Glandular hairs of Sigesbeckia jorullensis Kunth (Asteraceae): morphology, histochemistry and composition of essential oil

Exploring the co-operativity of secretory structures for defense and pollination in flowering plants

MAIN CONCLUSION

In flowers multiple secretory systems cooperate to deliver specialized metabolites to support specific roles in defence and pollination. The collective roles of cell types, enzymes, and transporters are discussed. The interplay between reproductive strategies and defense mechanisms in flowering plants has long been recognized, with trade-offs between investment in defense and reproduction predicted. Glandular trichomes and secretory cavities or ducts, which are epidermal and internal structures, play a pivotal role in the secretion, accumulation, and transport of specialized secondary metabolites, and contribute significantly to defense and pollination. Recent investigations have revealed an intricate connection between these two structures, whereby specialized volatile and non-volatile metabolites are exchanged, collectively shaping their respective ecological functions. However, a comprehensive understanding of this profound integration remains largely elusive. In this review, we explore the secretory systems and associated secondary metabolism primarily in Asteraceous species to propose potential shared mechanisms facilitating the directional translocation of these metabolites to diverse destinations. We summarize recent advances in our understanding of the cooperativity between epidermal and internal secretory structures in the biosynthesis, secretion, accumulation, and emission of terpenes, providing specific well-documented examples from pyrethrum (Tanacetum cinerariifolium). Pyrethrum is renowned for its natural pyrethrin insecticides, which accumulate in the flower head, and more recently, for emitting an aphid alarm pheromone. These examples highlight the diverse specializations of secondary metabolism in pyrethrum and raise intriguing questions regarding the regulation of production and translocation of these compounds within and between its various epidermal and internal secretory systems, spanning multiple tissues, to serve distinct ecological purposes. By discussing the cooperative nature of secretory structures in flowering plants, this review sheds light on the intricate mechanisms underlying the ecological roles of terpenes in defense and pollination.

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.

Phytochemical Composition of Combretum molle (R. Br. ex G. Don.) Engl. & Diels Leaf and Stem Extracts

The demand for medicinal plants is on a rise due to their affordability, accessibility and relatively non-toxic nature. Combretum molle (Combretaceae) is used in African traditional medicine to treat a number of diseases. This study aimed to screen the phytochemical composition of the hexane, chloroform and methanol extracts of C. molle leaves and stems using qualitative phytochemical screening. Additionally, the study aimed to identify the functional phytochemical groups, determine the elemental composition and provide a fluorescence characterization of the powdered leaves and stems by performing Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray (EDX) microanalyses and fluorescence microscopy. Phytochemical screening revealed the presence of alkaloids, flavonoids, phenolic compounds, polyphenols, terpenoids, tannins, coumarins, saponins, phytosterols, gums, mucilage, carbohydrates, amino acids and proteins within all leaf and stem extracts. Lipids and fixed oils were additionally present within the methanol extracts. FTIR demonstrated significant peaks in absorption frequency in the leaf at wavelengths of 3283.18, 2917.81, 1617.72, 1318.83, 1233.97, 1032.32 and 521.38 cm^-1, and in the stem at 3318.91, 1619.25, 1317.13, 1032.68, 780.86 and 516.39 cm^-1. These corresponded to the functional groups of chemical compounds including alcohols, phenols, primary amines, alkyl halides, alkanes and alkyl aryl ethers, corroborating the presence of the detected phytochemicals within the plant. EDX microanalyses showed the elemental composition of the powdered leaves (68.44% C, 26.72% O, 1.87% Ca, 0.96% Cl, 0.93% Mg, 0.71% K, 0.13% Na, 0.12 % Mn and 0.10% Rb) and stems (54.92% C, 42.86% O, 1.7% Ca, 0.43% Mg and 0.09% Mn). Fluorescence microscopy provided a characteristic evaluation of the plant in its powdered form and revealed distinct colour changes in the material when treated with various reagents and viewed under ultraviolet light. In conclusion, the phytochemical constituents of the leaves and stems of C. molle confirm the suitability of this species for use in traditional medicine. The findings from this study suggest the need to validate the use of C. molle in the development of modern medicines.

Secretory Products in Petals of Centaurea cyanus L. Flowers: A Histochemistry, Ultrastructure, and Phytochemical Study of Volatile Compounds

(1) Background: Centaurea cyanus L. is a medicinal plant whose flowers are widely used in herbal medicine. The aim of the study was to localise flower tissues that are responsible for the production of secretory products in petals and to analyse the volatile compounds. The volatile compounds of the flowers of this species have not been investigated to date. (2) Methods: Light, fluorescence, scanning and transmission electron microscopy techniques were used in the study. Lipophilic compounds were localised in the tissues using histochemical assays. Volatile compounds were determined with the use of solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). (3) Results: The study showed production of secretion in the petal parenchyma, whose ultrastructure has features of a secretory tissue. The lipophilic secretion was localised in the cells and intercellular spaces of the parenchyma and in the walls and surface of epidermal cells, where it accumulated after release through cuticle microchannels. Sesquiterpenes were found to constitute the main group of volatile compounds, with the highest content of β-caryophyllene (26.17%) and α-humulene (9.77%). (4) Conclusions: Given the presence of some volatile components that are often found in resins (caryophyllene, delta-cadinene) and the abundant secretion residues on the epidermal surface, we suppose that the C. cyanus secretion released by the flowers is a resinaceous mixture (oleoresin), which is frequently found in plants, as shown by literature data. This secretion may play an important role in the therapeutic effects of C. cyanus flowers.

Micropropagation of Rare Scutellaria havanensis Jacq. and Preliminary Studies on Antioxidant Capacity and Anti-Cancer Potential

We report the development of in vitro propagation protocols through an adventitious shoot induction pathway for a rare and medicinal Scutellaria havanensis. In vitro propagation studies using nodal explants showed MS medium supplemented with 10 µM 6-Benzylaminopurine induced the highest number of adventitious shoots in a time-dependent manner. A ten-day incubation was optimum for shoot bud induction as longer exposures resulted in hyperhydricity of the explants and shoots induced. We also report preliminary evidence of Agrobacterium tumefaciens EHA105-mediated gene transfer transiently expressing the green fluorescent protein in this species. Transformation studies exhibited amenability of various explant tissues, internode being the most receptive. As the plant has medicinal value, research was carried out to evaluate its potential antioxidant capacity and the efficacy of methanolic leaf extracts in curbing the viability of human colorectal cancer cell line HCT116. Comparative total polyphenol and flavonoid content measurement of fresh and air-dried leaf extract revealed that the fresh leaf extracts contain higher total polyphenol and flavonoid content. The HCT 116 cell viability was assessed by colorimetric assay using a 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide, showed a steady growth inhibition after 24 h of incubation. Scanning electron microscopy of leaf surface revealed a high density of glandular and non-glandular trichomes. This research provides a basis for the conservation of this rare plant and future phytochemical screening and clinical research.

Chemical Composition of Miscanthus sinensis var. purpurascens Flower Absolute and Its Beneficial Effects on Skin Wound Healing and Melanogenesis-Related Cell Activities

Miscanthus sinensis var. purpurascens (MSP, flame grass) is found in Korea, Japan, and China, and its biological activities include anti-cancer, detoxifying, vasodilatory, antipyretic, and diuretic effects. However, no study has investigated the effects of MSP on skin-related biological activities. In this study, we explored the effects of the absolute extracted from the MSP flowers (MSPFAb) on skin wound healing- and whitening-related responses in keratinocytes or melanocytes. MSPFAb contained 6 components and induced the proliferation, migration, and syntheses of type I and IV collagens in keratinocytes. MSPFAb also increased the phosphorylations of serine/threonine-specific protein kinase, p38 mitogen-activated protein kinase, and extracellular signal-regulated kinase1/2 in keratinocytes. In addition, treatment with MSPFAb decreased serum-induced melanoma cell proliferation and inhibited tyrosinase activity and melanin contents in α-MSH-stimulated melanoma cells. Taken together, this study indicates MSPFAb may promote wound healing- and whitening-associated activities in dermal cells, and suggests that it has potential use as a wound healing and skin whitening agent.

Comparative ultrastructure of trichomes on various organs of Rosa roxburghii

Chestnut rose, R. roxburghii Tratt. (Rosaceae) (RR) is an important crop in China due to its nutritional and medicinal values. RR frequently produces trichomes on the surfaces of a diverse range of organs, however a genetic component exists to the control of trichome development, with some cultivars having significantly fewer trichomes to others. Certain varieties have fruits that are thickly covered with macroscopic trichomes, which is an undesirable trait for fruit processing and consumption. However, smooth-fruit cultivars exist, such as R. roxburghii Tratt. f. esetosa Ku (RRE). Despite their economic importance, the anatomical features of trichomes have not been explored in detail for these two chestnut rose germplasms. Here, we investigate the ultrastructure of trichomes distributed on the stem, sepal, and fruit of RR and RRE using transmission electron microscopy (TEM). The internal structure of stem prickle trichomes in RR and RRE was oval in shape and did not contain nucleoli or other organelles. The cell walls of stem prickles in RR are thick and the intercellular spaces occupied with liquid, whereas the cells wall of stem prickles in RRE are thin and have air-filled intercellular spaces. The cells of sepal acicular trichomes in RR and glandular trichomes (GTs) of sepals in RRE had similar vacuole sizes, cytoplasm content, intercellular spaces, and arrangement of plastids within cells. However, there were osmiophilic granules present in the GTs of RRE. The flagelliform trichomes in the sepals of the two germplasms are composed of oval or rod-shaped cells. Although the flagelliform trichomes in the sepals of the two germplasms had a similar internal structure, and both contained starch grains and plastids with visible thylakoid membranes, the flagelliform trichomes in the sepals of RR had a thinner cell wall and a higher proportion of cytoplasm which was more evenly distributed across the cell. There were granules that stained heavily with osmium tetroxide which occurred infrequently in the flagelliform trichomes of sepals in RRE but were not observed in RR. On the acicular trichomes of fruit in RR, the flagelliform trichomes and the GTs of fruit in RRE shared similar cell morphology, arrangement and vacuole size as well as intercellular space. Both the fruit flagelliform trichomes and GTs in RRE contain granules which stain heavily with osmium tetroxide, and the GTs contain plastids and starch grains. These differences in trichome cell ultrastructure may be related to developmental processes or biological functions of the trichomes. These results also suggest that the two chestnut rose germplasms are good candidates for further study of trichome ontogeny in the genus and subsequent breeding of the smooth organ trait in this species.

Changes in the morphological traits and the essential oil content of sweet basil (Ocimum basilicum L.) as induced by cadmium and lead treatments

The purpose of this study was to inspect the suitability of sweet basil (Ocimum basilicum L.) for phytoremediation of heavy metal contaminated areas in urban areas of Egypt through study the impact of the most common pollutants in Egypt: cadmium (Cd) and lead (Pb) on morphological traits, and essential oil (EOs) content of sweet basil. The experiment was done under greenhouse conditions. The applied treatments tested in this experiment for Cd concentrations were 5, 10, 15, 20, 25 ppm and for Pb: 100, 350, 750, 1,000, 1,500 ppm. The composition and amount of EOs had been determined using gas chromatography-mass spectrometry (GC-MS). The results indicated that the contaminated soil had a negative impact on morphological traits development of basil (e.g., the decline in fresh and dry weight). The increase in Cd and Pb concentrations enhanced the EOs yield of sweet basil. In general, basil cultivation in the Cd and Pb contaminated soils could cause undesirable impacts on morphological traits; however, the presence of these metals has a major influence on the EOs yield, composition, and phytoremediation of the soil.

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.

Chemical profile and phytotoxic action of Onopordum acanthium essential oil

The potential of utilizing Onopordum acanthium essential oil and its major constituents as environment friendly herbicides was investigated. In total 29, 25, and 18 compounds were identified from flower, leaf, and stem oils, representing 94.77%, 80.02%, and 90.74% of the total oil, respectively. Flower and stem oils were found to be rich in n-alkanes, which accounted for 57.33% in flower oil, and 82.33% in stem oil. Flower oil exerted potent inhibitory activity on both receiver species, Amaranthus retroflexus and Poa annua, which nearly completely suppressed seed germination at 5 mg/mL, and β-eudesmol is the most likely responsible compound for its phytotoxicity; in comparison, leaf and stem oils exhibited much weaker inhibitory activity on A. retroflexus, and stimulatory effect on P. annua when tested concentration was below 2.5 mg/mL. Alkanes in the oils were found to exert relatively weak plant growth regulatory activity. This report is the first on the chemical profile and phytotoxic action of O. acanthium oil as well as the phytotoxicity of β-eudesmol.

Microstructural and histochemical analysis of aboveground organs of Centaurea cyanus used in herbal medicine

Centaurea cyanus L. is a valuable source of many different bioactive substances. It is used in herbal medicine, but the structure of its organs used as raw material and secretory tissues has been insufficiently examined. The aim of this paper was to investigate the microstructure of C. cyanus flowers, bracts, stems and leaves with particular emphasis on secretory structures. Moreover, the main classes of secondary metabolites present in the secretion were identified and the taxonomic value of some micromorphological and anatomical features was analysed. Histochemical, micromorphological and ultrastructural analyses of aboveground organs of C. cyanus were carried out using light, fluorescence, scanning and transmission electron microscopy. The analyses revealed the presence of petal papillae and a characteristic cuticular pattern on the petals, stamens and stylar hairs. There were four types of non-glandular trichomes on the bracts, leaves and stem surfaces. The epidermal cells of the bracts contained prismatic calcium oxalate crystals. Two kinds of secretory structures, i.e. glandular trichomes and ducts, were observed in the C. cyanus organs. The glandular trichomes were located on the bract and stem surfaces, and the ducts were detected in the leaves and stems. Ultrastructural analyses of the epithelium of the ducts showed the presence of strongly osmiophilic insoluble phenolic material in vacuoles as well as moderately osmiophilic insoluble lipidic material in elaioplasts and vesicles. The results of histochemical assays showed a heterogeneous nature of the duct secretion, which contained essential oil, lipids, flavonoids, tannins and terpenes containing steroids.

Essential Oil from Arnica Montana L. Achenes: Chemical Characteristics and Anticancer Activity

Mountain arnica Arnica montana L. is a source of several metabolite classes with diverse biological activities. The chemical composition of essential oil and its major volatile components in arnica may vary depending on the geographical region, environmental factors, and plant organ. The objective of this study was to characterize the chemical composition of essential oil derived from A. montana achenes and to investigate its effect on induction of apoptosis and autophagy in human anaplastic astrocytoma MOGGCCM and glioblastoma multiforme T98G cell lines. The chemical composition of essential oil extracted from the achenes was examined with the use of Gas Chromatography–Mass Spectrometry GC-MS. Only 16 components of the essential oil obtained from the achenes of 3-year-old plants and 18 components in the essential oil obtained from the achenes of 4-year-old plants constituted ca. 94.14% and 96.38% of the total EO content, respectively. The main components in the EO from the arnica achenes were 2,5-dimethoxy-p-cymene (39.54 and 44.65%), cumene (13.24 and 10.71%), thymol methyl ether (8.66 and 8.63%), 2,6-diisopropylanisole (8.55 and 8.41%), decanal (7.31 and 6.28%), and 1,2,2,3-tetramethylcyclopent-3-enol (4.33 and 2.94%) in the 3- and 4-year-old plants, respectively. The essential oils were found to exert an anticancer effect by induction of cell death in anaplastic astrocytoma and glioblastoma multiforme cells. The induction of apoptosis at a level of 25.7–32.7% facilitates the use of this secondary metabolite in further studies focused on the development of glioma therapy in the future. Probably, this component plays a key role in the anticancer activity against the MOGGCCM and T98G cell lines. The present study is the first report on the composition and anticancer activities of essential oil from A. montana achenes, and further studies are required to explore its potential for future medicinal purposes.

Mechanistic investigations on multiproduct β-himachalene synthase from Cryptosporangium arvum

A bacterial terpene synthase from Cryptosporangium arvum was characterised as a multiproduct β-himachalene synthase. In vitro studies showed not only a high promiscuity with respect to its numerous sesquiterpene products, including the structurally demanding terpenes longicyclene, longifolene and α-longipinene, but also to its substrates, as additional activity was observed with geranyl- and geranylgeranyl diphosphate. In-depth mechanistic investigations using isotopically labelled precursors regarding the stereochemical course of both 1,11-cyclisation and 1,3-hydride shift furnished a detailed catalytic model suggesting the molecular basis of the observed low product selectivity. The enzyme’s synthetic potential was also exploited in the preparation of sesquiterpene isotopomers, which provided insights into their EIMS fragmentation mechanisms.

Glandular trichomes of the leaves in three Doronicum species (Senecioneae, Asteraceae): morphology, histochemistry, and ultrastructure

Two types of glandular tichomes (GTs) develop on the leaves in three Doronicum species. The purpose of the work was to establish common and distinctive morphological, anatomical, histochemical, and ultrustructural features of the trichomes. It turned out that differences between types of trichomes are more significant than interspecific ones. For each Doronicum species, differences between GTs of two types include the dimensions, intensity of coloration by histochemical dyes, as well as ultrastructural features of the cells. The GTs of the first type are higher than GTs of the second type. Two to three upper cell layers of the first trichomes develop histochemical staining, whereas in the second ones, only apical cells give a positive histochemical reaction. In all trichomes, polysaccharides, polyphenols, and terpenoids are detected. In the GTs of the first type, polysaccharides are synthesized in larger quantity; in the GTs of the second type, synthesis of the secondary metabolites predominates. Main ultrastructural features of the GTs of the first type include proliferation of RER and an activity of Golgi apparatus denoting the synthesis of enzymes and pectin; however, development of SER, diversiform leucoplasts with reticular sheaths, and chloroplasts with peripheral plastid reticulum also demonstrate the synthesis of lipid substances. The ultrastructural characteristics of the second type GTs indicate the primary synthesis of lipid components. Secretion is localized in a periplasmic space of the upper cell layers. The secretory products pass through the cell wall, accumulate in the subcuticular cavity, and rupture it.

5-Deoxynevadensin, a Novel Flavone in Sunflower and Aspects of Biosynthesis during Trichome Development

Fluorescent microscopy of sunflower glandular trichomes from anther appendages showed a strong autofluorescence, caused by flavonoids. Chemical analysis with NMR and HRMS lead to the identification of 7-hydroxy-6,8,4′-trimethoxyflavone, a novel 5-deoxyflavone, exclusively found in capitate glandular trichomes. 5-Deoxy flavones are rarely found in Asteraceae and this is the first report for the genus Helianthus. Semiquantitative RT-PCR showed the presence of transcripts for phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS) in all biosynthetically active trichome stages. PAL and CHS are key steps in flavonoid biosynthesis indicating that flavonoid production occurs directly within the trichomes. This offers the possibility for examination of 5-deoxyflavone biosynthesis within Asteraceae.

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.

Morphology of glandular trichomes of Japanese catnip (Schizonepeta tenuifolia Briquet) and developmental dynamics of their secretory activity

Schizonepeta tenuifolia Briquet, commonly known as Japanese catnip, is used for the treatment of colds, headaches, fevers, and skin rashes in traditional Asian medicine (China, Japan and Korea). The volatile oil and its constituents have various demonstrated biological activities, but there is currently limited information regarding the site of biosynthesis. Light microscopy and scanning electron microscopy indicated the presence of three distinct glandular trichome types which, based on their morphological features, are referred to as peltate, capitate and digitiform glandular trichomes. Laser scanning microscopy and 3D reconstruction demonstrated that terpenoid-producing peltate glandular trichomes contain a disk of twelve secretory cells. The oil of peltate glandular trichomes, collected by laser microdissection or using custom-made micropipettes, was demonstrated to contain (-)-pulegone, (+)-menthone and (+)-limonene as major constituents. Digitiform and capitate glandular trichomes did not contain appreciable levels of terpenoid volatiles. The yield of distilled oil from spikes was significantly (44%) higher than that from leaves, while the composition of oils was very similar. Oils collected directly from leaf peltate glandular trichomes over the course of a growing season contained primarily (-)-pulegone (>80% at 32 days after germination) in young plants, while (+)-menthone began to accumulate later (>75% at 80 days after germination), at the expense of (-)-pulegone (the levels of (+)-limonene remained fairly stable at 3-5%). The current study establishes the morphological and chemical characteristics of glandular trichome types of S. tenuifolia, and also provides the basis for unraveling the biosynthesis of essential oil in this popular medicinal plant.

Cuticular Waxes of Arabidopsis thaliana Shoots: Cell-Type-Specific Composition and Biosynthesis

It is generally assumed that all plant epidermis cells are covered with cuticles, and the distinct surface geometries of pavement cells, guard cells, and trichomes imply functional differences and possibly different wax compositions. However, experiments probing cell-type-specific wax compositions and biosynthesis have been lacking until recently. This review summarizes new evidence showing that Arabidopsis trichomes have fewer wax compound classes than pavement cells, and higher amounts of especially long-chain hydrocarbons. The biosynthesis machinery generating this characteristic surface coating is discussed. Interestingly, wax compounds with similar, long hydrocarbon chains had been identified previously in some unrelated species, not all of them bearing trichomes.

Glandular trichomes of Tussilago Farfara (Senecioneae, Asteraceae)

The glandular trichomes are developed on the aerial organs of Tussilago farfara ; they produce phenols and terpenoids. Smooth endoplasmic reticulum and leucoplasts are the main organelles of the trichome secretory cells. The aim of this study was to characterise the morphology, anatomy, histochemistry and ultrastructure of the trichomes in Tussilago farfara as well as to identify composition of the secretory products. Structure of trichomes located on the peduncles, bracts, phyllaries, and leaves were studied by light and electron microscopy. The capitate glandular trichomes consist of a multicellular head and a biseriate long stalk. Histochemical tests and fluorescence microscopy reveal phenols and terpenoids in the head cells. During secretory stage, the head cells contain smooth and rough endoplasmic reticulum, Golgi apparatus, diversiform leucoplasts with opaque contents in lamellae, chloroplasts, mitochondria, and microbodies. In the capitate glandular trichomes of T. farfara subcuticular cavity is absent, unlike glandular trichomes in other Asteraceae species. For the first time, content of metabolites in the different vegetative and reproductive organs as well as in the isolated capitate glandular trichomes was identified by GC-MS. Forty-five compounds, including organic acids, sugars, polyols, phenolics, and terpenoids were identified. It appeared that metabolite content in the methanol extracts from peduncles, bracts and phyllaries is biochemically analogous, and similar to the metabolites from leaves, in which photosynthesis happens. At the same time, the metabolites from trichome extracts essentially differ and refer to the above-mentioned secondary substances. The study has shown that the practical value of the aerial organs of coltsfoot is provided with flavonoids produced in the capitate glandular trichomes.

Trichomes and chemical composition of the volatile oil of Trichogonia cinerea (Gardner) R. M. King & H. Rob. (Eupatorieae, Asteraceae)

Trichogonia cinerea is endemic to Brazil and occurs in areas of cerrado and campo rupestre. In this study, we characterized the glandular and non-glandular trichomes on the aerial parts of this species, determined the principal events in the development of the former, and identified the main constituents of the volatile oil produced in its aerial organs. Fully expanded leaves, internodes, florets, involucral bracts, and stem apices were used for the characterization of trichomes. Leaves, internodes, florets, and involucral bracts were examined by light microscopy and scanning electron microscopy, whereas stem apices were examined only by light microscopy. Branches in the reproductive phase were used for the extraction and determination of the composition of the volatile oil. The species has three types of glandular trichomes, biseriate vesicular, biseriate pedunculate, and multicellular uniseriate, which secrete volatile oils and phenolic compounds. The major components identified in the volatile oil were 3,5-muuroladiene (39.56%) and butylated hydroxytoluene (13.07%).

Bidirectional secretions from glandular trichomes of pyrethrum enable immunization of seedlings

Glandular trichomes are currently known only to store mono- and sesquiterpene compounds in the subcuticular cavity just above the apical cells of trichomes or emit them into the headspace. We demonstrate that basipetal secretions can also occur, by addressing the organization of the biosynthesis and storage of pyrethrins in pyrethrum (Tanacetum cinerariifolium) flowers. Pyrethrum produces a diverse array of pyrethrins and sesquiterpene lactones for plant defense. The highest concentrations accumulate in the flower achenes, which are densely covered by glandular trichomes. The trichomes of mature achenes contain sesquiterpene lactones and other secondary metabolites, but no pyrethrins. However, during achene maturation, the key pyrethrin biosynthetic pathway enzyme chrysanthemyl diphosphate synthase is expressed only in glandular trichomes. We show evidence that chrysanthemic acid is translocated from trichomes to pericarp, where it is esterified into pyrethrins that accumulate in intercellular spaces. During seed maturation, pyrethrins are then absorbed by the embryo, and during seed germination, the embryo-stored pyrethrins are recruited by seedling tissues, which, for lack of trichomes, cannot produce pyrethrins themselves. The findings demonstrate that plant glandular trichomes can selectively secrete in a basipetal direction monoterpenoids, which can reach distant tissues, participate in chemical conversions, and immunize seedlings against insects and fungi.

Transcriptome analysis approaches for the isolation of trichome-specific genes from the medicinal plant Cistus creticus subsp. creticus

Cistus creticus subsp. creticus is a plant of intrinsic scientific interest due to the distinctive pharmaceutical properties of its resin. Labdane-type diterpenes, the main constituents of the resin, exhibit considerable antibacterial and cytotoxic activities. In this study chemical analysis of isolated trichomes from different developmental stages revealed that young leaves of 1-2 cm length displayed the highest content of labdane-type diterpenes (80 mg/g fresh weight) whereas trichomes from older leaves (2-3 or 3-4 cm) exhibited gradual decreased concentrations. A cDNA library was constructed enriched in transcripts from trichomes isolated from young leaves, which are characterized by high levels of labdane-type diterpenes. Functional annotation of 2,022 expressed sequence tags (ESTs) from the trichome cDNA library based on homology to A. thaliana genes suggested that 8% of the putative identified sequences were secondary metabolism-related and involved primarily in flavonoid and terpenoid biosynthesis. A significant proportion of the ESTs (38%) displayed no significant similarity to any other DNA deposited in databases, indicating a yet unknown function. Custom DNA microarrays constructed with 1,248 individual clones from the cDNA library facilitated transcriptome comparisons between trichomes and trichome-free tissues. In addition, gene expression studies in various Cistus tissues and organs for one of the genes highlighted as the most differentially expressed by the microarray experiments revealed a putative sesquiterpene synthase with a trichome-specific expression pattern. Full length cDNA isolation and heterologous expression in E. coli followed by biochemical analysis, led to the characterization of the produced protein as germacrene B synthase.

Phenolic Compounds Produced by Secretory Structures in Plants: A Brief Review

The purpose of this brief review has been to provide more recent data regarding the production of phenolic compounds by secretory structures. Although morphology and histochemistry of glands are well documented, meagre information concerning phenolics is available in the surveyed literature. Two major groups of glands are found regarding phenolic compounds synthesis: 1. secretory cells producing mainly phenolics, 2. secretory cells producing phenolics coupled with other compounds. In the former group, phenolic compounds remain in mature organs, and prevail in the material produced by epidermis, hypodermis, idioblasts, and sheath encircling vascular bundles and ducts. The latter group is constituted of trichomes, cavities, ducts, laticifers, colleters, nuptial nectaries, osmophores and stigma system, which synthesize complex mixtures of terpenes, phenolic compounds, polysaccharides and other compounds. In vegetative organs, the secretion of these glands might provide chemical defence against damage by UV radiation, against pathogen activities, and play a role in the herbivory deterrence. Additional functions ascribed to phenolics produced by floral glands are associated with pollination, pollen germination and pollen-tube elongation.

Morphology and histochemistry of the glandular trichomes of Lippia scaberrima (Verbenaceae)

BACKGROUND AND AIMS

Lippia scaberrima, an aromatic indigenous South African plant, with medicinal application, potentially has economic value. The production of essential oil from this plant has not been optimized, and this study of the chemico-morphological characteristics was aimed at determining the location of oil production within the plant. Furthermore, the locality of other secondary metabolites important in medicinal applications needed to be ascertained. This information would be useful in deciding the protocol required for isolation of such compounds.

METHODS

The morphology of the glandular trichomes was investigated using a combination of scanning electron and light microscopy. Concurrently, the chemical content was studied by applying various chemical reagents and fluorescence microscopy.

KEY RESULTS

Three types of trichomes were distinguished on the material investigated. Large, bulbous peltate glands containing compounds of terpenoid nature are probably the main site of essential oil accumulation. Small glands were found to be both peltate and capitate and fluorescent stain indicated the possible presence of phenolic compounds. The third type was a slender tapered seta with an ornamented surface and uniseriate base, and evidently secretory in nature.

CONCLUSIONS

This study linking the chemical content and morphology of the glandular trichomes of L. scaberrima has contributed to the knowledge and understanding of secretory structures of Lippia spp. in general.

Evidence of protocarnivory in triggerplants (Stylidium spp.; Stylidiaceae)

Australian triggerplants (Stylidium spp.; Stylidiaceae) trap small insects using mucilage-secreting glandular hairs held at various points on their inflorescence stems and flower parts. Triggerplants are generally found in habitats also containing genera of plants already accepted as carnivorous, two of which (Drosera, Byblis) use the same basic mechanism as Stylidium to trap their prey. In the herbarium, sheets of triggerplants and of accepted groups of carnivorous plants held similar numbers of trapped insects, and in the field, trapping of small prey per unit of glandular surface area was the same at a given site for triggerplants and for nearby carnivorous plants at three sites in northern Australia. Even more important, protease activity was produced by glandular regions of both triggerplants and Drosera after induction with yeast extract. A panel of negative and positive controls, including use 1) of plants grown in tissue culture, which therefore lack surface microorganisms, and 2) of protease inhibitors, shows that this activity 1) is generated by the glandular regions of the triggerplant itself, not by organisms that might reside on the surface of the plants, and 2) is due to proteases. All of this evidence taken together provides strong evidence of protocarnivory in Stylidium, something not previously suggested in the scientific literature, though the insect trapping has been noted informally. Experiments remain to be done to determine nutrient uptake, so triggerplants may well be fully carnivorous.