the biosynthetic pathway in the control of flavan-3-ol formation, which results in catechin and gallocatechin as two of the major building blocks for Cistus proanthocyanidins. Additionally, there is a decline in ethylene production rates during non-climacteric Cistus fruit maturation, which coincides with the downregulation of the majority of flavonoid- and ethylene-related biosynthetic genes and corresponding transcription factors as well as with the decline in flavonoid content. Finally, functional characterization of a Cistus flavonoid hydroxylase (F3'5'H) was performed for the first time.

Reduction in organ-organ friction is critical for corolla elongation in morning glory

In complex structures such as flowers, organ-organ interactions are critical for morphogenesis. The corolla plays a central role in attracting pollinators: thus, its proper development is important in nature, agriculture, and horticulture. Although the intraorgan mechanism of corolla development has been studied, the importance of organ-organ interactions during development remains unknown. Here, using corolla mutants of morning glory described approximately 200 years ago, we show that glandular secretory trichomes (GSTs) regulate floral organ interactions needed for corolla morphogenesis. Defects in GST development in perianth organs result in folding of the corolla tube, and release of mechanical stress by sepal removal restores corolla elongation. Computational modeling shows that the folding occurs because of buckling caused by mechanical stress from friction at the distal side of the corolla. Our results suggest a novel function of GSTs in regulating the physical interaction of floral organs for macroscopic morphogenesis of the corolla.

Comparative transcriptome analysis infers bulb derived in vitro cultures as a promising source for sipeimine biosynthesis in Fritillaria cirrhosa D. Don (Liliaceae, syn. Fritillaria roylei Hook.) - High value Himalayan medicinal herb

Fritillaria cirrhosa D. Don (Liliaceae, syn. Fritillaria roylei Hook.) is a critically endangered medicinal herb of immense importance due to its pharmaceutical bioactive compound, especially sipeimine, used for the treatment of chronic respiratory disorders. However, the industrial demand for sipeimine solely depends on its endangered natural habitat. Therefore; there is an utmost need for its biodiversity conservation as well as for the sustainable utilization of phytochemicals. Plant cell culture and transcriptomics-based molecular bioprospection of key regulatory genes involved in sipeimine biosynthesis as such will play a crucial role in exploring the unexplored traits, that are in supply crisis or nearly in extinction stage. De novo comparative transcriptome sequencing of the bulb (in vivo), callus, and regenerated plantlets (in vitro) resulted in more than 150 million high-quality paired-end clean reads that assembled into final 31,428 transcripts. Functional annotation and unigenes classification with multiple public databases such as KEGG, Refseq, Uniprot, TAIR, GO, and COG, etc. along with chemical structures and functional biocatalytic activity analysis of different steroidal alkaloids facilitated the identification of 30 unigenes specific to sipeimine biosynthesis. Additionally, ABC transporters and TFs like bHLH, MYC, MYB, and WRKY suggests their possible role in metabolite translocation and regulation in vivo as well as in vitro tissues. Differential gene expression and quantitative analysis revealed that the MVA pathway probably the predominant route for 5C intermediate (IPP & DMAPP) biosynthesis. Further, the genes involved in the downstream biosynthesis pathway viz. SQLE, CAS1, SMT1, SMO1, SMO2, SC5DL, DHCR7, DHCR24, CYP710A, 3β-HSD, CYP90D2, and CYP374A6 shown similar expression pattern with RNA-Seq and qRT-PCR findings. The positive correlation between higher expression of proposed biosynthetic pathway genes and relatively higher accumulation of sipeimine in differentiated naturally grown bulb tissues (in vivo), undifferentiated cells (callus), and de-differentiated tissues i.e. regenerated plantlets (in vitro) has been evident from the present study. Comprehensive genomic resources created in F. cirrhosa will provide strong evidence of bulb derived in vitro culture as an alternative promising source for steroidal alkaloids biosynthesis and metabolite upscaling through genetic and metabolic engineering.

The Heat Shock Protein 40-Type Chaperone MASH Supports the Endoplasmic Reticulum-Associated Degradation E3 Ubiquitin Ligase MAKIBISHI1 in Medicago truncatula

Jasmonates (JA) are oxylipin-derived phytohormones that trigger the production of specialized metabolites that often serve in defense against biotic stresses. In Medicago truncatula, a JA-induced endoplasmic reticulum-associated degradation (ERAD)-type machinery manages the production of bioactive triterpenes and thereby secures correct plant metabolism, growth, and development. This machinery involves the conserved RING membrane-anchor (RMA)-type E3 ubiquitin ligase MAKIBISHI1 (MKB1). Here, we discovered two additional members of this protein control apparatus via a yeast-based protein-protein interaction screen and characterized their function. First, a cognate E2 ubiquitin-conjugating enzyme was identified that interacts with MKB1 to deliver activated ubiquitin and to mediate its ubiquitination activity. Second, we identified a heat shock protein 40 (HSP40) that interacts with MKB1 to support its activity and was therefore designated MKB1-supporting HSP40 (MASH). MASH expression was found to be co-regulated with that of MKB1. The presence of MASH is critical for MKB1 and ERAD functioning because the dramatic morphological, transcriptional, and metabolic phenotype of MKB1 knock-down M. truncatula hairy roots was phenocopied by silencing of MASH. Interaction was also observed between the Arabidopsis thaliana (Arabidopsis) homologs of MASH and MKB1, suggesting that MASH represents an essential and plant-specific component of this vital and conserved eukaryotic protein quality control machinery.