Deep learning-based quantification and transcriptomic profiling reveal a methyl jasmonate-mediated glandular trichome formation pathway in Cannabis sativa

Cannabis glandular trichomes (GTs) are economically and biotechnologically important structures that have a remarkable morphology and capacity to produce, store, and secrete diverse classes of secondary metabolites. However, our understanding of the developmental changes and the underlying molecular processes involved in cannabis GT development is limited. In this study, we developed Cannabis Glandular Trichome Detection Model (CGTDM), a deep learning-based model capable of differentiating and quantifying three types of cannabis GTs with a high degree of efficiency and accuracy. By profiling at eight different time points, we captured dynamic changes in gene expression, phenotypes, and metabolic processes associated with GT development. By integrating weighted gene co-expression network analysis with CGTDM measurements, we established correlations between phenotypic variations in GT traits and the global transcriptome profiles across the developmental gradient. Notably, we identified a module containing methyl jasmonate (MeJA)-responsive genes that significantly correlated with stalked GT density and cannabinoid content during development, suggesting the existence of a MeJA-mediated GT formation pathway. Our findings were further supported by the successful promotion of GT development in cannabis through exogenous MeJA treatment. Importantly, we have identified CsMYC4 as a key transcription factor that positively regulates GT formation via MeJA signaling in cannabis. These findings provide novel tools for GT detection and counting, as well as valuable information for understanding the molecular regulatory mechanism of GT formation, which has the potential to facilitate the molecular breeding, targeted engineering, informed harvest timing, and manipulation of cannabinoid production.

Artemisia arborescens (Vaill.) L.: Micromorphology, Essential Oil Composition, and Its Potential as an Alternative Biocontrol Product

Artemisia arborescens is a Mediterranean evergreen shrub, with silver grey-green tomentose leaves and a strong scent. It has various ethnopharmacological uses and its secondary metabolites have demonstrated antimicrobial, antiviral, pharmaceutical, phytotoxic, and insecticidal activities. Different extracts obtained from aerial parts of this species are known for their allelopathic effect, but similar studies on its essential oil (EO) are lacking. Therefore, we carried out a pharmacognostic study, obtaining the characterization of the secretory structures and the EO produced. Trans-thujone and camphor are the main components, followed by aromadendrene, camphene, and 8-cedren-13-ol. EO phytotoxic activity was tested on weed plants (Lolium multiflorum Lam. and Sinapis arvensis L.) and crops (Raphanus sativus L. and Cucumis sativus L.), showing inhibition on both germination and radical growth of the two weeds tested. The effects of the EO against the bacterial plant pathogens Xanthomonas campestris pv. campestris (Gram-) and Pseudomonas syringae pv. tomato (Gram+) was also assayed. The minimum inhibitory concentration (MIC) was observed when it was used undiluted [100% v/v], and growth inhibition when diluted at different doses. The antimicrobial activity was also confirmed by the cellular material release and biofilm formation assays. The overall data show that A. arborescens EO can find application as a potential alternative biocontrol product against weeds and plant pathogens. This goal is particularly important from the perspective of replacing synthetic pesticides with natural products, which safeguard both the environment and the health of consumers.

The evolution of glandularity as a defense against herbivores in the tarweed clade

PREMISE

Glandular trichomes are implicated in direct and indirect defense of plants. However, the degree to which glandular and non-glandular trichomes have evolved as a consequence of herbivory remains unclear, because their heritability, their association with herbivore resistance, their trade-offs with one another, and their association with other functions are rarely quantified.

METHODS

We conducted a phylogenetic comparison of trichomes and herbivore resistance against the generalist caterpillar, Heliothis virescens, among tarweed species (Asteraceae: Madiinae) and a genetic correlation study comparing those same traits among maternal half-sibs of three tarweed species.

RESULTS

Within a tarweed species, we found no evidence that herbivore growth rate decreased on tarweed individuals or maternal sib groups with more glandularity or denser trichomes. However, tarweed species with more glandularity and fewer non-glandular trichomes resulted in slower-growing herbivores. Likewise, a trade-off between glandular and non-glandular trichomes was apparent among tarweed species, but not among individuals or sib groups within a species.

CONCLUSIONS

Our results suggest that this key herbivore does not select for trichomes as a direct defense in tarweed species. However, trichomes differed substantially among species and likely affect herbivore pressure on those species. Our results demonstrate that trade-offs among plant traits, as well as inference on the function of those traits, can depend on scale.

An Integrative Approach to Selected Species of Tanacetum L. (Asteraceae): Insights into Morphology and Phytochemistry

In this work, we studied Tanacetum vulgare, Tanacetum parthenium, and Tanacetum corymbosum (Asteraceae) cultivated at the Ghirardi Botanic Garden (Toscolano Maderno, Brescia, Northern Italy) of the University of Milan. An integrative research approach was adopted: microscopic and histochemical, with special focus on the secretory structures responsible for the productivity of secondary metabolites; phytochemical, with the analysis of the essential oil (EO) profiles from the air-dried, flowered aerial parts collected in June 2021; bio-ecological, with emphasis, based on literature data, on the ecology and biological activity of the main EO components. In all three species, two basic trichome morphotypes (flagellar non-glandular and biseriate glandular) occurred with different distribution patterns. The glandular ones produced terpenes, along with flavonoids. A high level of chemical variability in the EO compositions emerged, specifically for qualitative data. T. vulgare profile was more complex and heterogeneous than those obtained from T. parthenium and T. corymbosum, with camphor as the predominant compound, followed by farnesol and α-santalone, respectively. Finally, the obtained scientific findings were made available to the visitors of the botanic garden through new dissemination labeling that highlights the "invisible", microscopic features of the plants, from an Open Science perspective ("Botanic Garden, factories of molecules…work in progress"-Lombardy Region Project Lr. 25/2016, year 2021).

Herb and Flowers of Achillea millefolium subsp. millefolium L.: Structure and Histochemistry of Secretory Tissues and Phytochemistry of Essential Oils

Achillea millefolium L. herb and flowers have high biological activity; hence, they are used in medicine and cosmetics. The aim of this study was to perform morpho-anatomical analyses of the raw material, including secretory tissues, histochemical assays of the location of lipophilic compounds, and quantitative and qualitative analysis of essential oil (EO). Light and scanning electron microscopy techniques were used to analyse plant structures. The qualitative analyses of EO were carried out using gas chromatography-mass spectrometry (GC/MS). The results of this study showed the presence of exogenous secretory structures in the raw material, i.e., conical cells (papillae) on the adaxial surface of petal teeth and biseriate glandular trichomes on the surface flowers, bracts, stems, and leaves. Canal-shaped endogenous secretory tissue was observed in the stems and leaves. The histochemical assays revealed the presence of total, acidic, and neutral lipids as well as EO in the glandular trichome cells. Additionally, papillae located at the petal teeth contained neutral lipids. Sesquiterpenes were detected in the glandular trichomes and petal epidermis cells. The secretory canals in the stems were found to contain total and neutral lipids. The phytochemical assays demonstrated that the A. millefolium subsp. millefolium flowers contained over 2.5-fold higher amounts of EO (6.1 mL/kg) than the herb (2.4 mL/kg). The EO extracted from the flowers and herb had a similar dominant compounds: β-pinene, bornyl acetate, (E)-nerolidol, 1,8-cineole, borneol, sabinene, camphor, and α-pinene. Both EO samples had greater amounts of monoterpenes than sesquiterpenes. Higher amounts of oxygenated monoterpenes and oxygenated sesquiterpenoids were detected in the EO from the herb than from the flowers.

Shifts in carbon partitioning by photosynthetic activity increase terpenoid synthesis in glandular trichomes

Several commercially important secondary metabolites are produced and accumulated in high amounts by glandular trichomes, giving the prospect of using them as metabolic cell factories. Due to extremely high metabolic fluxes through glandular trichomes, previous research focused on how such flows are achieved. The question regarding their bioenergetics became even more interesting with the discovery of photosynthetic activity in some glandular trichomes. Despite recent advances, how primary metabolism contributes to the high metabolic fluxes in glandular trichomes is still not fully elucidated. Using computational methods and available multi-omics data, we first developed a quantitative framework to investigate the possible role of photosynthetic energy supply in terpenoid production and next tested experimentally the simulation-driven hypothesis. With this work, we provide the first reconstruction of specialised metabolism in Type-VI photosynthetic glandular trichomes of Solanum lycopersicum. Our model predicted that increasing light intensities results in a shift of carbon partitioning from catabolic to anabolic reactions driven by the energy availability of the cell. Moreover, we show the benefit of shifting between isoprenoid pathways under different light regimes, leading to a production of different classes of terpenes. Our computational predictions were confirmed in vivo, demonstrating a significant increase in production of monoterpenoids while the sesquiterpenes remained unchanged under higher light intensities. The outcomes of this research provide quantitative measures to assess the beneficial role of chloroplast in glandular trichomes for enhanced production of secondary metabolites and can guide the design of new experiments that aim at modulating terpenoid production.

glandular trichomes for transmission electron microscopy

Cannabis glandular trichomes produce and store an abundance of lipidic specialised metabolites (e.g. cannabinoids and terpenes) that are consumed by humans for medicinal and recreational purposes. Due to a lack of genetic resources and inherent autofluorescence of cannabis glandular trichomes, our knowledge of cannabinoid trafficking and secretion is limited to transmission electron microscopy (TEM). Advances in cryofixation methods has resulted in ultrastructural observations closer to the 'natural state' of the living cell, and recent reports of cryofixed cannabis trichome ultrastructure challenge the long-standing model of cannabinoid trafficking proposed by ultrastructural reports using chemically fixed samples. Here, we compare the ultrastructural morphology of cannabis glandular trichomes preserved using conventional chemical fixation and ultrarapid cryofixation. We show that chemical fixation results in amorphous metabolite inclusions surrounding the organelles of glandular trichomes that were not present in cryofixed samples. Vacuolar morphology in cryofixed samples exhibited homogenous electron density, while chemically fixed samples contained a flocculent electron dense periphery and electron lucent lumen. In contrast to the apparent advantages of cryopreservation, fine details of cell wall fibre orientation could be observed in chemically fixed glandular trichomes that were not seen in cryofixed samples. Our data suggest that chemical fixation results in intracellular artefacts that impact the interpretation of lipid production and trafficking, while enabling greater detail of extracellular polysaccharide organisation.

Differentiation and Propagation Potential of Arnica montana L. Achenes as a Consequence of the Morphological Diversity of Flowers and the Position of Flower Heads on the Plant

Arnica montana L. is a very important medicinal plant and simultaneously a European endemic endangered plant species. The morphological features and details of seed development and achene variability are poorly recognized. The aim of this study was to determine the impact of the achene position in the infructescence and the location of the inflorescence on the plant on the (i) morphological characteristics and germination ability of achenes, and (ii) recruitment of seedlings and their biometric features. Infructescences containing fully ripe achenes were randomly collected from A. montana individuals for the measurements and the germination experiment. Scanning electron microscopy, fluorescence microscopy, and light microscopy were used for characterization of flowers and achenes. The morphological traits of achenes and reproductive characteristics of A. montana were determined by the position of the achenes in the infructescence and the location of the inflorescence on the plant. The surface of arnica achenes is equipped with non-glandular and glandular trichomes, which is very rarely presented in species of the family Asteraceae. It is possible that the fluid-containing glandular trichomes are a source of essential oils. The peripherally located achenes were longer, thinner, and lighter. They were characterized by lower embryo weight, lower embryo/achene weight ratio, and lower germination capacity in comparison to the centrally located ones. The results presented in this article fill the gap in the knowledge of the morphology of achenes and the biology of the species, and provide information that can help in breeding programs, active protection, and field cultivation.

Two homeologous MATE transporter genes, NtMATE21 and NtMATE22, are involved in the modulation of plant growth and flavonol transport in Nicotiana tabacum

The multidrug and toxic compound extrusion (MATE) protein family has been implicated in the transport of a diverse range of molecules, including specialized metabolites. In tobacco (Nicotiana tabacum), only a limited number of MATE transporters have been functionally characterized, and no MATE transporter has been studied in the context of flavonoid transport in this plant species so far. In the present study, we characterize two homeologous tobacco MATE genes, NtMATE21 and NtMATE22, and demonstrate their role in flavonol transport and in plant growth and development. The expression of these two genes was reported to be up-regulated in trichomes as compared with the trichome-free leaf. The transcript levels of NtMATE21 and NtMATE22 were found to be higher in flavonol overproducing tobacco transgenic lines as compared with wild type tobacco. The two transporters were demonstrated to be localized to the plasma membrane. Genetic manipulation of NtMATE21 and NtMATE22 led to altered growth phenotypes and modulated flavonol contents in N. tabacum. The β-glucuronidase and green fluorescent protein fusion transgenic lines of promoter regions suggested that NtMATE21 and NtMATE22 are exclusively expressed in the trichome heads in the leaf tissue and petals. Moreover, in a transient transactivation assay, NtMYB12, a flavonol-specific MYB transcription factor, was found to transactivate the expression of NtMATE21 and NtMATE22 genes. Together, our results strongly suggest the involvement of NtMATE21 and NtMATE22 in flavonol transport as well as in the regulation of plant growth and development.

Exogenous hydrogen sulphide alleviates copper stress impacts in Artemisia annua L.: Growth, antioxidant metabolism, glandular trichome development and artemisinin biosynthesis

A supply of plant micronutrients (some of which are metals) is necessary to regulate many plant processes; their excess, however, can have detrimental consequences and can hamper plant growth, physiology and metabolism. Artemisia annua is an important crop plant used in the treatment of malaria. In this investigation, the physio-biochemical mechanisms involved in exogenous hydrogen sulphide-mediated (H₂ S) alleviation of copper (Cu) stress in A. annua were assessed.. Two different levels of Cu (20, 40 mg·kg^-1 ), one H₂ S treatment (200 µm) and their combinations were introduced while one set of plants was retained as control. Results showed that the presence of excess Cu in the soil reduced growth and biomass, photosynthetic parameters, chlorophyll content and fluorescence, gas exchange parameters and induced antioxidant enzyme activity. Copper stress enhanced the production of thiobarbituric acid reactive substances (TBARS) and increased Cu content in both roots and shoots of affected plants. Exogenous application of H₂ S restored the physio-biochemical characteristics of Cu-treated A. annua plants by reducing lipid peroxidation and enhancing the activity of antioxidant enzymes in Cu-stressed plants as compared with the controls. Hydrogen sulphide also reduced the Cu content in different plant parts, increased photosynthetic efficiency, trichome density, average area of trichomes and artemisinin content. Therefore, our results provide a comprehensive assessment of the defensive role of H₂ S in Cu-stressed A. annua.

Volatile Organic Compound Composition and Glandular Trichome Characteristics of In Vitro Propagated Clinopodium pulegium (Rochel) Bräuchler: Effect of Carbon Source

Clinopodium pulegium (Rochel) Bräuchler (Lamiaceae) is an endangered species endemic to the Southern Carpathians. It is characterized by the production of high amounts of essential oils, which emit volatile organic compounds (VOCs) that have an essential role in biotic and abiotic stress responses and in plant-plant and plant-insect interactions. The present study was initiated to phytochemically examine the influence of different carbon sources in the nutrition medium on VOC emissions of micropropagated C. pulegium plants, using gas chromatography-mass spectrometry analysis of headspace VOCs. The volatile profiles were subjected to multivariate analysis with respect to the presence, concentration and type of carbon source in the nutrient medium. In addition, the effect of different carbohydrates on the density and size of the leaf glandular trichomes, the main structures involved in the emission of VOCs, was determined. A total of 19 VOCs, primarily belonging to mono- and sesquiterpenes previously described in plants, were tentatively identified. Six VOCs were produced at levels higher than 2% of the total VOC emission, dominated by pulegone, ß-pinene and menthone. Inclusion of the carbohydrates in the culture media affected the production of the main leaf trichome-associated volatile allelochemicals although the qualitative composition of the volatiles changed only slightly. Multivariate analysis showed that the concentration, rather than the carbohydrate type, influenced the VOC profile.

Stinging Trichomes in Apocynaceae and Their Evolution in Angiosperms

Stinging trichomes are rare in plants, occurring only in angiosperms, where they are reported for a few genera belonging to six families. Although there is no report of stinging trichomes in Apocynaceae, previous fieldwork collections of Fischeria and Matelea caused us a mild allergic reaction on the skin when we contacted the dense indumentum of the plants. This fact associated with the well-known presence of glandular trichomes with acute apex in both genera raised suspicions that stinging trichomes could be present in the family. Hence, this study aimed to investigate the likely occurrence of stinging trichomes in Fischeria and Matelea. We analyzed vegetative shoots and leaves of Fischeria stellata and Matelea denticulata through the usual procedures of light and scanning electron microscopy. We also performed several histochemical tests to investigate the chemical composition of trichome secretion. We detected that glandular trichomes occur throughout the surface of the leaf and stem. They are multicellular, uniseriate with an apical secretory cell, which has a dilated base and a needle-shaped apex. The secretion is compressed into the acuminate portion of the apical cell by a large vacuole, and crystals are deposited in the cell wall in a subapical position, providing a preferential site of rupture. The secretion, composed of amino acids and/or proteins, is released under mechanical action, causing skin irritation. Based on our detailed morphological and anatomical analyses, and in the functional aspects observed, we concluded that the glandular trichomes in Fischeria and Matelea can indeed be classified as stinging. Thus, Apocynaceae is the seventh family for which this type of trichome has been reported. We also compiled information on stinging trichomes in all families of angiosperms. Their phylogenetic distribution indicates that they have evolved at least 12 times during angiosperm evolution and may represent an evolutionary convergence of plant defense against herbivory.

Characterization of the detrimental effects of type IV glandular trichomes on the aphid Macrosiphum euphorbiae in tomato

BACKGROUND

Glandular trichomes are essential in plants' defence against pests however, the mechanisms of action are not completely understood. While there is considerable evidence of feeding and movement impairment by trichomes, the effect on other traits is less clear. We combined laboratory and greenhouse experiments with molecular analysis to understand how glandular trichomes affect the behavior, population growth, and the expression of biomarkers involved in detoxification, primary metabolism, and developmental pathways of the aphid Macrosiphum euphorbiae. We used two isogenic tomato lines that differ in the presence of type IV glandular trichomes and production of acylsucroses; i.e.,Solanum lycopersicum cv. 'Moneymaker' and an introgressed line from Solanum pimpinellifolium (with trichomes type IV).

RESULTS

Type IV glandular trichomes affected host selection and aphid proliferation with aphids avoiding, and showing impaired multiplication on the genotype with trichomes. The exposure to type IV glandular trichomes resulted in the overexpression of detoxication markers (i.e., Hsp70, Hsp17, Hsp10); the repression of the energetic metabolism (GAPDH), and the activation of the ecdysone pathway; all these, underlying the key adaptations and metabolic trade-offs in aphids exposed to glandular trichomes.

CONCLUSION

Our results demonstrate the detrimental effect of glandular trichomes (type IV) on the aphid and put forward their mode of action. Given the prevalence of glandular trichomes in wild and cultivated Solanaceae; and of the investigated molecular biomarkers in insects in general, our results provide relevant mechanisms to understand the effect of trichomes not only on herbivorous insects but also on other trophic levels.

AaWRKY9 contributes to light- and jasmonate-mediated to regulate the biosynthesis of artemisinin in Artemisia annua

Artemisinin, isolated from Artemisia annua, is recommended as the preferred drug to fight malaria. Previous research showed that jasmonate (JA)-mediated promotion of artemisinin accumulation depended on light. However, the mechanism underlying the interaction of light and JA in regulating artemisinin accumulation is still unknown. We identified a WRKY transcription factor, AaWRKY9, using transcriptome analysis. The glandular trichome-specific AaWRKY9 positively regulates artemisinin biosynthesis by directly binding to the promoters of AaDBR2 and AaGSW1. The key regulator in the light pathway AaHY5 activates the expression of AaWRKY9 by binding to its promoter. In addition, AaWRKY9 interacts with AaJAZ9, a repressor in the JA signalling pathway. AaJAZ9 represses the transcriptional activation activity of AaWRKY9 in the absence of methyl jasmonate. Notably, in the presence of methyl jasmonate, the transcriptional activation activity of AaWRKY9 is increased. Taken together, our results reveal a novel molecular mechanism underlying AaWRKY9 contributes to light-mediated and jasmonate-mediated to regulate the biosynthesis of artemisinin in A. annua. Our study provides new insights into integrating the two signalling pathways to regulate terpene biosynthesis in plants.

argyi]

The basic features of glandular and non-glandular trichomes on leaves of Artemisia argyi( germplasms from Qichun,Ningbo,Tangyin,and Anguo,respectively) and related species A. stolonifera were observed by scanning electron microscopy( SEM)and compared. There were significant differences in trichome characteristics of leaves at all parts of A. argyi and A. stolonifera,which were closely related to the difference in chemical components. The length of non-glandular trichomes and size of glandular trichomes on middle leaves were the stablest. A. argyi and A. stolonifera can be distinguished by the density of glandular trichome. Additionally,the four germplasms of A. argyi can be discriminated via the density and curvature of non-glandular trichome. The density of non-glandular trichomes was the highest in A. stolonifera. For A. argyi,the germplasm from Qichun had the highest density of non-glandular trichomes on the abaxial surfaces of upper leaves and that from Ningbo had the largest non-glandular trichome curvature. With regard to the germplasm from Anguo,the T-shaped non-glandular trichomes of long stalks on the adaxial surfaces of the middle leaves were lodging-susceptible,and those with slender heads were wave-like. Statistics results of A. argyi and A. stolonifera are as follows: largest glandular trichomes on the adaxial and abaxial surfaces and highest glandular trichome density on the abaxial surfaces of the lower leaves in A. argyi germplasm from Ningbo,highest density of non-glandular trichomes on the abaxial surfaces of upper leaves in A. stolonifera,and highest density of glandular trichomes and non-glandular trichomes on the adaxial surfaces of the upper leaves in A. argyi germplasm from Qichun. According to the observation result under fluorescence microscope( FM),flavonoids were closely related to the size and density of non-glandular trichomes and size of glandular trichomes. The fluorescence intensity was the strongest and fluorescence area was the largest for flavonoids in A. argyi germplasms from Qichun and Tangyin,while the fluorescence for flavonoids was the weakest in A. stolonifera. It was the first time to observe and analyze the trichome ultrastructure of A. argyi leaves at different positions by SEM and FM. This study clarifies the differences between A. stolonifera and four famous A. argyi germplasms,which provides new evidence for the microscopic identification of A. argyi and its related species and serves as a reference for the study of the relationship of A. argyi structure with its components and functions.

Micromorphological Traits of Balcanic Micromeria and Closely Related Clinopodium Species (Lamiaceae)

A study of the trichomes types and distribution and pollen morphology was carried out in nine Micromeria taxa (M. cristata ssp. cristata, M. cristata ssp. kosaninii, M. croatica, M. graeca ssp. graeca, M. graeca ssp. fruticulosa, M. juliana, M. kerneri, M. longipedunculata and M. microphylla) and five closely related Clinopodium species (C. dalmaticum, C. frivaldszkyanum, C. pulegium, C. serpyllifolium and C. thymifolium) from the Lamiaceae family of the Balkan Peninsula. By scanning electron microscope, non-glandular trichomes, peltate and capitate trichomes were observed on the calyx, leaves and stem of the studied species. Two subtypes of capitate trichomes were observed in Micromeria species: subtype 1 (consisting of a basal epidermal cell and an elliptically shaped head cell) and subtype 2 (consisting of a basal epidermal cell, two to three stalk cells and a round head cell). In Clinopodium species, three types of capitate trichomes were observed: subtype 1, subtype 3 (consisting of a basal epidermal cell, a short peduncle cell, and a single round head cell), and subtype 4 (consisting of a basal epidermal cell, a stalk cell, and an elongated head cell). These results support the recent transfer of Micromeria species from the section Pseudomelissa to the genus Clinopodium.

Plant biochemistry influences tropospheric ozone formation, destruction, deposition, and response

Tropospheric ozone (O₃) is among the most damaging air pollutant to plants. Plants alter the atmospheric O₃ concentration in two distinct ways: (i) by the emission of volatile organic compounds (VOCs) that are precursors of O_3; and (ii) by dry deposition, which includes diffusion of O₃ into vegetation through stomata and destruction by nonstomatal pathways. Isoprene, monoterpenes, and higher terpenoids are emitted by plants in quantities that alter tropospheric O₃. Deposition of O₃ into vegetation is related to stomatal conductance, leaf structural traits, and the detoxification capacity of the apoplast. The biochemical fate of O₃ once it enters leaves and reacts with aqueous surfaces is largely unknown, but new techniques for the tracking and identification of initial products have the potential to open the black box.

Isoprenoid Metabolism and Engineering in Glandular Trichomes of Lamiaceae

The isoprenoids play important ecological and physiological roles in plants. They also have a tremendous impact on human lives as food additives, medicines, and industrial raw materials, among others. Though some isoprenoids are highly abundant in nature, plants produce many at extremely low levels. Glandular trichomes (GT), which cover the aerial parts of more than 25% of vascular plants, have been considered as natural biofactories for the mass production of rare industrially important isoprenoids. In several plant genera (e.g., Lavandula and Mentha), GTs produce and store large quantities of the low molecular weight isoprenoids, in particular mono- and sesquiterpenes, as essential oil constituents. Within each trichome, a group of secretory cells is specialized to strongly and specifically express isoprenoid biosynthetic genes, and to synthesize and deposit copious amounts of terpenoids into the trichome's storage reservoir. Despite the abundance of certain metabolites in essential oils and defensive resins, plants, particularly those lacking glandular trichomes, accumulate small quantities of many of the biologically active and industrially important isoprenoids. Therefore, there is a pressing need for technologies to enable the mass production of such metabolites, and to help meet the ever-increasing demand for plant-based bioproducts, including medicines and renewable materials. Considerable contemporary research has focused on engineering isoprenoid metabolism in GTs, with the goal of utilizing them as natural biofactories for the production of valuable phytochemicals. In this review, we summarize recent advances related to the engineering of isoprenoid biosynthetic pathways in glandular trichomes.

Differences in Leaf Chemistry and Glandular Trichome Density between Wild Southwestern American Hop (Humulus neomexicanus) and Commercial Hop Cultivars

The female flowers ("cones") of the hop plant (Humulus L.) produce compounds that contribute to the flavor and other properties of beer. Hop leaves and cones produce many of the same compounds, which also confer agronomic traits such as insect and disease resistance. Targeted and untargeted ultraperformance liquid chromatography-quadrupole time-of-flight-mass spectrometry with Waters MS^E technology (UPLC-QTof-MS^E) metabolomics were used to compare leaf phytochemical compositions of greenhouse-grown southwestern American wild Humulus neomexicanus (A. Nelson and Cockerell) Rydb. against a group of commercial hop cultivars consisting of both pure European Humulus lupulus L. and European-North American hybrids. Principal component analysis showed a clear distinction in chemical profiles between the two groups. H. neomexicanus leaves had a significantly higher content of total α acids (p = 4.4 × 10^-9), total bitter acids (p = 2.6 × 10^-6), cohumulone (p = 1.0 × 10^-13), humulone + adhumulone (p = 9.1 × 10^-4), and the prenylflavonoids xanthohumol (p = 0.013) and desmethylxanthohumol (p = 0.029) as well as significantly higher densities of glandular trichomes (p = 1.3 × 10^-6), the biosynthetic site of those compounds. Most flavonol glycosides measured were also significantly more abundant in H. neomexicanus (p = 1.5 × 10^-22 to 0.0027), whereas phenolic acids were consistently, but generally nonsignificantly (p > 0.05), more abundant in the cultivars. The higher bitter acid, prenylflavonoid, and flavonol glycoside content of H. neomexicanus leaves may help to confer more favorable insect and disease-resistance properties.

Cytosolic geraniol and citronellol biosynthesis require a Nudix hydrolase in rose-scented geranium (Pelargonium graveolens)

Geraniol, citronellol and their esters are high-value acyclic monoterpenes used in food technology, perfumery and cosmetics. A major source of these compounds is the essential oil of rose-scented geraniums of the genus Pelargonium. We provide evidence that their biosynthesis mainly takes place in the cytosol of glandular trichomes via geranyl monophosphate (GP) through the action of a Nudix hydrolase. Protein preparations could convert geranyl diphosphate (GDP) to geraniol in in vitro assays, a process which could be blocked by inorganic phosphatase inhibitors, suggesting a two-step conversion of GDP to geraniol. Pelargonium graveolens chemotypes enriched in either geraniol or (-)-citronellol accumulate GP or citronellyl monophosphate (CP), respectively, the presumed precursors to their monoterpenoid end products. Geranyl monophosphate was highly enriched in isolated glandular trichomes of lines producing high amounts of geraniol. In contrast, (-)-isomenthone-rich lines are depleted in these prenyl monophosphates and monoterpene alcohols and instead feature high levels of GDP, the precursor to plastidic p-menthane biosynthesis. A Nudix hydrolase cDNA from Pelargonium glandular trichomes, dubbed PgNdx1, encoded a cytosolic protein capable of hydrolyzing GDP to GP with a KM of about 750 nm but is only weakly active towards farnesyl diphosphate. In citronellol-rich lines, GDP, GP and CP were detected in nearly equimolar amounts, while citronellyl diphosphate was absent, suggesting that citronellol biosynthesis may proceed by reduction of GP to CP in this species. These findings highlight the cytosol as a compartment that supports monoterpene biosynthesis and expands the roles of Nudix hydrolases in the biosynthesis of plant volatiles.

Observational evidence of herbivore-specific associational effects between neighboring conspecifics in natural, dimorphic populations of Datura wrightii

Associational effects-in which the vulnerability of a plant to herbivores is influenced by its neighbors-have been widely implicated in mediating plant-herbivore interactions. Studies of associational effects typically focus on interspecific interactions or pest-crop dynamics. However, associational effects may also be important for species with intraspecific variation in defensive traits. In this study, we observed hundreds of Datura wrightii-which exhibits dimorphism in its trichome phenotype-from over 30 dimorphic populations across California. Our aim was to determine whether a relationship existed between the trichome phenotype of neighboring conspecifics and the likelihood of being damaged by four species of herbivorous insects. We visited plants at three timepoints to assess how these effects vary both within and between growing seasons. We hypothesized that the pattern of associational effects would provide rare morphs (i.e., focal plants that are a different morph than their neighbors) with an advantage in the form of reduced herbivory, thereby contributing to the negative frequency-dependent selection previously documented in this system. We found the best predictor of herbivory/herbivore presence on focal plants was the phenotype of the focal plant. However, we also found some important neighborhood effects. The total number of plants near a focal individual predicted the likelihood and/or magnitude of herbivory by Tupiochoris notatus, Lema daturaphila, and Manduca sexta. We also found that velvety focal plants with primarily sticky neighbors are more susceptible to infestation by Tupiochoris notatus and Lema daturaphila. This does not align with the hypothesis that associational effects at the near-neighbor scale contribute to a rare-morph advantage in this system. Overall, the results of our study show that the number and trichome-morph composition of neighboring conspecifics impact interactions between D. wrightii and insect herbivores.

Strategies to Modulate Specialized Metabolism in Mediterranean Crops: From Molecular Aspects to Field

Plant specialized metabolites (SMs) play an important role in the interaction with the environment and are part of the plant defense response. These natural products are volatile, semi-volatile and non-volatile compounds produced from common building blocks deriving from primary metabolic pathways and rapidly evolved to allow a better adaptation of plants to environmental cues. Specialized metabolites include terpenes, flavonoids, alkaloids, glucosinolates, tannins, resins, etc. that can be used as phytochemicals, food additives, flavoring agents and pharmaceutical compounds. This review will be focused on Mediterranean crop plants as a source of SMs, with a special attention on the strategies that can be used to modulate their production, including abiotic stresses, interaction with beneficial soil microorganisms and novel genetic approaches.

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.

Endemic Veronica saturejoides Vis. ssp. saturejoides-Chemical Composition and Antioxidant Activity of Free Volatile Compounds

Chemical profile and antioxidant activity of the species Veronica saturejoides Vis. ssp. saturejoides (Plantaginaceae)-which is endemic to Croatia, Bosnia and Herzegovina and Montenegro -were investigated. Volatile compounds produced by glandular trichomes (composed of one stalk cell and two elliptically formed head cells according to scanning electron microscope investigation) were isolated from the plants collected in two locations. Additionally, as a part of specialized metabolites, total polyphenols, total tannins, total flavonoids and total phenolic acids were determined spectrophotometrically. In the lipophilic volatile fractions-essential oils, the most abundant compounds identified were hexahydrofarnesyl acetone, caryophyllene oxide and hexadecanoic acid. In total, the class of oxygenated sesquiterpenes and the group of fatty aldehydes, acids and alcoholic compounds dominated in the essential oils. In the hydrophilic volatile fractions-hydrosols, the most abundant compounds identified were trans-p-mentha-1(7),8-dien-2-ol, allo-aromadendrene and (E)-caryophyllene. A group of oxygenated monoterpenes and the sesquiterpene hydrocarbons dominated in the hydrosols. Antioxidant activity of essential oils and hydrosols was tested with two methods: 2,2'-diphenyl-1-picrylhydrazyl (DPPH) and oxygen radical absorbance capacity (ORAC). Essential oils showed higher antioxidant activity than hydrosols and showed similar antioxidant activity to Rosmarinus officinalis essential oil. Obtained results demonstrate that this genus is a potential source of volatiles with antioxidant activity.

Tools to Tie: Flower Characteristics, VOC Emission Profile, and Glandular Trichomes of Two Mexican Salvia Species to Attract Bees

A plant can combine physical and chemical tools to interact with other organisms. Some are designed for pollinator attraction (i.e., colors and volatile organic compounds-VOCs); others can act to discourage herbivores (i.e., non-glandular trichomes). Few studies fully address available tools in a single species; notwithstanding, this information can be pivotal in understanding new interactions out of the home range. We characterized flower traits, emission profiles of constitutive compounds from flowers and leaves, micro-morphology of the glandular trichomes, and listed flower visitors of two Mexican bird-pollinated Salvia species (S. blepharophylla and S. greggii), growing in an Italian botanical garden. Flowers were highly variable in their morphometric characteristics. In both species, four trichome morphotypes with similar histochemistry and distribution were documented for leaves and flowers except the calyx abaxial side. The vegetative emission profiles were qualitatively more complex than the floral ones; however, common compounds occurring in high relative percentages were β-caryophyllene and germacrene D. Floral bouquets were dominated by limonene and β-pinene in S. greggii and by 1,8-cineole in S. blepharophylla. Two potential (non-bird) pollinators were especially abundant: small bees belonging to the genus Lasioglossum and large bees belonging to the species Xylocopa violacea. Our study highlights the plasticity of these plants, as well as tools that can be conveniently used to establish novel interactions.

The Quality of Greek Oregano (O. vulgare L. Subsp. hirtum (Link) Ietswaart) and Common Oregano (O. vulgare L. Subsp. vulgare) Cultivated in the Temperate Climate of Central Europe

The purpose of the study was to determine the differences between two subspecies: O. vulgare L. subsp. hirtum (Link) Ietswaart (Greek oregano) and O. vulgare L. subsp. vulgare (common oregano) growing in cultivation conditions within temperate climate of Central Europe. The characteristic of the subspecies was undertaken in terms of selected morphological parameters and the quality of the raw material. The herb of both subspecies was evaluated on the content and composition of essential oil by hydrodistillation followed by GC-MS and GC-FID (gas chromatography coupled with mass spectrometry and flame ionization detector), the total content of phenolic acids (according to PP 6th ed.) and the content of rosmarinic acid (by HPLC). The sensory evaluation (QDA) was performed, as well. Greek oregano was distinguished by visibly higher number of glandular trichomes on the leaves (up to 4.85 per 1 mm2) followed by higher content of essential oil in the herb (up to 3.36 g × 100 g-1 DW) in comparison to common oregano. Based on the essential oil composition, Greek oregano was classified as mixed carvacrol/γ-terpinene chemotype, while common oregano as mixed sabinyl/cymyl type rich in sesquiterpenes. Greek oregano was also characterized by higher total content of phenolic acids (up to 6.16 g × 100 g-1 DW) and rosmarinic acid (up to 6787.2 mg × 100 g-1 DW) than common oregano. Essential oil content reached the maximum at the beginning of blooming (common oregano) and at the full blooming stage (Greek oregano). In turn, the amount of phenolic acids followed by rosmarinic acid was the highest at the beginning of seed-setting stage, in the case of both subspecies. The differences between subspecies concerning chemical composition (especially essential oil) were reflected in the sensory attributes, where both odor and taste notes were found at higher level for Greek oregano. Results of our work indicate that Greek oregano is well adapted to grow in the temperate zone conditions. Such adaptation was reflected mainly in the satisfied yield and maintaining characters typical for the Mediterranean plant, e.g., a high essential oil content followed by high carvacrol share, traits the most important from practice viewpoint.

A novel study approach on Scutellaria altissima L. cultivated at the Ghirardi Botanic Garden (Lombardy, Italy)

Within an Open Science project, research was carried out to describe to the public of the Ghirardi Botanic Garden (BS, Lombardy, Italy) the invisible features of plants. This work is dedicated to Scutellaria altissima L. (Lamiaceae). Micromorphological, histochemical and phytochemical investigations were conducted on the vegetative and reproductive organs to correlate the structures involved in the emission of substances and their unique productivity. This work reports volatile organic compound (VOC) profiles of leaves and flowers and the composition of essential oil (EO) obtained from aerial parts of plants cultivated in Italy that have never been described before. Three morphotypes of glandular trichomes were observed: peltate, short-stalked capitate and long-stalked capitate. Peltate trichomes were the main producers of terpenes, short-stalked capitates of polysaccharides and long-stalked capitates of terpenes and polyphenols. The leaf VOC profile showed heterogeneous composition, with non-terpene derivatives as the major chemical class (71.04%), while monoterpene hydrocarbons represented almost the totality of the flower (99.73%). The leaf presented a higher number of total (37 versus 11) and exclusive (33 versus 7) compounds. (Z)-3-Hexenol acetate was most abundant in the leaf and (E)-β-ocimene in the flower. Four common compounds were detected: β-pinene, β-caryophyllene, γ-muurolene and germacrene-D. The EO contaied 21 compounds, dominated by β-caryophyllene, linalool and hexahydrofarnesyl acetone. This research allowed us to correlate morphotypes of the secretory structures with the production of secondary metabolites, with the aim of providing the public of the Ghirardi Botanic Garden with a dedicated iconographic approach, which accounts for olfactory perception linked to S. altissima.

An Acylsucrose-Producing Tomato Line Derived from the Wild Species Solanum pimpinellifolium Decreases Fitness of the Whitefly Trialeurodes vaporariorum

Simple Summary

The greenhouse whitefly, Trialeurodes vaporariorum is an insect pest of many plant crops including tomato and is especially harmful because it is a vector for a number of plant viral diseases. In this paper, an improved tomato line bred to produce glandular trichomes that exudate the deterrent compounds acylsucroses, which was introgressed from a wild tomato species, was demonstrated to decrease fitness of the insect and showed as a means for controlling the pests and, indirectly, could be an aid to reduce virus transmission to tomato plants.

Abstract

A combination of biological control and host plant resistance would be desirable for optimally controlling the greenhouse whitefly, Trialeurodes vaporariorum in tomato crops. Whitefly settlement preference, oviposition, and survivorship were evaluated on ABL 10-4 and ‘Moneymaker’, two nearly-isogenic tomato lines with, and without, whitefly-resistance traits based on type IV leaf glandular trichomes derived from the tomato wild species Solanum pimpinellifolium, respectively. Significantly reduced preference of T. vaporariorum adult whiteflies for ABL 10-4 leaves was observed. Moreover, T. vaporariorum altered its abaxial–adaxial settling performance on leaves of ABL 10-4 plants. A significantly lower tendency to settle on abaxial leaf surface was observed in ABL 10-4 compared to Moneymaker plants. Furthermore, T. vaporariorum deposited fewer eggs and exhibited a significantly reduced egg to adult survivorship in ABL 10-4 than in Moneymaker plants. Therefore, reduced fitness and distorted performance were observed for T. vaporariorum on ABL 10-4 tomato plants supporting that type IV leaf glandular trichomes might protect them from this pest and, indirectly, from the viruses it transmits.

Effect of Shading on Development, Yield and Quality of Bastard Balm Herb (Melittis melissophyllum L.)

The aim of the study was to assess the effects of Melittis melissophyllum shading on its development and accumulation of phenolics. Their content (verbascoside, apiin, luteolin-7-O-glucoside, coumarin, 3,4-dihydroxycoumarin, o-coumaric acid 2-O-glucoside as well as o-coumaric, p-coumaric, chlorogenic, caffeic, ferulic and cichoric acid) was determined in the herb using HPLC-DAD. The results showed that the content of abovementioned flavonoids and phenolic acids was highest in plants grown under full sunlight. On the other hand, a higher content of coumarin was observed in shaded plants, especially after the seed-setting stage. A similar tendency was noted for the amount of chlorophyll a and b. The content of hydrogen peroxide and malondialdehyde, the activity of polyphenol oxidase and catalase and the antioxidant capacity of plant extracts (measured using DPPH, ABTS and FRAP assays) were found to be the highest in the plants grown in full sunlight. However, the plants grown in moderate (30%) shade were found to thrive best.

Glandular trichomes: micro-organs with model status?

Glandular trichomes are epidermal outgrowths that are the site of biosynthesis and storage of large quantities of specialized metabolites. Besides their role in the protection of plants against biotic and abiotic stresses, they have attracted interest owing to the importance of the compounds they produce for human use; for example, as pharmaceuticals, flavor and fragrance ingredients, or pesticides. Here, we review what novel concepts investigations on glandular trichomes have brought to the field of specialized metabolism, particularly with respect to chemical and enzymatic diversity. Furthermore, the next challenges in the field are understanding the metabolic network underlying the high productivity of glandular trichomes and the transport and storage of metabolites. Another emerging area is the development of glandular trichomes. Studies in some model species, essentially tomato, tobacco, and Artemisia, are now providing the first molecular clues, but many open questions remain: How is the distribution and density of different trichome types on the leaf surface controlled? When is the decision for an epidermal cell to differentiate into one type of trichome or another taken? Recent advances in gene editing make it now possible to address these questions and promise exciting discoveries in the near future.

Trichome Density in Relation to Volatiles Emission and 1,8-Cineole Synthase Gene Expression in Thymus albicans Vegetative and Reproductive Organs

1,8-Cineole is the main volatile produced by Thymus albicans Hoffmanns. & Link 1,8-cineole chemotype. To understand the contribution of distinct plant organs to the high 1,8-cineole production, trichome morphology and density, as well as emitted volatiles and transcriptional expression of the 1,8-cineole synthase (CIN) gene were determined separately for T. albicans leaves, bracts, calyx, corolla and inflorescences. Scanning electron microscopy (SEM) and stereoscope microscopy observations showed the highest peltate trichome density in leaves and bracts, significantly distinct from calyx and corolla. T. albicans volatiles were collected by solid phase micro extraction (SPME) and analyzed by gas chromatography-mass spectrometry (GC/MS) and by GC for component identification and quantification, respectively. Of the 23 components identified, 1,8-cineole was the dominant volatile (57-93 %) in all T. albicans plant organs. The relative amounts of emitted volatiles clearly separated vegetative from reproductive organs. Gene expression of CIN was assigned to all organs analyzed and was consistent with the relatively high emission of 1,8-cineole in leaves and bracts. Further studies will be required to analyze monoterpenoid biosynthesis by each type of glandular trichome.

Herbivore-mediated negative frequency-dependent selection underlies a trichome dimorphism in nature

Negative frequency-dependent selection (NFDS) has been shown to maintain polymorphism in a diverse array of traits. The action of NFDS has been confirmed through modeling, experimental approaches, and genetic analyses. In this study, we investigated NFDS in the wild using morph-frequency changes spanning a 20-year period from over 30 dimorphic populations of Datura wrightii. In these populations, plants either possess glandular (sticky) or non-glandular (velvety) trichomes, and the ratio of these morphs varies substantially among populations. Our method provided evidence that NFDS, rather than drift or migration, is the primary force maintaining this dimorphism. Most populations that were initially dimorphic remained dimorphic, and the overall mean and variance in morph frequency did not change over time. Furthermore, morph-frequency differences were not related to geographic distances. Together, these results indicate that neither directional selection, drift, or migration played a substantial role in determining morph frequencies. However, as predicted by negative frequency-dependent selection, we found that the rare morph tended to increase in frequency, leading to a negative relationship between the change in the frequency of the sticky morph and its initial frequency. In addition, we found that morph-frequency change over time was significantly correlated with the damage inflicted by two herbivores: Lema daturaphila and Tupiochoris notatus. The latter is a specialist on the sticky morph and damage by this herbivore was greatest when the sticky morph was common. The reverse was true for L. daturaphila, such that damage increased with the frequency of the velvety morph. These findings suggest that these herbivores contribute to balancing selection on the observed trichome dimorphism.

Distinct metabolic pathways drive monoterpenoid biosynthesis in a natural population of Pelargonium graveolens

Pelargonium graveolens is a wild predecessor to rose-scented geranium hybrids prized for their essential oils used as fragrances and flavorings. However, little is known about their biosynthesis. Here we present metabolic evidence that at least two distinct monoterpene biosynthetic pathways contribute to their volatile profiles, namely, cyclic p-menthanes such as (-)-isomenthone and acyclic monoterpene alcohols such as geraniol and (-)-citronellol and their derivatives (referred to here as citronelloid monoterpenes). We established their common origin via the 2C-methyl-d-erythritol-4-phosphate pathway but found no indication these pathways share common intermediates beyond geranyl diphosphate. Untargeted volatile profiling of 22 seed-grown P. graveolens lines demonstrated distinct chemotypes that preferentially accumulate (-)-isomenthone, geraniol, or (-)-citronellol along with approximately 85 minor volatile products. Whole plant 13CO2 isotopic labeling performed under physiological conditions permitted us to measure the in vivo rates of monoterpenoid accumulation in these lines and quantify differences in metabolic modes between chemotypes. We further determined that p-menthane monoterpenoids in Pelargonium are likely synthesized from (+)-limonene via (+)-piperitone rather than (+)-pulegone. Exploitation of this natural population enabled a detailed dissection of the relative rates of competing p-menthane and citronelloid pathways in this species, providing real time rates of monoterpene accumulation in glandular trichomes.

Cannabis glandular trichomes alter morphology and metabolite content during flower maturation

The cannabis leaf is iconic, but it is the flowers of cannabis that are consumed for the psychoactive and medicinal effects of their specialized metabolites. Cannabinoid metabolites, together with terpenes, are produced in glandular trichomes. Superficially, stalked and sessile trichomes in cannabis only differ in size and whether they have a stalk. The objectives of this study were: to define each trichome type using patterns of autofluorescence and secretory cell numbers, to test the hypothesis that stalked trichomes develop from sessile-like precursors, and to test whether metabolic specialization occurs in cannabis glandular trichomes. A two-photon microscopy technique using glandular trichome intrinsic autofluorescence was developed which demonstrated that stalked glandular trichomes possessed blue autofluorescence correlated with high cannabinoid levels. These stalked trichomes had 12-16 secretory disc cells and strongly monoterpene-dominant terpene profiles. In contrast, sessile trichomes on mature flowers and vegetative leaves possessed red-shifted autofluorescence, eight secretory disc cells and less monoterpene-dominant terpene profiles. Moreover, intrinsic autofluorescence patterns and disc cell numbers supported a developmental model where stalked trichomes develop from apparently sessile trichomes. Transcriptomes of isolated floral trichomes revealed strong expression of cannabinoid and terpene biosynthetic genes, as well as uncharacterized genes highly co-expressed with CBDA synthase. Identification and characterization of two previously unknown and highly expressed monoterpene synthases highlighted the metabolic specialization of stalked trichomes for monoterpene production. These unique properties and highly expressed genes of cannabis trichomes determine the medicinal, psychoactive and sensory properties of cannabis products.

Ultraviolet radiation exposure time and intensity modulate tomato resistance to herbivory through activation of jasmonic acid signaling

Ultraviolet (UV) radiation can modulate plant defenses against herbivorous arthropods. We investigated how different UV exposure times and irradiance intensities affected tomato (Solanum lycopersicum) resistance to thrips (Frankliniella occidentalis) by assessing UV effects on thrips-associated damage and host-selection, selected metabolite and phytohormone contents, expression of defense-related genes, and trichome density and chemistry, the latter having dual roles in defense and UV protection. Short UV daily exposure times increased thrips resistance in the cultivar 'Moneymaker' but this could not be explained by changes in the contents of selected leaf polyphenols or terpenes, nor by trichome-associated defenses. UV irradiance intensity also affected resistance to thrips. Further analyses using the tomato mutants def-1, impaired in jasmonic acid (JA) biosynthesis, od-2, defective in the production of functional type-VI trichomes, and their wild-type, 'Castlemart', showed that UV enhanced thrips resistance in Moneymaker and od-2, but not in def-1 and Castlemart. UV increased salicylic acid (SA) and JA-isoleucine concentrations, and increased expression of SA- and JA-associated genes in Moneymaker, while inducing expression of JA-defensive genes in od-2. Our results demonstrate that UV-mediated enhancement of tomato resistance to thrips is probably associated with the activation of JA-associated signaling, but not with plant secondary metabolism or trichome-related traits.

Trichomes and Allelochemicals in Tomato Genotypes Have Antagonistic Effects Upon Behavior and Biology of Tetranychus urticae

Tomato genotypes selected for their high foliar zingiberene (ZGB) contents in a segregating F₂ population were assessed to determine their effect on behavior and biology of Tetranychus urticae Koch, the putative resistance mechanisms involved and the role of trichomes on that resistance. Genotypes with contrasting ZGB content (RVTZ-09 = low ZGB, RVTZ-79 = high ZGB, RVTZ-142 = high ZGB, and RVTZ-331 = high ZGB) were selected from an interspecific cross between wild S. habrochaites var. hirsutum accession PI-127826 (high ZGB content and resistant to mites) and S. lycopersicum cv. Redenção (low ZGB content and susceptible to mites). To determine the effect of these genotypes on mite behavior and biology, free- and no-choice tests, as well as biological studies were performed. Types and densities of trichomes on the foliar surface and their correlation with ZGB contents was determined. Genotypes rich in ZGB (RVTZ-79, RVTZ-142, and RVTZ-331) presented a high number of types IV and VI glandular trichomes, and both type IV and VI densities were positively correlated with ZGB content. In the free-choice test, T. urticae showed a high preference toward S. lycopersicum cv. Redenção and the genotype RVTZ-09 (low ZGB content), whereas, genotypes with high ZBG content were less preferred. Moreover, on high ZGB genotypes, increase in the egg incubation period and in total mortality of nymphs, and decrease of fecundity rate were observed, indicating deleterious effects in mite biology. Results indicated that high ZGB/high glandular trichome densities genotypes present both non-preference and antibiosis mechanisms of resistance to the mite.

Glandular trichomes as a barrier against atmospheric oxidative stress: Relationships with ozone uptake, leaf damage, and emission of LOX products across a diverse set of species

There is a spectacular variability in trichome types and densities and trichome metabolites across species, but the functional implications of this variability in protecting from atmospheric oxidative stresses remain poorly understood. The aim of this study was to evaluate the possible protective role of glandular and non-glandular trichomes against ozone stress. We investigated the interspecific variation in types and density of trichomes and how these traits were associated with elevated ozone impacts on visible leaf damage, net assimilation rate, stomatal conductance, chlorophyll fluorescence, and emissions of lipoxygenase pathway products in 24 species with widely varying trichome characteristics and taxonomy. Both peltate and capitate glandular trichomes played a critical role in reducing leaf ozone uptake, but no impact of non-glandular trichomes was observed. Across species, the visible ozone damage varied 10.1-fold, reduction in net assimilation rate 3.3-fold, and release of lipoxygenase compounds 14.4-fold, and species with lower glandular trichome density were more sensitive to ozone stress and more vulnerable to ozone damage compared to species with high glandular trichome density. These results demonstrate that leaf surface glandular trichomes constitute a major factor in reducing ozone toxicity and function as a chemical barrier that neutralizes the ozone before it enters the leaf.

Detailed Phytochemical Analysis of High- and Low Artemisinin-Producing Chemotypes of Artemisia annua

Chemical derivatives of artemisinin, a sesquiterpene lactone produced by Artemisia annua, are the active ingredient in the most effective treatment for malaria. Comprehensive phytochemical analysis of two contrasting chemotypes of A. annua resulted in the characterization of over 80 natural products by NMR, more than 20 of which are novel and described here for the first time. Analysis of high- and low-artemisinin producing (HAP and LAP) chemotypes of A. annua confirmed the latter to have a low level of DBR2 (artemisinic aldehyde Δ¹¹⁽¹³⁾ reductase) gene expression. Here we show that the LAP chemotype accumulates high levels of artemisinic acid, arteannuin B, epi-deoxyarteannuin B and other amorpha-4,11-diene derived sesquiterpenes which are unsaturated at the 11,13-position. By contrast, the HAP chemotype is rich in sesquiterpenes saturated at the 11,13-position (dihydroartemisinic acid, artemisinin and dihydro-epi-deoxyarteannunin B), which is consistent with higher expression levels of DBR2, and also with the presence of a HAP-chemotype version of CYP71AV1 (amorpha-4,11-diene C-12 oxidase). Our results indicate that the conversion steps from artemisinic acid to arteannuin B, epi-deoxyarteannuin B and artemisitene in the LAP chemotype are non-enzymatic and parallel the non-enzymatic conversion of DHAA to artemisinin and dihyro-epi-deoxyarteannuin B in the HAP chemotype. Interestingly, artemisinic acid in the LAP chemotype preferentially converts to arteannuin B rather than the endoperoxide bridge containing artemisitene. In contrast, in the HAP chemotype, DHAA preferentially converts to artemisinin. Broader metabolomic and transcriptomic profiling revealed significantly different terpenoid profiles and related terpenoid gene expression in these two morphologically distinct chemotypes.

Plant Volatiles: Going 'In' but not 'Out' of Trichome Cavities

Plant glandular trichomes are able to secrete and store large amounts of volatile organic compounds (VOCs). VOCs typically accumulate in dedicated extracellular spaces, which can be either subcuticular, as in the Lamiaceae or Asteraceae, or intercellular, as in the Solanaceae. Volatiles are retained at high concentrations in these storage cavities with limited release into the atmosphere and without re-entering the secretory cells, where they would be toxic. This implies the existence of mechanisms allowing transport of VOCs to the cavity but preventing their diffusion out once they have been delivered. The cuticle and cell wall lining the cavity are likely to have key roles in retaining volatiles, but their exact composition and the potential molecular players involved are largely unknown.

Localisation of Two Bioactive Labdane Diterpenoids in the Peltate Glandular Trichomes of Leonurus japonicus by Laser Microdissection Coupled with UPLC-MS/MS

INTRODUCTION

Glandular trichomes of plants are biochemical factories that could produce, store and secrete copious pharmaceutically important natural products. The Labiatae plant Leonurus japonicus is an important traditional Chinese medicine used to treat gynecological diseases, and has abundant peltate glandular trichomes (PGTs), in which the secondary metabolites accumulated are still unknown.

OBJECTIVE

To study the secondary metabolites specifically accumulated in the PGTs of L. japonicus and their biological activities, and provide a new way to pinpoint bioactive natural products from plants.

METHODOLOGY

Morphology of the trichomes on L. japonicus were observed under a scanning electron microscope. The PGTs of L. japonicus were precisely collected using laser microdissection (LMD) and analysed for their secondary metabolites with ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Targeted compounds were isolated with classical phytochemical methods, and their structures were elucidated by spectroscopic analysis. Biological activities were evaluated by in vitro assays.

RESULTS

Two labdane diterpenoids, leoheterin (1) and galeopsin (2), were localised in the PGTs of L. japonicus. Antithrombotic activity of 1 in anti-platelet aggregation assay induced by arachidonic acid was observed. Both compounds showed potential anti-inflammatory activity by inhibiting proinflammatory cytokine TNF-α. In addition, anti-proliferative effect of both compounds on several cancer cell lines was also detected.

CONCLUSION

Two bioactive labdane diterpenoids were localised in the PGTs of L. japonicus. The findings suggested that it might be an efficient approach to explore bioactive natural products from the glandular trichomes of medicinal plants with LMD-UPLC/MS/MS. Copyright © 2017 John Wiley & Sons, Ltd.

Secretory structures and essential oil composition in Stachys officinalis (L.) Trevisan subsp. officinalis (Lamiaceae) from Italy

The secretory structures and the volatile fraction of Stachys officinalis (L.) Trevisan subsp. officinalis (Lamiaceae) from Italy were studied for the first time. Peltate and small capitate trichomes were observed on the whole plant (leaves and inflorescences). In the peltate trichomes, an unusual polyphenols content was evidenced by the histochemical methods. The volatile fraction was obtained by a solvent extract from the distillation water of leaves and inflorescences and analysed by GC-MS. Forty-four constituents for leaves, representing 94.1% of the total volatiles, and 57 compounds for flowers, accounting for 90.1% of the total volatiles, were identified. (E)-caryophyllene (20.1%), (E)-nerolidol (14.3%), caryophyllene oxide (6.1%) and γ-cadinene (5.7%) were recognised as the main constituents for the leaf volatile fraction, while caryophyllene oxide (16.5%), (E)-nerolidol (15.4%), humulene epoxide II (9.2%) and α-pinene (7.0%) were the main compounds for the flower volatile fraction.

In vitro reconstruction and analysis of evolutionary variation of the tomato acylsucrose metabolic network

Plant glandular secreting trichomes are epidermal protuberances that produce structurally diverse specialized metabolites, including medically important compounds. Trichomes of many plants in the nightshade family (Solanaceae) produce O-acylsugars, and in cultivated and wild tomatoes these are mixtures of aliphatic esters of sucrose and glucose of varying structures and quantities documented to contribute to insect defense. We characterized the first two enzymes of acylsucrose biosynthesis in the cultivated tomato Solanum lycopersicum. These are type I/IV trichome-expressed BAHD acyltransferases encoded by Solyc12g006330--or S. lycopersicum acylsucrose acyltransferase 1 (Sl-ASAT1)--and Solyc04g012020 (Sl-ASAT2). These enzymes were used--in concert with two previously identified BAHD acyltransferases--to reconstruct the entire cultivated tomato acylsucrose biosynthetic pathway in vitro using sucrose and acyl-CoA substrates. Comparative genomics and biochemical analysis of ASAT enzymes were combined with in vitro mutagenesis to identify amino acids that influence CoA ester substrate specificity and contribute to differences in types of acylsucroses that accumulate in cultivated and wild tomato species. This work demonstrates the feasibility of the metabolic engineering of these insecticidal metabolites in plants and microbes.

Diversity of fig glands is associated with nursery mutualism in fig trees

PREMISE OF THE STUDY

Fig trees (Moraceae) have remarkable enclosed inflorescences called figs or syconia. The flowers are pollinated by host-specific fig wasps that enter the fig to lay their eggs. This nursery pollination system is one of the most studied of tropical mutualism interactions, but the source of the volatiles that attract fig wasps to their specific host figs has not been confirmed. The fragrance is the basis of host selection and, therefore, of reproductive isolation among sympatric Ficus species. This study locates and characterizes the glands likely to be responsible for pollinator attraction and also protection from herbivory in the figs of nine Ficus species representing all the major lineages within the genus.

METHODS

Figs with receptive pistillate flowers were examined using light and scanning electron microscopy. Tests for histolocalization of substances were employed to detect glandular activity throughout the figs.

KEY RESULTS

A great diversity of glands is found throughout the fig, and for the first time, the sites producing fragrances are identified. Scent glands are present on the ostiolar bracts and the outer layers of the fig receptacle. Laticifers and phenolic-producing idioblasts, epidermis, and trichomes associated with fig protection occur on the ostiolar bracts, the fig receptacle, and floral tissues.

CONCLUSIONS

The volatiles produced by glands on the ostiolar bracts are candidate sources for the long-distance attraction of pollinator fig wasps. Scent glands on the outer layers of the receptacle may also play a role in chemical perception of the figs or may be related to their protection. The high cost to the plants if the figs are eaten and the temperature conditions required for nursery pollination are likely the factors that led to the selection of phenolic glands and laticifers during the group's evolution.

A universal glue: underwater adhesion of the secretion of the carnivorous flypaper plant Roridula gorgonias

Glandular trichomes of the carnivorous plant Roridula gorgonias release a viscous resinous secretion. Its adhesion to hydrophilic and hydrophobic glass surfaces was measured in air and underwater. The underwater adhesion reached up to 91% (on hydrophilic glass) and 28% (on hydrophobic glass) of that measured in the air. After being submersed for 24 h in water, trichomes did not lose their ability to adhere to both types of glass surfaces underwater. We assume that acylglycerides and triterpenoids, which have been demonstrated previously to be main compounds of the secretion, cause the predominantly non-polar character and the insolubility in water. The robustness of the secretion to a wet environment presumably enables the plant to maintain its trapping function also under humid conditions and during rainy weather.

Repellency and oviposition deterrence of wild tomato leaf extracts to spider mites, Tetranychus urticae Koch

Glandular trichomes on the leaves of wild tomato, Lycopersicon hirsutum f. hirsutum Mull, also known as Solanum habrochaites (Solanaceae) synthesize and accumulate high levels of methyl ketones (MKs). The potential of using MKs as alternatives to synthetic acaricides for controlling the twospotted spider mite, Tetranychus urticae Koch, is explored in this study. Plants of L. hirsutum accession LA 407 having high concentrations of MKs were grown from seeds under greenhouse conditions. The main objective of this investigation was to conduct bioassays that are quick and easy to implement, consistent over time (repeatable) with the ability to utilize small amounts of test material for testing repellency and fecundity (number of eggs laid by a female mite) of MKs in pure forms and in LA 407 crude extracts. Four MKs (2-tridecanone, 2-undecanone, 2-dodecanone, 2-pentadecanone) and their mixture were screened for their repellency and ability to alter fecundity of spider mites. All MKs repelled spider mites at the two periods tested. Following spraying of tomato leaf extracts prepared in ethanol (ethanol extracts), average number of eggs laid per female mite on bean leaf discs dropped from 0.8 to 0.3 and from 0.9 to 0.3 at 4 and 24 h after exposure representing 65 and 68% reduction, respectively. However, spraying of tomato leaf extracts prepared in water (water extracts) reduced number of eggs laid per female mite from 1.7 to 0.7 and from 2.6 to 0.9 at 4 and 24 h after exposure representing 60 and 67% reduction, respectively. We concluded that all MKs have repellent and egg laying deterrence activities against spider mites. This investigation suggests that ethanol and water extracts of LA 407 have a potential for repelling female spider mites and reducing their laid eggs which might be explored under field conditions for managing populations of spider mites, which could reduce reliance on synthetic acaricides.

Trichomes related to an unusual method of water retention and protection of the stem apex in an arid zone perennial species

It is well known that trichomes protect plant organs, and several studies have investigated their role in the adaptation of plants to harsh environments. Recent studies have shown that the production of hydrophilic substances by glandular trichomes and the deposition of this secretion on young organs may facilitate water retention, thus preventing desiccation and favouring organ growth until the plant develops other protective mechanisms. Lychnophora diamantinana is a species endemic to the Brazilian 'campos rupestres' (rocky fields), a region characterized by intense solar radiation and water deficits. This study sought to investigate trichomes and the origin of the substances observed on the stem apices of L. diamantinana. Samples of stem apices, young and expanded leaves were studied using standard techniques, including light microscopy and scanning and transmission electron microscopy. Histochemical tests were used to identify the major groups of metabolites present in the trichomes and the hyaline material deposited on the apices. Non-glandular trichomes and glandular trichomes were observed. The material deposited on the stem apices was hyaline, highly hydrophilic and viscous. This hyaline material primarily consists of carbohydrates that result from the partial degradation of the cell wall of uniseriate trichomes. This degradation occurs at the same time that glandular trichomes secrete terpenoids, phenolic compounds and proteins. These results suggest that the non-glandular trichomes on the leaves of L. diamantinana help protect the young organ, particularly against desiccation, by deposition of highly hydrated substances on the apices. Furthermore, the secretion of glandular trichomes probably repels herbivore and pathogen attacks.

Next generation sequencing unravels the biosynthetic ability of spearmint (Mentha spicata) peltate glandular trichomes through comparative transcriptomics

BACKGROUND

Plant glandular trichomes are chemical factories with specialized metabolic capabilities to produce diverse compounds. Aromatic mint plants produce valuable essential oil in specialised glandular trichomes known as peltate glandular trichomes (PGT). Here, we performed next generation transcriptome sequencing of different tissues of Mentha spicata (spearmint) to identify differentially expressed transcripts specific to PGT. Our results provide a comprehensive overview of PGT's dynamic metabolic activities which will help towards pathway engineering.

RESULTS

Spearmint RNAs from 3 different tissues: PGT, leaf and leaf stripped of PGTs (leaf-PGT) were sequenced by Illumina paired end sequencing. The sequences were assembled de novo into 40,587 non-redundant unigenes; spanning a total of 101 Mb. Functions could be assigned to 27,025 (67%) unigenes and among these 3,919 unigenes were differentially expressed in PGT relative to leaf - PGT. Lack of photosynthetic transcripts in PGT transcriptome indicated the high levels of purity of isolated PGT, as mint PGT are non-photosynthetic. A significant number of these unigenes remained unannotated or encoded hypothetical proteins. We found 16 terpene synthases (TPS), 18 cytochrome P450s, 5 lipid transfer proteins and several transcription factors that were preferentially expressed in PGT. Among the 16 TPSs, two were characterized biochemically and found to be sesquiterpene synthases.

CONCLUSIONS

The extensive transcriptome data set renders a complete description of genes differentially expressed in spearmint PGT. This will facilitate the metabolic engineering of mint terpene pathway to increase yield and also enable the development of strategies for sustainable production of novel or altered valuable compounds in mint.

Wild tomato leaf extracts for spider mite and cowpea aphid control

Glandular trichomes on the leaves of wild tomato, L. hirsutum f. hirsutum Mull, also known as Solanum habrochaites (Solanaceae), synthesize and accumulate high levels of methyl ketones (MKs). L. hirsutum accession LA 407, having high concentration of MKs, was grown from seeds under greenhouse conditions. Four MKs (2-undecanone, 2-dodecanone, 2-tridecanone, and 2-pentadecanone) were screened for their toxicity to spider mites, Tetranychus urticae Koch and cowpea aphids, Aphis craccivora Koch. The objectives of this investigation were to: (1) develop a bioassay for testing MKs on spider mite and cowpea aphid mortality and (2) compare the efficacies of wild tomato leaf crude extracts and pure standard materials of MKs against spider mite and cowpea aphid mortality. Our results revealed that spider mites are most sensitive to 2-tridecanone (LC50 = 0.08 μmole cm(-2) of treated leaf surface) and least sensitive to 2-undecanone (LC50 = 1.5 μmole cm(-2) of treated leaf surface) 4 h after treatment. Similarly, 2-tridecanone caused greatest mortality (LC50 = 0.2 μmole cm(-2) of treated leaf surface), whereas 2-undecanone caused the lowest morality (LC50 = 0.48 μmole cm(-2) of treated surface) of cowpea aphid. We concluded that all MKs tested in this investigation are toxic to spider mites and aphids. 2-Tridecanone is more effective in killing mites and aphids compared to other MKs. Toxicity of crude extracts, prepared from the leaves of L. hirsutum accession LA 407, to spider mites and cowpea aphids revealed greater mortality compared to a combined mixture of MKs standard material (used at the same concentration as found on LA 407 leaves). This indicates that in addition to MKs, other unidentified compounds in LA 407 leaf extract also have pesticidal properties. Accordingly, leaf extracts of LA 407 could be explored in crop protection, and they might open a new area of MK formulations and discovery of biorational alternatives for pest control in agricultural fields.