Glandular trichomes: what comes after expressed sequence tags?

Integrated multi-omics analysis reveals genes involved in flavonoid biosynthesis and trichome development of Artemisia argyi

Artemisia argyi is an herbaceous plant of the genus Artemisia. Its young and mature leaves are used as food and medicine, respectively. Glandular trichomes (GTs) are distributed on the leaf surface in A. argyi and are generally considered the location of flavonoid biosynthesis and accumulation. However, the mechanism of flavonoid biosynthesis and accumulation in A. argyi remains unclear. In this study, the coregulatory genes involved in flavonoid biosynthesis and trichome development in this species were screened and evaluated, and the biosynthetic pathways for key flavonoids in A. argyi were uncovered. AaMYB1 and AaYABBY1 were screened using weighted gene co-expression network analysis, and both genes were then genetically transformed into Nicotiana tabacum L. cv. K326 (tobacco). Simultaneously, AaYABBY1 was also genetically transformed into Arabidopsis thaliana. The total flavonoid and rutin contents were increased in tobacco plants overexpressing AaMYB1 and AaYABBY1, and the expression levels of genes participating in the flavonoid synthesis pathway, such as PAL, FLS, and F3H, were significantly up-regulated in plants overexpressing these genes. These results indicated that AaMYB1 and AaYABBY1 promote flavonoid biosynthesis in tobacco. Furthermore, compared to that in the wild-type, the trichome density was significantly increased in tobacco and A. thaliana plants overexpressing AaYABBY1. These results confirm that AaYABBY1 might be involved in regulating trichome formation in A. argyi. This indicates the potential genes involved in and provides new insights into the development of trichome cellular factories based on the "development-metabolism" interaction network and the cultivation of high-quality A. argyi.

The Chemical Landscape of Leaf Surfaces and Its Interaction with the Atmosphere

Atmospheric chemists have historically treated leaves as inert surfaces that merely emit volatile hydrocarbons. However, a growing body of evidence suggests that leaves are ubiquitous substrates for multiphase reactions-implying the presence of chemicals on their surfaces. This Review provides an overview of the chemistry and reactivity of the leaf surface's "chemical landscape", the dynamic ensemble of compounds covering plant leaves. We classified chemicals as endogenous (originating from the plant and its biome) or exogenous (delivered from the environment), highlighting the biological, geographical, and meteorological factors driving their contributions. Based on available data, we predicted ≫2 μg cm-2 of organics on a typical leaf, leading to a global estimate of ≫3 Tg for multiphase reactions. Our work also highlighted three major knowledge gaps: (i) the overlooked role of ambient water in enabling the leaching of endogenous substances and mediating aqueous chemistry; (ii) the importance of phyllosphere biofilms in shaping leaf surface chemistry and reactivity; (iii) the paucity of studies on the multiphase reactivity of atmospheric oxidants with leaf-adsorbed chemicals. Although biased toward available data, we hope this Review will spark a renewed interest in the leaf surface's chemical landscape and encourage multidisciplinary collaborations to move the field forward.

The multiple roles of trichomes in two Croton species

Trichomes are common in plants from dry environments, and despite their recognized role in protection and defense, little is known about their role as absorptive structures and in other aspects of leaf ecophysiology. We combine anatomical and ecophysiological data to evaluate how trichomes affect leaf gas exchange and water balance during drought. We studied two congeneric species with pubescent leaves which co-occur in Brazilian Caatinga: Croton blanchetianus (dense trichomes) and Croton adenocalyx (sparse trichomes). We found a novel foliar water uptake (FWU) pathway in C. blanchetianus composed of stellate trichomes and underlying epidermal cells and sclereids that interconnect the trichomes from both leaf surfaces. The water absorbed by these trichomes is redistributed laterally by pectin protuberances on mesophyll cell walls. This mechanism enables C. blanchetianus leaves to absorb water more efficiently than C. adenocalyx. Consequently, the exposure of C. blanchetianus to dew during drought improved its leaf gas exchange and water status more than C. adenocalyx. C. blanchetianus trichomes also increase their leaf capacity to reflect light and maintain lower temperatures during drought. Our results emphasize the multiple roles that trichomes might have on plant functioning and the importance of FWU for the ecophysiology of Caatinga plants during drought.

Bioprospecting of Artemisia genus: from artemisinin to other potentially bioactive compounds

Species from genus Artemisia are widely distributed throughout temperate regions of the northern hemisphere and many cultures have a long-standing traditional use of these plants as herbal remedies, liquors, cosmetics, spices, etc. Nowadays, the discovery of new plant-derived products to be used as food supplements or drugs has been pushed by the exploitation of bioprospection approaches. Often driven by the knowledge derived from the ethnobotanical use of plants, bioprospection explores the existing biodiversity through integration of modern omics techniques with targeted bioactivity assays. In this work we set up a bioprospection plan to investigate the phytochemical diversity and the potential bioactivity of five Artemisia species with recognized ethnobotanical tradition (A. absinthium, A. alba, A. annua, A. verlotiorum and A. vulgaris), growing wild in the natural areas of the Verona province. We characterized the specialized metabolomes of the species (including sesquiterpenoids from the artemisinin biosynthesis pathway) through an LC-MS based untargeted approach and, in order to identify potential bioactive metabolites, we correlated their composition with the in vitro antioxidant activity. We propose as potential bioactive compounds several isomers of caffeoyl and feruloyl quinic acid esters (e.g. dicaffeoylquinic acids, feruloylquinic acids and caffeoylferuloylquinic acids), which strongly characterize the most antioxidant species A. verlotiorum and A. annua. Morevoer, in this study we report for the first time the occurrence of sesquiterpenoids from the artemisinin biosynthesis pathway in the species A. alba.

Cannabis sativa: origin and history, glandular trichome development, and cannabinoid biosynthesis

Is Cannabis a boon or bane? Cannabis sativa has long been a versatile crop for fiber extraction (industrial hemp), traditional Chinese medicine (hemp seeds), and recreational drugs (marijuana). Cannabis faced global prohibition in the twentieth century because of the psychoactive properties of ∆9-tetrahydrocannabinol; however, recently, the perspective has changed with the recognition of additional therapeutic values, particularly the pharmacological potential of cannabidiol. A comprehensive understanding of the underlying mechanism of cannabinoid biosynthesis is necessary to cultivate and promote globally the medicinal application of Cannabis resources. Here, we comprehensively review the historical usage of Cannabis, biosynthesis of trichome-specific cannabinoids, regulatory network of trichome development, and synthetic biology of cannabinoids. This review provides valuable insights into the efficient biosynthesis and green production of cannabinoids, and the development and utilization of novel Cannabis varieties.

Humic Substances Isolated from Recycled Biomass Trigger Jasmonic Acid Biosynthesis and Signalling

Intensive agriculture maintains high crop yields through chemical inputs, which are well known for their adverse effects on environmental quality and human health. Innovative technologies are required to reduce the risk generated by the extensive and harmful use of pesticides. The plant biostimulants made from humic substances isolated from recyclable biomass offer an alternative approach to address the need for replacing conventional agrochemicals without compromising the crop yield. The stimulatory effects of humic substances are commonly associated with plant hormones, particularly auxins. However, jasmonic acid (JA) is crucial metabolite in mediating the defence responses and governing plant growth and development. This work aimed to evaluate the changes in the biosynthesis and signalling pathway of JA in tomato seedlings treated with humic acids (HA) isolated from vermicompost. We use the tomato model system cultivar Micro-Tom (MT) harbouring a reporter gene fused to a synthetic promoter that responds to jasmonic acid (JERE::GUS). The transcript levels of genes involved in JA generation and activity were also determined using qRT-PCR. The application of HA promoted plant growth and altered the JA status, as revealed by both GUS and qRT-PCR assays. Both JA enzymatic synthesis (LOX, OPR3) and JA signalling genes (JAZ and JAR) were found in higher transcription levels in plants treated with HA. In addition, ethylene (ETR4) and auxin (ARF6) signalling components were positively modulated by HA, revealing a hormonal cross-talk. Our results prove that the plant defence system linked to JA can be emulated by HA application without growth inhibition.

Plant resistance against whitefly and its engineering

Plants face constant threats from insect herbivores, which limit plant distribution and abundance in nature and crop productivity in agricultural ecosystems. In recent decades, the whitefly Bemisia tabaci, a group of phloem-feeding insects, has emerged as pests of global significance. In this article, we summarize current knowledge on plant defenses against whitefly and approaches to engineer plant resistance to whitefly. Physically, plants deploy trichome and acylsugar-based strategies to restrain nutrient extraction by whitefly. Chemically, toxic secondary metabolites such as terpenoids confer resistance against whitefly in plants. Moreover, the jasmonate (JA) signaling pathway seems to be the major regulator of whitefly resistance in many plants. We next review advances in interfering with whitefly-plant interface by engineering of plant resistance using conventional and biotechnology-based breeding. These breeding programs have yielded many plant lines with high resistance against whitefly, which hold promises for whitefly control in the field. Finally, we conclude with an outlook on several issues of particular relevance to the nature and engineering of plant resistance against whitefly.

The Foliar Anatomy and Micromorphology of Cyphostemma hypoleucum (Vitaceae)

Cyphostemma hypoleucum (Harv.) Desc. ex Wild & R.B. Drumm is a perennial climber, indigenous to Southern Africa, and belongs to the Vitaceae. Although there have been many studies of Vitaceae micromorphology, only a few taxa have been described in detail. This study aimed to characterize the micro-morphology of the leaf indumentum and determining its possible functions. Stereo microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to produce images. Micrographs of stereomicroscopy and SEM showed the presence of non-glandular trichomes. In addition, pearl glands were observed on the abaxial surface using a stereo microscope and SEM. These were characterized by a short stalk and a spherical- shaped head. The density of trichomes decreased on both surfaces of leaves as the leaf expanded. Idioblasts that contained raphide crystals were also detected in tissues. The results obtained from various microscopy techniques confirmed that non-glandular trichomes serve as the main external appendages of the leaves. Additionally, their functions may include serving as a mechanical barrier against environmental factors such as low humidity, intense light, elevated temperatures, as well as herbivory and insect oviposition. Our results may also be added to the existing body of knowledge with regard to microscopic research and taxonomic applications.

Development and Biomechanics of Grewia lasiocarpa E. Mey. Ex Harv. Trichomes Exudate

Grewia lasiocarpa E. Mey. Ex Harv., Malvaceae (forest raisin) is a tropical small tree or shrub valued for its ecological importance as well as its nutritional, antioxidant, antibacterial, and anti-cancer properties as well as its ecological and ornamental importance. Glandular and non-glandular trichomes are present on the fruits, stem bark and leaves of G. lasiocarpa and these trichomes are the first line of defense. They are important structures that plants use to combat biotic and abiotic stress. The development of G. lasiocarpa trichomes and the biomechanics of the exudates present in the glandular (capitate) trichome were investigated for the first time using advanced microscopy techniques [Scanning electron microscope (SEM) and Transmission electron microscope (TEM)]. The pressurized cuticular striations may play a role in the exudates' biomechanics, i.e., releasing secondary metabolites present in the capitate trichome, which was observed to be multidirectional. The presence of many glandular trichomes on a plant implies an increase in the amount of phytometabolites. A common precursor for the development of trichomes (non-glandular and glandular) was observed to be DNA synthesis associated with a periclinal cell division, thus the final fate of the cell is determined by cell cycle regulation, polarity, and expansion. The glandular trichomes of G. lasiocarpa are multicellular and polyglandular, while the non-glandular (glandless) trichomes are either single-celled or multicellular. Since, trichomes 'house' phytocompounds of medicinal, nutritional, and agronomical benefits; the molecular and genetic study of the glandular trichomes of Grewia lasiocarpa will be beneficial to humanity.

AaSEPALLATA1 integrates jasmonate and light-regulated glandular secretory trichome initiation in Artemisia annua

Glandular secretory trichomes (GSTs) can secrete and store a variety of specific metabolites. By increasing GST density, valuable metabolites can be enhanced in terms of productivity. However, the comprehensive and detailed regulatory network of GST initiation still needs further investigation. By screening a complementary DNA library derived from young leaves of Artemisia annua, we identified a MADS-box transcription factor, AaSEPALLATA1 (AaSEP1), that positively regulates GST initiation. Overexpression of AaSEP1 in A. annua substantially increased GST density and artemisinin content. The HOMEODOMAIN PROTEIN 1 (AaHD1)-AaMYB16 regulatory network regulates GST initiation via the jasmonate (JA) signaling pathway. In this study, AaSEP1 enhanced the function of AaHD1 activation on downstream GST initiation gene GLANDULAR TRICHOME-SPECIFIC WRKY 2 (AaGSW2) through interaction with AaMYB16. Moreover, AaSEP1 interacted with the JA ZIM-domain 8 (AaJAZ8) and served as an important factor in JA-mediated GST initiation. We also found that AaSEP1 interacted with CONSTITUTIVE PHOTOMORPHOGENIC 1 (AaCOP1), a major repressor of light signaling. In this study, we identified a MADS-box transcription factor that is induced by JA and light signaling and that promotes the initiation of GST in A. annua.

Integrative analysis of metabolite and transcriptome reveals biosynthetic pathway and candidate genes for eupatilin and jaceosidin biosynthesis in Artemisia argyi

Artemisia argyi (A. argyi) is a medicinal plant belonging to the Asteraceae family and Artemisia genus. Flavonoids abundant in A. argyi are associated with anti-inflammatory, anticancer, and antioxidative effects. Eupatilin and jaceosidin are representative polymethoxy flavonoids with medicinal properties significant enough to warrant the development of drugs using their components. However, the biosynthetic pathways and related genes of these compounds have not been fully explored in A. argyi. This study comprehensively analyzed the transcriptome data and flavonoids contents from four different tissues of A. argyi (young leaves, old leaves, trichomes collected from stems, and stems without trichomes) for the first time. We obtained 41,398 unigenes through the de-novo assembly of transcriptome data and mined promising candidate genes involved in the biosynthesis of eupatilin and jaceosidin using differentially expressed genes, hierarchical clustering, phylogenetic tree, and weighted gene co-expression analysis. Our analysis led to the identification of a total of 7,265 DEGs, among which 153 genes were annotated as flavonoid-related genes. In particular, we were able to identify eight putative flavone-6-hydroxylase (F6H) genes, which were responsible for providing a methyl group acceptor into flavone basic skeleton. Furthermore, five O-methyltransferases (OMTs) gene were identified, which were required for the site-specific O-methylation during the biosynthesis of eupatilin and jaceosidin. Although further validation would be necessary, our findings pave the way for the modification and mass-production of pharmacologically important polymethoxy flavonoids through genetic engineering and synthetic biological approaches.

Vernonia polyanthes Less. (Asteraceae Bercht. & Presl), a Natural Source of Bioactive Compounds with Antibiotic Effect against Multidrug-Resistant Staphylococcus aureus

Vernonia polyanthes is a medicinal plant used to treat many disorders, including infectious diseases. This study investigated the chemical constituents and the antibacterial activity of V. polyanthes leaf rinse extract (Vp-LRE). The chemical characterization of Vp-LRE was established using ultra-high performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UHPLC/Q-TOF-MS), and glaucolide A was identified through ¹H and ¹³C nuclear magnetic resonance (NMR) and mass fragmentation. The cytotoxicity was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). The antibacterial activity was assessed by minimal inhibitory concentration and minimal bactericidal concentration. Interactions between ligands and beta-lactamase were evaluated via molecular docking. UHPLC/Q-TOF-MS detected acacetin, apigenin, chrysoeriol, isorhamnetin, isorhamnetin isomer, kaempferide, 3',4'-dimethoxyluteolin, 3,7-dimethoxy-5,3',4'-trihydroxyflavone, piptocarphin A and glaucolide A. Vp-LRE (30 µg/mL) and glaucolide A (10 and 20 μg/mL) were cytotoxic against RAW 264.7 cells. Glaucolide A was not active, but Vp-LRE inhibited the Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), Escherichia coli, Salmonella Choleraesuis and Typhimurium, with a bacteriostatic effect. The compounds (glaucolide A, 3',4'-dimethoxyluteolin, acacetin and apigenin) were able to interact with beta-lactamase, mainly through hydrogen bonding, with free energy between -6.2 to -7.5 kcal/mol. These results indicate that V. polyanthes is a potential natural source of phytochemicals with a significant antibiotic effect against MRSA strains.

A HD-ZIP transcription factor specifies fates of multicellular trichomes via dosage-dependent mechanisms in tomato

Hair-like structures are shared by most living organisms. The hairs on plant surfaces, commonly referred to as trichomes, form diverse types to sense and protect against various stresses. However, it is unclear how trichomes differentiate into highly variable forms. Here, we show that a homeodomain leucine zipper (HD-ZIP) transcription factor named Woolly controls the fates of distinct trichomes in tomato via a dosage-dependent mechanism. The autocatalytic reinforcement of Woolly is counteracted by an autoregulatory negative feedback loop, creating a circuit with a high or low Woolly level. This biases the transcriptional activation of separate antagonistic cascades that lead to different trichome types. Our results identify the developmental switch of trichome formation and provide mechanistic insights into the progressive fate specification in plants, as well as a path to enhancing plant stress resistance and the production of beneficial chemicals.

Chromosome-level assembly and analysis of the Thymus genome provide insights into glandular secretory trichome formation and monoterpenoid biosynthesis in thyme

Thyme has medicinal and aromatic value because of its potent antimicrobial and antioxidant properties. However, the absence of a fully sequenced thyme genome limits functional genomic studies of Chinese native thymes. Thymus quinquecostatus Čelak., which contains large amounts of bioactive monoterpenes such as thymol and carvacrol, is an important wild medicinal and aromatic plant in China. Monoterpenoids are abundant in glandular secretory trichomes. Here, high-fidelity and chromatin conformation capture technologies were used to assemble and annotate the T. quinquecostatus genome at the chromosome level. The 13 chromosomes of T. quinquecostatus had a total length of 528.66 Mb, a contig N50 of 8.06 Mb, and a BUSCO score of 97.34%. We found that T. quinquecostatus had experienced two whole-genome duplications, with the most recent event occurring ∼4.34 million years ago. Deep analyses of the genome, in conjunction with comparative genomic, phylogenetic, transcriptomic, and metabonomic studies, uncovered many regulatory factors and genes related to monoterpenoids and glandular secretory trichome development. Genes encoding terpene synthase (TPS), cytochrome P450 monooxygenases (CYPs), short-chain dehydrogenase/reductase (SDR), R2R3-MYB, and homeodomain-leucine zipper (HD-ZIP) IV were among those present in the T. quinquecostatus genome. Notably, Tq02G002290.1 (TqTPS1) was shown to encode the terpene synthase responsible for catalyzing production of the main monoterpene product γ-terpinene from geranyl diphosphate (GPP). Our study provides significant insight into the mechanisms of glandular secretory trichome formation and monoterpenoid biosynthesis in thyme. This work will facilitate the development of molecular breeding tools to enhance the production of bioactive secondary metabolites in Lamiaceae.

Single-cell transcriptome of Nepeta tenuifolia leaves reveal differentiation trajectories in glandular trichomes

The peltate glandular trichomes (PGTs) on Nepeta tenuifolia leaves can secrete and store bioactive essential oils. ScRNA-seq is a powerful tool for uncovering heterogeneous cells and exploring the development and differentiation of specific cells. Due to leaves rich in PGTs, the young leaves were used to isolated protoplasts and successfully captured 33,254 protoplasts for sequencing purposes. After cell type annotation, all the cells were partitioned into six broad populations with 19 clusters. Cells from PGTs were identified based on the expression patterns of trichome-specific genes, monoterpene biosynthetic genes, and metabolic analysis of PGT secretions. The developmental trajectories of PGTs were delineated by pseudotime analysis. Integrative analysis of scRNA-seq data from N. tenuifolia leaves and Arabidopsis thaliana shoot revealed that PGTs were specific to N. tenuifolia. Thus, our results provide a promising basis for exploring cell development and differentiation in plants, especially glandular trichome initiation and development.

Cloning, characterization and functional analysis of NtMYB306a gene reveals its role in wax alkane biosynthesis of tobacco trichomes and stress tolerance

Trichomes are specialized hair-like organs found on epidermal cells of many terrestrial plants, which protect plant from excessive transpiration and numerous abiotic and biotic stresses. However, the genetic basis and underlying mechanisms are largely unknown in Nicotiana tabacum (common tobacco), an established model system for genetic engineering and plant breeding. In present study, we identified, cloned and characterized an unknown function transcription factor NtMYB306a from tobacco cultivar K326 trichomes. Results obtained from sequence phylogenetic tree analysis showed that NtMYB306a-encoded protein belonged to S1 subgroup of the plants' R2R3-MYB transcription factors (TFs). Observation of the green fluorescent signals from NtMYB306a-GFP fusion protein construct exhibited that NtMYB306a was localized in nucleus. In yeast transactivation assays, the transformed yeast containing pGBKT7-NtMYB306a construct was able to grow on SD/-Trp-Ade+X-α-gal selection media, signifying that NtMYB306a exhibits transcriptional activation activity. Results from qRT-PCR, in-situ hybridization and GUS staining of transgenic tobacco plants revealed that NtMYB306a is primarily expressed in tobacco trichomes, especially tall glandular trichomes (TGTs) and short glandular trichomes (SGTs). RNA sequencing (RNA-seq) and qRT-PCR analysis of the NtMYB306a-overexpressing transgenic tobacco line revealed that NtMYB306a activated the expression of a set of key target genes which were associated with wax alkane biosynthesis. Gas Chromatography-Mass Spectrometry (GC-MS) exhibited that the total alkane contents and the contents of n-C28, n-C29, n-C31, and ai-C31 alkanes in leaf exudates of NtMYB306a-OE lines (OE-3, OE-13, and OE-20) were significantly greater when compared to WT. Besides, the promoter region of NtMYB306a contained numerous stress-responsive cis-acting elements, and their differential expression towards salicylic acid and cold stress treatments reflected their roles in signal transduction and cold-stress tolerance. Together, these results suggest that NtMYB306a is necessarily a positive regulator of alkane metabolism in tobacco trichomes that does not affect the number and morphology of tobacco trichomes, and that it can be used as a candidate gene for improving stress resistance and the quality of tobacco.

Integrative omics approaches for biosynthetic pathway discovery in plants

Covering: up to 2022With the emergence of large amounts of omics data, computational approaches for the identification of plant natural product biosynthetic pathways and their genetic regulation have become increasingly important. While genomes provide clues regarding functional associations between genes based on gene clustering, metabolome mining provides a foundational technology to chart natural product structural diversity in plants, and transcriptomics has been successfully used to identify new members of their biosynthetic pathways based on coexpression. Thus far, most approaches utilizing transcriptomics and metabolomics have been targeted towards specific pathways and use one type of omics data at a time. Recent technological advances now provide new opportunities for integration of multiple omics types and untargeted pathway discovery. Here, we review advances in plant biosynthetic pathway discovery using genomics, transcriptomics, and metabolomics, as well as recent efforts towards omics integration. We highlight how transcriptomics and metabolomics provide complementary information to link genes to metabolites, by associating temporal and spatial gene expression levels with metabolite abundance levels across samples, and by matching mass-spectral features to enzyme families. Furthermore, we suggest that elucidation of gene regulatory networks using time-series data may prove useful for efforts to unwire the complexities of biosynthetic pathway components based on regulatory interactions and events.

A high-efficiency trichome collection system by laser capture microdissection

Trichomes, which are classified as glandular or non-glandular, are hair-like epidermal structures that are present on aerial parts of most plant species. Glandular secretory trichomes (GSTs) have the capacity to secrete and store specialized metabolites, which are widely used as natural pesticides, food additives, fragrance ingredients or pharmaceuticals. Isolating individual trichomes is an essential way for identifying trichome-specific gene functions and discovering novel metabolites. However, the isolation of trichomes is difficult and time-consuming. Here, we report a method to isolate the GSTs from leaf epidermis dispense with fixation using laser capture microdissection (LCM). In this study, 150 GSTs were captured efficiently from Artemisia annua leaves and enriched for artemisinin measurement. UPLC analysis of microdissected samples indicated specific accumulation of secondary metabolites could be detected from a small number of GSTs. In addition, qRT-PCR revealed that the GST-specific structural genes involved in artemisinin biosynthesis pathway were highly expressed in GSTs. Taken together, we developed an efficient method to collect comparatively pure GSTs from unfixed leaved, so that the metabolites were relatively obtained intact. This method can be implemented in metabolomics research of purely specific plant cell populations and has the potential to discover novel secondary metabolites.

Exogenous application of methyl jasmonate affects the emissions of volatile compounds in lavender (Lavandula angustifolia)

The plant hormone, methyl jasmonate (MeJA), is an orthodox elicitor of secondary metabolites, including terpenoids. Lavandula angustifolia is an important aromatic plant generating, yet few studies have been performed to evaluate the function of MeJA on the biosynthesis of terpenoids in lavender. Five treatments (with concentrations of 0, 0.4, 4, 8, and 16 mM) were set, and the physiological indicators of each group were determined after 0, 6, 12, 24, 48, and 72 h. The results illustrate that (1) MeJA could affect the diurnal rhythm of the emission of volatiles and MeJA acted in a dose-dependent and time-dependent manner; (2) 8 mM MeJA treatment increased the total content of the volatiles, and the contents of monoterpenoids and sesquiterpenoids were up-regulated 0.46- and 0.74- fold than the control at 24 h and 12 h, respectively; (3) after MeJA treatment, all the genes expression analyzed changed to varying degrees, of which 3-carene synthase (La3CARS) gene changed most significantly (7.66- to 38.02- fold than the control); (4) MeJA application was associated with a rise in glandular trichome density. The positive effects of MeJA indicate that the exogenous application of MeJA could be a beneficial mean for studies on the biosynthesis of terpenoids in lavender.

Identification and characterization of a novel gene involved in glandular trichome development in Nepeta tenuifolia

Nepeta tenuifolia is a medicinal plant rich in terpenoids and flavonoids with antiviral, immunoregulatory, and anti-inflammatory activities. The peltate glandular trichome (PGT) is a multicellular structure considered to be the primary storage organ for monoterpenes; it may serve as an ideal model for studying cell differentiation and the development of glandular trichomes (GTs). The genes that regulate the development of GTs have not yet been well studied. In this study, we identified NtMIXTA1, a GT development-associated gene from the R2R3 MYB SBG9 family. NtMIXTA1 overexpression in tobacco resulted in the production of longer and denser GTs. Virus-induced gene silencing of NtMIXTA1 resulted in lower PGT density, a significant reduction in monoterpene concentration, and the decreased expression of genes related to monoterpene biosynthesis. Comparative transcriptome and widely targeted metabolic analyses revealed that silencing NtMIXTA1 significantly influenced the expression of genes, and the production of metabolites involved in the biosynthesis of terpenoids, flavonoids, and lipids. This study provides a solid foundation describing a mechanism underlying the regulation of GT development. In addition, this study further deepens our understanding of the regulatory networks involved in GT development and GT development-associated metabolite flux, as well as provides valuable reference data for studying plants with a high medicinal value without genetic transformation.

Identification of Gene Biomarkers for Tigilanol Tiglate Content in Fontainea picrosperma

Tigilanol tiglate (EBC-46) is a small-molecule natural product under development for the treatment of cancers in humans and companion animals. The drug is currently produced by purification from the Australian rainforest tree Fontainea picrosperma (Euphorbiaceae). As part of a selective-breeding program to increase EBC-46 yield from F. picrosperma plantations, we investigated potential gene biomarkers associated with biosynthesis of EBC-46. Initially, we identified individual plants that were either high (>0.039%) or low EBC-46 (<0.008%) producers, then assessed their differentially expressed genes within the leaves and roots of these two groups by quantitative RNA sequencing. Compared to low EBC-46 producers, high-EBC-46-producing plants were found to have 145 upregulated genes and 101 downregulated genes in leaves and 53 upregulated genes and 82 downregulated genes in roots. Most of these genes were functionally associated with defence, transport, and biosynthesis. Genes identified as expressed exclusively in either the high or low EBC-46-producing plants were further validated by quantitative PCR, showing that cytochrome P450 94C1 in leaves and early response dehydration 7.1 and 2-alkenal reductase in roots were consistently and significantly upregulated in high-EBC-46 producers. In summary, this study has identified biomarker genes that may be used in the selective breeding of F. picrosperma.

Exploring the Relationship between Trichome and Terpene Chemistry in Chrysanthemum

Chrysanthemum is a popular ornamental plant with a long history of cultivation. Both the leaf and flowerhead of Chrysanthemum are known to produce diverse secondary metabolites, particularly terpenoids. Here we aimed to determine the relationship between terpene chemistry and the trichome traits in Chrysanthemum. In our examination of three cultivars of C. morifilium and three accessions of C. indicum, all plants contained T-shaped trichomes and biseriate peltate glandular trichomes. The biseriate peltate glandular trichome contained two basal cells, two stalk cells, six secondary cells and a subcuticular space, while the non-glandular T-shaped trichome was only composed of stalk cells and elongated cells. Histochemical staining analysis indicated that the biseriate peltate glandular trichome contained terpenoids and lipid oil droplets but not the T-shaped trichome. Next, experiments were performed to determine the relationship between the accumulation and emission of the volatile terpenoids and the density of trichomes on the leaves and flowerheads in all six Chrysanthemum cultivars\accessions. A significant correlation was identified between the monoterpenoid and sesquiterpenoid content and the density of glandular trichomes on the leaves, with the correlation coefficients being 0.88, 0.86 and 0.90, respectively. In contrast, there was no significant correlation between the volatile terpenoid content and the density of T-shaped trichomes on the leaves. In flowerheads, a significant correlation was identified between the emission rate of terpenoids and the number of glandular trichomes on the disc florets, with a correlation coefficient of 0.95. Interestingly, the correlation between the density of glandular trichomes and concentrations of terpenoids was insignificant. In summary, the relationship between trichomes and terpenoid chemistry in Chrysanthemum is clearly established. Such knowledge may be helpful for breeding aromatic Chrysanthemum cultivars by modulating the trichome trait.

The Genetic Complexity of Type-IV Trichome Development Reveals the Steps towards an Insect-Resistant Tomato

The leaves of the wild tomato Solanum galapagense harbor type-IV glandular trichomes (GT) that produce high levels of acylsugars (AS), conferring insect resistance. Conversely, domesticated tomatoes (S. lycopersicum) lack type-IV trichomes on the leaves of mature plants, preventing high AS production, thus rendering the plants more vulnerable to insect predation. We hypothesized that cultivated tomatoes engineered to harbor type-IV trichomes on the leaves of adult plants could be insect-resistant. We introgressed the genetic determinants controlling type-IV trichome development from S. galapagense into cv. Micro-Tom (MT) and created a line named “Galapagos-enhanced trichomes” (MT-Get). Mapping-by-sequencing revealed that five chromosomal regions of S. galapagense were present in MT-Get. Further genetic mapping showed that S. galapagense alleles in chromosomes 1, 2, and 3 were sufficient for the presence of type-IV trichomes on adult organs but at lower densities. Metabolic and gene expression analyses demonstrated that type-IV trichome density was not accompanied by the AS production and exudation in MT-Get. Although the plants produce a significant amount of acylsugars, those are still not enough to make them resistant to whiteflies. We demonstrate that type-IV glandular trichome development is insufficient for high AS accumulation. The results from our study provided additional insights into the steps necessary for breeding an insect-resistant tomato.

Characterisation of the Mentha canadensis R2R3-MYB transcription factor gene McMIXTA and its involvement in peltate glandular trichome development

BACKGROUND

Mentha canadensis L. has important economic value for the production of essential oils, which are synthesised, secreted and stored in peltate glandular trichomes. As a typical multicellular secretory trichome, glandular trichomes are important biological factories for the synthesis of some specialised metabolites. However, little is known about the molecular mechanism of glandular trichome development in M. canadensis.

RESULTS

In this study, the R2R3-MYB transcription factor gene McMIXTA was isolated to investigate its function in glandular trichome development. Bioinformatics analysis indicated that McMIXTA belonged to the subgroup 9 R2R3-MYB, with a R2R3 DNA-binding domain and conserved subgroup 9 motifs. A subcellular localisation assay indicated that McMIXTA was localised in the nucleus. Transactivation analysis indicated that McMIXTA was a positive regulator, with transactivation regions located between positions N253 and N307. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that McMIXTA formed a complex with McHD-Zip3, a trichome development-related HD-ZIP IV transcription factor. Overexpression of McMIXTA in Mentha × piperita L. caused an increase in peltate glandular trichomes density of approximately 25% on the leaf abaxial surface.

CONCLUSIONS

Our results demonstrated that the subgroup 9 R2R3-MYB transcription factor McMIXTA has a positive effect on regulating peltate glandular trichome development and the MIXTA/HD-ZIP IV complexes might be conserved regulators for glandular trichome initiation. These results provide useful information for revealing the regulatory mechanism of multicellular glandular trichome development.

Morphogenesis, ultrastructure, and chemical profiling of trichomes in Artemisia argyi H. Lév. & Vaniot (Asteraceae)

Glandular trichomes of Artemisia argyi H. Lév. & Vaniot are the key tissues for the production of flavonoid and terpenoid metabolites. Artemisia argyi H. Lév. & Vaniot is an herbaceous perennial plant that has been widely used in traditional medicine for thousands of years. Glandular trichomes (GTs) and nonglandular trichomes (NGTs) have been reported on the leaf surface of A. argyi. The aim of this study was to elucidate the morphogenetic process and to analyze the metabolites of trichomes in A. argyi. The morphogenesis of GTs and NGTs was characterized using light, scanning, and transmission electron microscopy. The constituents of GTs were analyzed using laser microdissection combined with gas and liquid chromatography-mass spectrometry. Five developmental stages of two types of GTs and four developmental stages of one type of NGTs were observed. Two types of mature GT and one type of NGT were composed of 10, 5, and 4-6 cells, respectively. A large storage cavity was detected between the cuticle and cell walls in the first type of mature GT. Large nuclei, nucleoli, and mitochondria were observed in the basal and intermediate cells of the second type of GT. In addition, large vacuoles were observed in the basal and apical cells, and large nuclei were observed in the middle cells of NGTs. One monoterpene and seven flavonoids were identified in GTs of A. argyi. We suggest that GTs are the key tissues for the production of bioactive metabolites in A. argyi. This study provides an important theoretical basis and technical approach for clarifying the regulatory mechanisms for trichome development and bioactive metabolite biosynthesis in A. argyi.

Engineering of tomato type VI glandular trichomes for trans-chrysanthemic acid biosynthesis, the acid moiety of natural pyrethrin insecticides

Glandular trichomes, known as metabolic cell factories, have been proposed as highly suitable for metabolically engineering the production of plant high-value specialized metabolites. Natural pyrethrins, found only in Dalmatian pyrethrum (Tanacetum cinerariifolium), are insecticides with low mammalian toxicity and short environmental persistence. Type I pyrethrins are esters of the monoterpenoid trans-chrysanthemic acid with one of the three rethrolone-type alcohols. To test if glandular trichomes can be made to synthesize trans-chrysanthemic acid, we reconstructed its biosynthetic pathway in tomato type VI glandular trichomes, which produce large amounts of terpenoids that share the precursor dimethylallyl diphosphate (DMAPP) with this acid. This was achieved by coexpressing the trans-chrysanthemic acid pathway related genes including TcCDS encoding chrysanthemyl diphosphate synthase and the fusion gene of TcADH2 encoding the alcohol dehydrogenase 2 linked with TcALDH1 encoding the aldehyde dehydrogenase 1 under the control of a newly identified type VI glandular trichome-specific metallocarboxypeptidase inhibitor promoter. Whole tomato leaves harboring type VI glandular trichomes expressing all three aformentioned genes had a concentration of total trans-chrysanthemic acid that was about 1.5-fold higher (by mole number) than the levels of β-phellandrene, the dominant monoterpene present in non-transgenic leaves, while the levels of β-phellandrene and the representative sesquiterpene β-caryophyllene in transgenic leaves were reduced by 96% and 81%, respectively. These results suggest that the tomato type VI glandular trichome is an alternative platform for the biosynthesis of trans-chrysanthemic acid by metabolic engineering.

Subtle interplay between trichome development and cuticle formation in plants

Trichomes and cuticles are key protective epidermal specializations. This review highlights the genetic interplay existing between trichome and cuticle formation in a variety of species. Controlling trichome development, the biosynthesis of trichome-derived specialized metabolites as well as cuticle biosynthesis and deposition can be viewed as different aspects of a common defensive strategy adopted by plants to protect themselves from environmental stresses. Existence of such interplay is based on the mining of published transcriptomic data as well as on phenotypic observations in trichome or cuticle mutants where the morphology of both structures often appear to be concomitantly altered. Given the existence of several trichome developmental pathways depending on the plant species and the types of trichomes, genetic interactions between cuticle formation and trichome development are complex to decipher and not easy to generalize. Based on our review of the literature, a schematic overview of the gene network mediating this transcriptional interplay is presented for two model plant species: Arabidopsis thaliana and Solanum lycopersicum. In addition to fundamental new insights on the regulation of these processes, identifying key transcriptional switches controlling both processes could also facilitate more applied investigations aiming at improving much desired agronomical traits in plants.

An advanced method for the release, enrichment and purification of high-quality Arabidopsis thaliana rosette leaf trichomes enables profound insights into the trichome proteome

BACKGROUND

Rosette leaf trichomes of Arabidopsis thaliana have been broadly used to study cell development, cell differentiation and, more recently, cell wall biogenesis. However, trichome-specific biochemical or -omics analyses require a proper separation of trichomes from residual plant tissue. Thus, different strategies were proposed in the past for trichome isolation, which mostly rely on harsh conditions and suffer from low yield, thereby limiting the spectrum of downstream analyses.

RESULTS

To take trichome-leaf separation to the next level, we revised a previously proposed method for isolating A. thaliana trichomes by optimizing the mechanical and biochemical specifications for trichome release. We additionally introduced a density gradient centrifugation step to remove residual plant debris. We found that prolonged, yet mild seedling agitation increases the overall trichome yield by more than 60% compared to the original protocol. We noticed that subsequent density gradient centrifugation further visually enhances trichome purity, which may be advantageous for downstream analyses. Gene expression analysis by quantitative reverse transcriptase-polymerase chain reaction validated a substantial enrichment upon purification of trichomes by density gradient centrifugation. Histochemical and biochemical investigation of trichome cell wall composition indicated that unlike the original protocol gentle agitation during trichome release largely preserves trichome integrity. We used enriched and density gradient-purified trichomes for proteomic analysis in comparison to trichome-depleted leaf samples and present a comprehensive reference data set of trichome-resident and -enriched proteins. Collectively we identified 223 proteins that are highly enriched in trichomes as compared to trichome-depleted leaves. We further demonstrate that the procedure can be applied to retrieve diverse glandular and non-glandular trichome types from other plant species.

CONCLUSIONS

We provide an advanced method for the isolation of A. thaliana leaf trichomes that outcompetes previous procedures regarding yield and purity. Due to the large amount of high-quality trichomes our method enabled profound insights into the so far largely unexplored A. thaliana trichome proteome. We anticipate that our protocol will be of use for a variety of downstream analyses, which are expected to shed further light on the biology of leaf trichomes in A. thaliana and possibly other plant species.

Silicon-based induced resistance in maize against fall armyworm [Spodoptera frugiperda (Lepidoptera: Noctuidae)]

The fall armyworm (Spodoptera frugiperda) is a major economic pest in the United States and has recently become a significant concern in African and Asian countries. Due to its increased resistance to current management strategies, including pesticides and transgenic corn, alternative management techniques have become more necessary. Currently, silicon (Si) is being used in many pest control systems due to its ability to increase plant resistance to biotic and abiotic factors and promote plant growth. The current experiments were carried out at the College of Plant Protection, Gansu Agricultural University, Lanzhou, China, to test the effect of Si on lifetable parameters and lipase activity of fall armyworm and vegetative and physiological parameters of maize plants. Two sources of Si (silicon dioxide: SiO2 and potassium silicate: K2SiO3) were applied on maize plants with two application methods (foliar application and soil drenching). The experiment results revealed that foliar applications of SiO2 and K2SiO3 significantly (P≤0.05) increased mortality percentage and developmental period and decreased larval and pupal biomass of fall armyworm. Similarly, both Si sources significantly (P≤0.05) reduced lipase activity of larvae, and fecundity of adults, whereas prolonged longevity of adults. Among plant parameters, a significant increase in fresh and dry weight of shoot, stem length, chlorophyll content, and antioxidant activity was observed with foliar applications of Si. Root fresh and dry weight was significantly (P ≤ 0.05) higher in plants treated with soil drenching of SiO2 and K2SiO3. Moreover, SiO2 performed better for all parameters as compared to K2SiO3 and control treatment. The study conclusively demonstrated a significant negative effect on various biological parameters of fall armyworm when plants were treated with Si, so it can be a promising strategy to control this pest.

Structural traits of leaf epidermis correspond to metal tolerance in Rumex acetosella populations growing on metal-contaminated soils

The pseudometallophyte Rumex acetosella L. occupies habitats with normal and high soil concentrations of zinc (Zn), lead (Pb), and copper (Cu). It remains unclear if the plants respond to the toxic metals by altering their morphology and increasing the resilience of their cells. We compared plants growing on soils contaminated with Zn/Pb (populations Terézia, Lintich), or Cu (populations Špania Dolina, Staré Hory), with those from non-contaminated soil (Dúbravka) in Slovakia, and analysed leaf structure, physiology, and metal contents by light and electron microscopy, element localization by energy-dispersive X-ray analysis (EDX) in scanning electron microscope, and by specific fluorescence dyes. In control population, the epidermis of the amphistomatic leaves of R. acetosella contained capitate glandular trichomes, consisting of four head (secretory), two stalk, and two basal cells. The ultrastructure of secretory cells revealed fine wall ingrowths bordered by plasma membrane protruding into the cytoplasm. The metallicolous populations had higher contents of Zn and Cu in the epidermal and glandular cells, and a higher density of both stomata and trichomes. Extensive cell wall labyrinth was present in the trichome secretory cells. Their abnormal number and elevated metal contents might indicate effects of heavy metals, especially of Cu, on mitosis and cell plate formation. Differences in leaf physiology were indicated by significantly higher cytoplasmic tolerance to Zn and Cu in metallicolous populations and by structural properties of glandular heads suggesting secretion of toxic metals. Our findings are suggestive of plant reactions to metal stress, which facilitate the populations to occupy the metal-contaminated sites.

Sclareol and linalyl acetate are produced by glandular trichomes through the MEP pathway

Sclareol, an antifungal specialized metabolite produced by clary sage, Salvia sclarea, is the starting plant natural molecule used for the hemisynthesis of the perfume ingredient ambroxide. Sclareol is mainly produced in clary sage flower calyces; however, the cellular localization of the sclareol biosynthesis remains unknown. To elucidate the site of sclareol biosynthesis, we analyzed its spatial distribution in the clary sage calyx epidermis using laser desorption/ionization mass spectrometry imaging (LDI-FTICR-MSI) and investigated the expression profile of sclareol biosynthesis genes in isolated glandular trichomes (GTs). We showed that sclareol specifically accumulates in GTs' gland cells in which sclareol biosynthesis genes are strongly expressed. We next isolated a glabrous beardless mutant and demonstrate that more than 90% of the sclareol is produced by the large capitate GTs. Feeding experiments, using 1-¹³C-glucose, and specific enzyme inhibitors further revealed that the methylerythritol-phosphate (MEP) biosynthetic pathway is the main source of isopentenyl diphosphate (IPP) precursor used for the biosynthesis of sclareol. Our findings demonstrate that sclareol is an MEP-derived diterpene produced by large capitate GTs in clary sage emphasing the role of GTs as biofactories dedicated to the production of specialized metabolites.

An HD-ZIP-MYB complex regulates glandular secretory trichome initiation in Artemisia annua

Plant glandular secretory trichomes (GSTs) produce various specialized metabolites. Increasing GST density represents a strategy to enhance the yield of these chemicals; however, the gene regulatory network that controls GST initiation remains unclear. In a previous study of Artemisia annua L., we found that a HD-ZIP IV transcription factor, AaHD1, promotes GST initiation by directly regulating AaGSW2. Here, we identified two AaHD1-interacting transcription factors, namely AaMIXTA-like 2 (AaMYB16) and AaMYB5. Through the generation and characterization of transgenic plants, we found that AaMYB16 is a positive regulator of GST initiation, whereas AaMYB5 has the opposite effect. Notably, neither of them regulates GST formation independently. Rather, they act competitively, by interacting and modulating AaHD1 promoter binding activity. Additionally, the phytohormone jasmonic acid (JA) was shown to be associated with the AaHD1-AaMYB16/AaMYB5 regulatory network through transcriptional regulation via a JASMONATE-ZIM DOMAIN (JAZ) protein repressor. These results bring new insights into the mechanism of GST initiation through regulatory complexes, which appear to have similar functions in a range of vascular plant taxa.

The scarecrow-like transcription factor SlSCL3 regulates volatile terpene biosynthesis and glandular trichome size in tomato (Solanum lycopersicum)

Tomato (Solanum lycopersicum L.) type VI glandular trichomes that occur on the surface of leaves, stems, young fruits and flowers produce and store a blend of volatile monoterpenes and sesquiterpenes. These compounds play important roles in the interaction with pathogens and herbivorous insects. Although the function of terpene synthases in the biosynthesis of volatile terpenes in tomato has been comprehensively investigated, the deciphering of their transcriptional regulation is only just emerging. We selected transcription factors that are over-expressed in trichomes based on existing transcriptome data and silenced them individually by virus-induced gene silencing. Of these, SlSCL3, a scarecrow-like (SCL) subfamily transcription factor, led to a significant decrease in volatile terpene content and expression of the corresponding terpene synthase genes when its transcription level was downregulated. Overexpression of SlSCL3 dramatically increased both the volatile terpene content and glandular trichome size, whereas its homozygous mutants showed reduced terpene biosynthesis. However, its heterozygous mutants also showed a significantly elevated volatile terpene content and enlarged glandular trichomes, similar to the overexpression plants. SlSCL3 modulates the expression of terpene biosynthetic pathway genes by transcriptional activation, but neither direct protein-DNA binding nor interaction with known regulators was observed. Moreover, transcript levels of the endogenous copy of SlSCL3 were decreased in the overexpression plants but increased in the heterozygous and homozygous mutants, suggesting feedback repression of its own promoter. Taken together, our results provide new insights into the role of SlSCL3 in the complex regulation of volatile terpene biosynthesis and glandular trichome development in tomato.

Ultrastructural Alterations in Cells of Sunflower Linear Glandular Trichomes during Maturation

Sunflower and related taxa are known to possess a characteristic type of multicellular uniseriate trichome which produces sesquiterpenes and flavonoids of yet unknown function for this plant. Contrary to the metabolic profile, the cytological development and ultrastructural rearrangements during the biosynthetic activity of the trichome have not been studied in detail so far. Light, fluorescence and transmission electron microscopy were employed to investigate the functional structure of different trichome cells and their subcellular compartmentation in the pre-secretory, secretory and post-secretory phase. It was shown that the trichome was composed of four cell types, forming the trichome basis with a basal and a stalk cell, a variable number (mostly from five to eight) of barrel-shaped glandular cells and the tip consisting of a dome-shaped apical cell. Metabolic activity started at the trichome tip sometimes accompanied by the formation of small subcuticular cavities at the apical cell. Subsequently, metabolic activity progressed downwards in the upper glandular cells. Cells involved in the secretory process showed disintegration of the subcellular compartments and lost vitality in parallel to deposition of fluorescent and brownish metabolites. The subcuticular cavities usually collapsed in the early secretory stage, whereas the colored depositions remained in cells of senescent hairs.

Introgression of the sesquiterpene biosynthesis from Solanum habrochaites to cultivated tomato offers insights into trichome morphology and arthropod resistance

Cultivated tomatoes harboring the plastid-derived sesquiterpenes from S. habrochaites have altered type-VI trichome morphology and unveil additional genetic components necessary for piercing-sucking pest resistance. Arthropod resistance in the tomato wild relative Solanum habrochaites LA1777 is linked to specific sesquiterpene biosynthesis. The Sesquiterpene synthase 2 (SsT2) gene cluster on LA1777 chromosome 8 controls plastid-derived sesquiterpene synthesis. The main genes at SsT2 are Z-prenyltransferase (zFPS) and Santalene and Bergamotene Synthase (SBS), which produce α-santalene, β-bergamotene, and α-bergamotene in LA1777 round-shaped type-VI glandular trichomes. Cultivated tomatoes have mushroom-shaped type-VI trichomes with much smaller glands that contain low levels of monoterpenes and cytosolic-derived sesquiterpenes, not presenting the same pest resistance as in LA1777. We successfully transferred zFPS and SBS from LA1777 to cultivated tomato (cv. Micro-Tom, MT) by a backcrossing approach. The trichomes of the MT-Sst2 introgressed line produced high levels of the plastid-derived sesquiterpenes. The type-VI trichome internal storage-cavity size increased in MT-Sst2, probably as an effect of the increased amount of sesquiterpenes, although it was not enough to mimic the round-shaped LA1777 trichomes. The presence of high amounts of plastid-derived sesquiterpenes was also not sufficient to confer resistance to various tomato piercing-sucking pests, indicating that the effect of the sesquiterpenes found in the wild S. habrochaites can be insect specific. Our results provide for a better understanding of the morphology of S. habrochaites type-VI trichomes and paves the way to obtain insect-resistant tomatoes.

Cannabinoids and Terpenes: How Production of Photo-Protectants Can Be Manipulated to Enhance Cannabis sativa L. Phytochemistry

Cannabis sativa L. is cultivated for its secondary metabolites, of which the cannabinoids have documented health benefits and growing pharmaceutical potential. Recent legal cannabis production in North America and Europe has been accompanied by an increase in reported findings for optimization of naturally occurring and synthetic cannabinoid production. Of the many environmental cues that can be manipulated during plant growth in controlled environments, cannabis cultivation with different lighting spectra indicates differential production and accumulation of medically important cannabinoids, including Δ9-tetrahydrocannabinol (Δ9-THC), cannabidiol (CBD), and cannabigerol (CBG), as well as terpenes and flavonoids. Ultraviolet (UV) radiation shows potential in stimulating cannabinoid biosynthesis in cannabis trichomes and pre-harvest or post-harvest UV treatment merits further exploration to determine if plant secondary metabolite accumulation could be enhanced in this manner. Visible LED light can augment THC and terpene accumulation, but not CBD. Well-designed experiments with light wavelengths other than blue and red light will provide more insight into light-dependent regulatory and molecular pathways in cannabis. Lighting strategies such as subcanopy lighting and varied light spectra at different developmental stages can lower energy consumption and optimize cannabis PSM production. Although evidence demonstrates that secondary metabolites in cannabis may be modulated by the light spectrum like other plant species, several questions remain for cannabinoid production pathways in this fast-paced and growing industry. In summarizing recent research progress on light spectra and secondary metabolites in cannabis, along with pertinent light responses in model plant species, future research directions are presented.

In vitro cultivation of tansy (Tanacetum vulgare L.): a tool for the production of potent pharmaceutical agents

In this study, tansy (Tanacetum vulgare L.) in vitro culture was established from seeds collected from natural populations. The multiplication of plantlets was conducted through shoot tips that exhibited potent apical growth and regeneration capacities on basal medium (BM), without the addition of any plant growth regulators (PGRs). PGRs were also omitted for the establishment and cultivation of tansy root cultures. Both abaxial and adaxial leaf surfaces of in vitro micropropagated plantlets were covered with glandular biseriate trichomes. Histochemical staining showed that glandular secretions were rich in lipid and terpene compounds, confirmed by GC-MS analysis of essential oil (EO). In the total EO, similar portions of oxygenated monoterpenes (38.5% m/m) and oxygenated sesquiterpenes (22.6% m/m) were detected. Chemical profiles of methanol extracts of in vitro cultured tansy shoots and roots varied in quantity and quality from those obtained from wild-growingtansy. HPLC analysis indicated that the methanol extracts of in vitro cultured roots were the richest in 3,5-O-dicaffeoylquinic acid (3,5-O-DCQA), in which the concentration was 6 times higher (10.220 mg/g DW) than that in the extract obtained from roots of wild-growing tansy (1.684 mg/g DW). This result is noticeable in the manner of industrial production of biologically active 3,5-O-DCQA that has been shown to have antioxidant, hepatoprotective, antiviral, antimutagenic, and immunomodulatory activity. Biotechnological interventions on secondary metabolite production taking place in trichomes could further enhance the production of some important tansy metabolites and further investigation will be directed toward the elucidation of the pharmaceutical potential of tansy in vitro obtained metabolites, as mixtures or single moieties.

The Lanata trichome mutation increases stomatal conductance and reduces leaf temperature in tomato

Trichomes are epidermal structures with a large variety of ecological functions and economic applications. Glandular trichomes produce a rich repertoire of secondary metabolites, whereas non-glandular trichomes create a physical barrier on the epidermis: both operate in tandem against biotic and abiotic stressors. A deeper understanding of trichome development and function would enable the breeding of more resilient crops. However, little is known about the impact of altered trichome density on leaf photosynthesis, gas exchange and energy balance. Previous work has compared multiple, closely related species differing in trichome density. Here, we analysed monogenic trichome mutants in the same tomato genetic background (Solanum lycopersicum cv. 'Micro-Tom'). We determined growth parameters, leaf spectral properties, gas exchange and leaf temperature in the hairs absent (h), Lanata (Ln) and Woolly (Wo) trichome mutants. Shoot dry weight, leaf area, leaf spectral properties and cuticular conductance were not affected by the mutations. However, the Ln mutant showed increased net carbon assimilation rate (A_n), associated with higher stomatal conductance (g_s), with no differences in stomatal density or stomatal index between genotypes. Leaf temperature was furthermore reduced in Ln in the hottest, early hours of the afternoon. We show that a single monogenic mutation that modifies trichome density, a desirable trait for crop breeding, concomitantly improves leaf gas exchange and reduces leaf temperature.

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.

A single cytochrome P450 oxidase from Solanum habrochaites sequentially oxidizes 7-epi-zingiberene to derivatives toxic to whiteflies and various microorganisms

Secretions from glandular trichomes potentially protect plants against a variety of aggressors. In the tomato clade of the Solanum genus, glandular trichomes of wild species produce a rich source of chemical diversity at the leaf surface. Previously, 7-epi-zingiberene produced in several accessions of Solanum habrochaites was found to confer resistance to whiteflies (Bemisia tabaci) and other insect pests. Here, we report the identification and characterisation of 9-hydroxy-zingiberene (9HZ) and 9-hydroxy-10,11-epoxyzingiberene (9H10epoZ), two derivatives of 7-epi-zingiberene produced in glandular trichomes of S. habrochaites LA2167. Using a combination of transcriptomics and genetics, we identified a gene coding for a cytochrome P450 oxygenase, ShCYP71D184, that is highly expressed in trichomes and co-segregates with the presence of the zingiberene derivatives. Transient expression assays in Nicotiana benthamiana showed that ShCYP71D184 carries out two successive oxidations to generate 9HZ and 9H10epoZ. Bioactivity assays showed that 9-hydroxy-10,11-epoxyzingiberene in particular exhibits substantial toxicity against B. tabaci and various microorganisms including Phytophthora infestans and Botrytis cinerea. Our work shows that trichome secretions from wild tomato species can provide protection against a wide variety of organisms. In addition, the availability of the genes encoding the enzymes for the pathway of 7-epi-zingiberene derivatives makes it possible to introduce this trait in cultivated tomato by precision breeding.

The WRKY transcription factor AaGSW2 promotes glandular trichome initiation in Artemisia annua

Glandular secreting trichomes (GSTs) synthesize and secrete large quantities of secondary metabolites, some of which have well-established commercial value. An example is the anti-malarial compound artemisinin, which is synthesized in the GSTs of Artemisia annua. Accordingly, there is considerable interest in understanding the processes that regulate GST density as a strategy to increase artemisinin production. In this study, we identified a GST-specific WRKY transcription factor from A. annua, AaGSW2, which is positively regulated by the direct binding of the homeodomain proteins AaHD1 and AaHD8 to the L1-box of the AaGSW2 promoter. Overexpression of AaGSW2 in A. annua significantly increased GST density, while AaGSW2 knockdown lines showed impaired GST initiation. Ectopic expression of AaGSW2 homologs from two mint cultivars, Mentha spicata and Mentha haplocalyx, in A. annua also induced GST formation. These results reveal a molecular mechanism involving homeodomain and WRKY proteins that controls glandular trichome initiation, at least part of which is shared by A. annua and mint.

Using Gene Expression to Study Specialized Metabolism-A Practical Guide

Plants produce a vast array of chemical compounds that we use as medicines and flavors, but these compounds' biosynthetic pathways are still poorly understood. This paucity precludes us from modifying, improving, and mass-producing these specialized metabolites in suitable bioreactors. Many of the specialized metabolites are expressed in a narrow range of organs, tissues, and cell types, suggesting a tight regulation of the responsible biosynthetic pathways. Fortunately, with unprecedented ease of generating gene expression data and with >200,000 publicly available RNA sequencing samples, we are now able to study the expression of genes from hundreds of plant species. This review demonstrates how gene expression can elucidate the biosynthetic pathways by mining organ-specific genes, gene expression clusters, and applying various types of co-expression analyses. To empower biologists to perform these analyses, we showcase these analyses using recently published, user-friendly tools. Finally, we analyze the performance of co-expression networks and show that they are a valuable addition to elucidating multiple the biosynthetic pathways of specialized metabolism.

An R2R3-MYB Transcription Factor Positively Regulates the Glandular Secretory Trichome Initiation in Artemisia annua L

Artemisia annua L. is known for its specific product "artemisinin" which is an active ingredient for curing malaria. Artemisinin is secreted and accumulated in the glandular secretory trichomes (GSTs) on A. annua leaves. Earlier studies have shown that increasing GST density is effective in increasing artemisinin content. However, the mechanism of GST initiation is not fully understood. To this end, we isolated and characterized an R2R3-MYB gene, AaMYB17, which is expressed specifically in the GSTs of shoot tips. Overexpression of AaMYB17 in A. annua increased GST density and enhanced the artemisinin content, whereas RNA interference of AaMYB17 resulted in the reduction of GST density and artemisinin content. Additionally, neither overexpression lines nor RNAi lines showed an abnormal phenotype in plant growth and the morphology of GSTs. Our study demonstrates that AaMYB17 is a positive regulator of GSTs' initiation, without influencing the trichome morphology.

Glandular trichomes of Robinia viscosa Vent. var. hartwigii (Koehne) Ashe (Faboideae, Fabaceae)-morphology, histochemistry and ultrastructure

MAIN CONCLUSION

Permanent glandular trichomes of Robinia viscosa var. hartwigii produce viscous secretion containing several secondary metabolites, as lipids, mucilage, flavonoids, proteins and alkaloids. Robinia viscosa var. hartwigii (Hartweg's locust) is an ornamental tree with high apicultural value. It can be planted in urban greenery and in degraded areas. The shoots, leaves, and inflorescences of this plant are equipped with numerous persistent glandular trichomes producing sticky secretion. The distribution, origin, development, morphology, anatomy, and ultrastructure of glandular trichomes of Hartweg's locust flowers as well as the localisation and composition of their secretory products were investigated for the first time. To this end, light, scanning, and transmission electron microscopy combined with histochemical and fluorescence techniques were used. The massive glandular trichomes differing in the distribution, length, and stage of development were built of a multicellular and multiseriate stalk and a multicellular head. The secretory cells in the stalk and head had large nuclei with nucleoli, numerous chloroplasts with thylakoids and starch grains, mitochondria, endoplasmic reticulum profiles, Golgi apparatus, vesicles, and multivesicular bodies. Many vacuoles contained phenolic compounds dissolved or forming various condensed deposits. The secretion components were transported through symplast elements, and the granulocrine and eccrine modes of nectar secretion were observed. The secretion was accumulated in the subcuticular space at the trichome apex and released through a pore in the cuticle. Histochemical and fluorescence assays showed that the trichomes and secretion contained lipophilic and polyphenol compounds, polysaccharides, proteins, and alkaloids. We suggest that these metabolites may serve an important function in protection of plants against biotic stress conditions and may also be a source of phytopharmaceuticals in the future.

Fascinating impact of silicon and silicon transporters in plants: A review

Silicon (Si) is a metalloid which is gaining worldwide attention of plant scientists due to its ameliorating impact on plants' growth and development. The beneficial response of Si is observed predominantly under numerous abiotic and biotic stress conditions. However, under favorable conditions, most of the plant can grow without it. Therefore, Si has yet not been fully accepted as essential element rather it is being considered as quasi-essential for plants' growth. Si is also known to enhance resilience in plants by reducing the plant's stress. Besides its second most abundance on the earth crust, most of the soils lack plant available form of Si i.e. silicic acid. In this regard, understanding the role of Si in plant metabolism, its uptake from roots and transport to aerial tissues along with its ionomics and proteomics under different circumstances is of great concern. Plants have evolved a well-optimized Si-transport system including various transporter proteins like Low silicon1 (Lsi1), Low silicon2 (Lsi2), Low silicon3 (Lsi3) and Low silicon6 (Lsi6) at specific sub-cellular locations along with the expression profiling that creates precisely coordinated network among these transporters, which also facilitate uptake and accumulation of Si. Though, an ample amount of information is available pertinent to the solute specificity, active sites, transcriptional and post-transcriptional regulation of these transporter genes. Similarly, the information regarding transporters involved in Si accumulation in different organelles is also available particularly in silica cells occurred in poales. But in this review, we have attempted to compile studies related to plants vis à vis Si, its role in abiotic and biotic stress, its uptake in various parts of plants via different types of Si-transporters, expression pattern, localization and the solute specificity. Besides these, this review will also provide the compiled knowledge about the genetic variation among crop plants vis à vis enhanced Si uptake and related benefits.

Genome wide annotation and characterization of young, intact long terminal repeat retrotransposons (In-LTR-RTs) of seven legume species

Availability of genome sequence of different legume species has provided an opportunity to characterize the abundance, distribution, and divergence of canonical intact long terminal retrotransposons (In-LTR-RT) superfamilies. Among seven legume species, Arachis ipaensis (Aip) showed the highest number of full-length canonical In-LTR-RTs (3325), followed by Glycine max (Gma, 2328), Vigna angularis (Van, 1625), Arachis durensis (Adu, 1348), Lotus japonicus (Lja, 1294), Medicago truncatula (Mtr, 788), and Circer arietinum (Car, 124). Divergence time analysis demonstrated that the amplification timeframe of LTR-RTs dramatically varied in different families. The average insertion time of Copia element varied from 0.51 (Van) to 1.37 million years ago (Mya) (Adu, and Aip), whereas that of Gypsy was between 0.22 (Mtr) and 1.82 Mya (Adu). Bayesian phylogenetic tree analysis suggested that the 1397 and 1917 reverse transcriptase (RT) domains of Copia and Gypsy families of the seven legume species were clustered into 7 and 14 major groups, respectively. The highest proportion (approximately 94.79-100%) of transposable element (TE)-associated genes assigned to pathways was mapped to metabolism-related pathways in all species. The results enabled the structural understanding of full-length In-LTR-RTs and will be valuable resource for the further study of the impact of TEs on gene structure and expression in legume species.

Enhanced monoterpene emission in transgenic orange mint (Mentha × piperita f. citrata) overexpressing a tobacco lipid transfer protein (NtLTP1)

MAIN CONCLUSION

Overexpression of the tobacco lipid transfer protein (NtLTP1) gene in transgenic orange mint resulted in enhanced accumulation of monoterpenes in the cavity of head cells of glandular trichomes, which resulted in enhanced emission of monoterpenes from transgenic orange mints. Plants in the genus Mentha (Lamiaceae) produce volatile oils that accumulate in peltate glandular trichomes in the aerial parts of plants. A lipid transfer protein (NtLTP1) in tobacco showed glandular trichome-specific expression and supported the secretion of diterpenoid lipids from head cells of glandular trichomes (Choi et al., Plant J 70:480-491,2012). Here, we constructed transgenic orange mint (Mentha × piperita f. citrata) overexpressing the tobacco NtLTP1 gene via Agrobacterium-mediated transformation. Transgenic lines of orange mint overexpressing NtLTP1 were confirmed by genomic PCR and RT-PCR. Immunoblotting analysis using an NtLTP1 polyclonal antibody showed clear dark spots at the position of the lipid exudates from tobacco glandular trichomes and the squeezed out lipids from the glandular trichomes of transgenic orange mint. Heads of glandular trichomes in transgenic plants overexpressing the NtLTP1 gene showed a larger diameter than those of the wild-type control. The enhanced size of trichome heads in transgenic orange mint was confirmed by scanning electron microscopy. Volatile components were extracted from wild-type and transgenic orange mint by solid-phase microextraction (SPME) and analyzed by headspace-gas chromatography-mass spectrometry (HS/GC/MS). Linalyl acetate was the most abundant component among the eleven identified monoterpenes in the volatile compounds extracted from both the wild-type and transgenic lines of orange mint. Overexpression of NtLTP1 in transgenic orange mint plants resulted in enhanced emission of volatile monoterpenoids compared with that of volatile monoterpenoids in the wild-type control plants.

Metabolic Engineering of the Native Monoterpene Pathway in Spearmint for Production of Heterologous Monoterpenes Reveals Complex Metabolism and Pathway Interactions

Spearmint produces and stores large amounts of monoterpenes, mainly limonene and carvone, in glandular trichomes and is the major natural source of these compounds. Towards producing heterologous monoterpenes in spearmint, we first reduced the flux into the native limonene pathway by knocking down the expression of limonene synthase (MsLS) by RNAi method. The MsLS RNAi lines exhibited a huge reduction in the synthesis of limonene and carvone. Detailed GC-MS and LC-MS analysis revealed that MsLS RNAi plants also showed an increase in sesquiterpene, phytosterols, fatty acids, flavonoids, and phenolic metabolites, suggesting an interaction between the MEP, MVA shikimate and fatty acid pathways in spearmint. Three different heterologous monoterpene synthases namely, linalool synthase and myrcene synthase from Picea abies and geraniol synthase from Cananga odorata were cloned and introduced independently into the MsLS RNAi mutant background. The expression of these heterologous terpene synthases resulted mainly in production of monoterpene derivatives. Of all the introduced monoterpenes geraniol showed the maximum number of derivatives. Our results provide new insights into MEP pathway interactions and regulation and reveals the existence of mechanisms for complex metabolism of monoterpenes in spearmint.