Capitate glandular trichomes of Helianthus annuus (Asteraceae): ultrastructure and cytological development

Targeted Screening and Quantification of Characteristic Sesquiterpene Lactones in Ambrosia artemisiifolia L. at Different Growth Stages

Sesquiterpene lactones are specialized plant metabolites with promising pharmacological activities. These metabolites are characteristic marker compounds for the aerial parts of Ambrosia artemisiifolia. Numerus sesquiterpene lactones have been isolated from ragweed; however, there is no information on their bioproduction and quantification throughout the life cycle of the plant. The sesquiterpene lactone content of ragweed samples collected in Szeged and Nyíri was analyzed using HPLC. Significant differences in the amount and bioproduction rhythm of sesquiterpene lactones were found between the two sets of samples. The samples collected near Szeged contained significantly lower amounts of the investigated compounds compared to the Nyíri samples. Sesquiterpene lactone production in the samples peaked at the end of July or in August; the trend of the change in sesquiterpene lactones might correlate with precipitation and temperature. Geographical location and geoclimatic factors might exert significant influence on the production of sesquiterpene lactones in ragweed.

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.

A comprehensive phytochemical and pharmacological review on sesquiterpenes from the genus Ambrosia

Sesquiterpenes are bitter secondary metabolites characteristic to the genus Ambrosia (Asteraceae) and constitute one of the most diverse classes of terpenoids. These compounds exhibit broad-spectrum bioactivities, such as antiproliferative, cytotoxic, antimicrobial, anti-inflammatory, molluscicidal, schistomicidal, larvicidal, and antiprotozoal activities. This review compiles and discusses the chemistry and pharmacology of sesquiterpenes of the Ambrosia species covering the period between 1950 and 2021. The review identified 158 sesquiterpenes previously isolated from 23 different Ambrosia species collected from across the American, African, and Asian continents. These compounds have guaiane, pseudoguaiane, seco-pseudoguaiane, daucane, germacrane, eudesmane, oplopane, clavane, and aromadendrane carbon skeletons. Most sesquiterpene compounds predominantly harbor the pseudoguaiane skeleton, whereas the eudesmanes have the most varied substituents. Antiproliferative and antiprotozoal activities are the most promising bioactivities of sesquiterpenes in Ambrosia and could lead to new pathways toward drug discovery.

Ultrastructural Alterations in Cells of Sunflower Linear Glandular Trichomes during Maturation

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

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.

CYP71BL9, the missing link in costunolide synthesis of sunflower

Despite intensive research in recent years, the biosynthetic route to costunolide in sunflower so far remained obscured. Additional P450 sequences from public sunflower transcriptomic database were screened to search for candidate enzymes which are able to introduce the 6α-hydroxy-group required for the esterification with the carboxy group of germacarane A acid, the final step in costunolide formation. CYP71BL9, a new P450 enzyme from sunflower was shown to catalyze this hydroxylation, hence being identified as HaCOS. Phylogentically, HaCOS is closer related to HaG8H than to any other known costunolide synthase in Asteraceae.The enzyme was successfully employed to reconstruct the sunflower biosynthesis of costunolide in transformed tobacco. Contrary, in yeast, only minor amounts of sesquiterpene lactone was produced, while 5-hydroxyfarnesylic acid was formed instead. HaCOS in combination with HaG8H produced 8β-hydroxycostunolide (eupatolide) in transformed plants, thus indicating that sunflower possesses two independent modes of eupatolide synthesis via HaCOS and via HaES. The lack of HaCOS expression and of costunolide in trichomes suggests that the enzyme triggers the costunolied synthesis of the inner tissues of sunflower and might be linked to growth regulation processes.

Metabolomic and Gene Expression Studies Reveal the Diversity, Distribution and Spatial Regulation of the Specialized Metabolism of Yacón (Smallanthus sonchifolius, Asteraceae)

Smallanthus sonchifolius, also known as yacón, is an Andean crop species commercialized for its nutraceutical and medicinal properties. The tuberous roots of yacón accumulate a diverse array of probiotic and bioactive metabolites including fructooligosaccharides and caffeic acid esters. However, the metabolic diversity of yacón remains unexplored, including the site of biosynthesis and accumulation of key metabolite classes. We report herein a multidisciplinary approach involving metabolomics, gene expression and scanning electron microscopy, to provide a comprehensive analysis of the diversity, distribution and spatial regulation of the specialized metabolism in yacón. Our results demonstrate that different metabolic fingerprints and gene expression patterns characterize specific tissues, organs and cultivars of yacón. Manual inspection of mass spectrometry data and molecular networking allowed the tentative identification of 71 metabolites, including undescribed structural analogues of known bioactive compounds. Imaging by scanning electron microscopy revealed the presence of a new type of glandular trichome in yacón bracts, with a distinctive metabolite profile. Furthermore, the high concentration of sesquiterpene lactones in capitate glandular trichomes and the restricted presence of certain flavonoids and caffeic acid esters in underground organs and internal tissues suggests that these metabolites could be involved in protective and ecological functions. This study demonstrates that individual organs and tissues make specific contributions to the highly diverse and specialized metabolome of yacón, which is proving to be a reservoir of previously undescribed molecules of potential significance in human health.

Spatial and developmental synthesis of endogenous sesquiterpene lactones supports function in growth regulation of sunflower

Tissue-specific occurrence and formation of endogenous sesquiterpene lactones has been assessed and suggests physiological function as antagonists of auxin-induced plant growth in sunflower. Sunflower, Helianthus annuus, accumulate high concentrations of bioactive sesquiterpene lactones (STL) in glandular trichomes, but in addition, structurally different STL occur in only trace amounts in the inner tissues. The spatial and temporal production of these endogenous STL during early phases of plant development is widely unknown and their physiological function as putative natural growth regulators is yet speculative. By means of HPLC and MS analysis it was shown that costunolide, dehydrocostuslactone, 8-epixanthatin and tomentosin are already present in dry seeds and can be extracted in low amounts from cotyledons, hypocotyls and roots of seedlings during the first days after germination. Semi-quantitative and RT-qPCR experiments with genes of the key enzymes of two independent routes of the endogenous STL biosynthesis confirmed the early and individual expression in these organs and revealed a gradual down regulation during the first 72-96 h after germination. Light irradiation of the plants led to a fast, but transient increase of STL in parts of the hypocotyl which correlated with growth retardation of the stem. One-sided external application of costunolide on hypocotyls conferred reduced growth of the treated side, thus resulting in the curving of the stem towards the side of the application. This indicates the inhibiting effects of STL on plant growth. The putative function of endogenous STL in sunflower as antagonists of auxin in growth processes is discussed.

Histochemical assays of secretory trichomes and the structure and content of mineral nutrients in Rubus idaeus L. leaves

Leaves of Rubus idaeus are a raw material, ingredients of herbal blend, and a source of antioxidants. There are no data concerning histochemistry of trichomes, and little is known about the leaf structure of this species. The aim of this study was to determine the histochemistry of active compounds and the structure of glandular trichomes, micromorphology, anatomy, and ultrastructure of leaves as well as content of elements. To determine the histochemistry of glandular trichomes, different chemical compounds were used. The leaf structure was analysed using light, scanning, and transmission electron microscopes. The content of elements was determined with atomic absorption spectrometry, and the microanalysis of the epidermis ultrastructure was carried out with a transmission electron microscope equipped with a digital X-ray analyser. In glandular trichomes, polyphenols, terpenes, lipids, proteins, and carbohydrates were identified. The main elements in the ultrastructure of the epidermis were Na, Mo, Se, Ca, and Mg. In dry matter of leaves, K, Mg, Ca, P, and Fe were dominant. Infusions from leaves are safe for health in terms of the Cd and Pb concentrations. Leaves can be a valuable raw material. Non-glandular trichomes prevent clumping of mixed raw materials in herbal mixtures.

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.

Biosynthesis of Eupatolide-A Metabolic Route for Sesquiterpene Lactone Formation Involving the P450 Enzyme CYP71DD6

Sesquiterpene lactones are a class of natural compounds well-known for their bioactivity and are characteristic for the Asteraceae family. Most sesquiterpene lactones are considered derivatives of germacrene A acid (GAA). GAA can be stereospecifically hydroxylated by the cytochrome P450 enzymes (CYP) Lactuca sativa costunolide synthase CYP71BL2 (LsCOS) and Helianthus annuus GAA 8β-hydroxylase CYP71BL1 (HaG8H) at C6 (in α-orientation) or C8 (in β-orientation), respectively. Spontaneous subsequent lactonization of the resulting 6α-hydroxy-GAA leads to costunolide, whereas 8β-hydroxy-GAA has not yet been reported to cyclize to a sesquiterpene lactone. Sunflower and related species of the Heliantheae tribe contain sesquiterpene lactones mainly derived from inunolide (7,8-cis lactone) and eupatolide (8β-hydroxy-costunolide) precursors. However, the mechanism of 7,8-cis lactonization in general, and the 6,7-trans lactone formation in the sunflower tribe, remain elusive. Here, we show that, in plant cells, heterologous expression of CYP71BL1 leads to the formation of inunolide. Using a phylogenetic analysis of enzymes from the CYP71 family involved in sesquiterpenoid metabolism, we identified the CYP71DD6 gene, which was able to catalyze the 6,7-trans lactonization in sunflowers, using as a substrate 8β-hydroxy-GAA. Consequently, CYP71DD6 resulted in the synthesis of eupatolide, thus called HaES ( Helianthus annuus eupatolide synthase). Thus, our study shows the entry point for the biosynthesis of two distinct types of sesquiterpene lactones in sunflowers: the 6,7-trans lactones derived from eupatolide and the 7,8-cis lactones derived from inunolide. The implications for tissue-specific localization, based on expression studies, are discussed.

Genetic Architecture of Capitate Glandular Trichome Density in Florets of Domesticated Sunflower (Helianthus annuus L.)

Capitate glandular trichomes (CGT), one type of glandular trichomes, are most common in Asteraceae species. CGT can produce various secondary metabolites such as sesquiterpene lactones (STLs) and provide durable resistance to insect pests. In sunflower, CGT-based host resistance is effective to combat the specialist pest, sunflower moth. However, the genetic basis of CGT density is not well understood in sunflower. In this study, we identified two major QTL controlling CGT density in sunflower florets by using a F₄ mapping population derived from the cross HA 300 × RHA 464 with a genetic linkage map constructed from genotyping-by-sequencing data and composed of 2121 SNP markers. One major QTL is located on chromosome 5, which explained 11.61% of the observed phenotypic variation, and the second QTL is located on chromosome 6, which explained 14.06% of the observed phenotypic variation. The QTL effects and the association between CGT density and QTL support interval were confirmed in a validation population which included 39 sunflower inbred lines with diverse genetic backgrounds. We also identified two strong candidate genes in the QTL support intervals, and the functions of their orthologs in other plant species suggested their potential roles in regulating capitate glandular trichome density in sunflower. Our results provide valuable information to sunflower breeding community for developing host resistance to sunflower insect pests.

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.

Morphogenesis and developmental ultrastructure of Nicotiana tabacum short glandular trichomes

The anatomy and ultrastructure of the short glandular trichomes occurring on young expanding leaves of Nicotiana tabacum were investigated using light and transmission electron microscopy. The objective of the present research was to characterize the cellular changes that occur during morphogenesis of short glandular trichomes, from initiation to senescence. Ultrastructural analysis of their secretory cells revealed characteristics common to gland cells: numerous mitochondria in highly organized cytoplasm, large nuclei, and an elaborate network of endoplasmic reticulum. Initial changes in nuclear and plastidial organization were observed at a more advanced secretory stage, marking the onset of senescence. During trichome senescence, gradual reduction of the cytoplasm density occurred along with structural changes of the plastids and the tonoplast. As a result of inward blebbing of the cytoplasm into the vacuole, membrane bound vesicular structures appeared in the vacuolar space. At the late secretory stage, marked by an increase in vacuolation and extraplasmic space, degenerative changes included further fragmentation of the cytoplasm and deterioration of the tonoplast. Multimembrane myelin bodies observed in the vacuolar space were indicative of membrane digestion although plasma membrane did not appear massively degraded.

Direct Analyses of Secondary Metabolites by Mass Spectrometry Imaging (MSI) from Sunflower (Helianthus annuus L.) Trichomes

Helianthus annuus (sunflower) displays non-glandular trichomes (NGT), capitate glandular trichomes (CGT), and linear glandular trichomes (LGT), which reveal different chemical compositions and locations in different plant tissues. With matrix-assisted laser desorption/ionization (MALDI) and laser desorption/ionization (LDI) mass spectrometry imaging (MSI) techniques, efficient methods were developed to analyze the tissue distribution of secondary metabolites (flavonoids and sesquiterpenes) and proteins inside of trichomes. Herein, we analyzed sesquiterpene lactones, present in CGT, from leaf transversal sections using the matrix 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (CHCA) (mixture 1:1) with sodium ions added to increase the ionization in positive ion mode. The results observed for sesquiterpenes and polymethoxylated flavones from LGT were similar. However, upon desiccation, LGT changed their shape in the ionization source, complicating analyses by MSI mainly after matrix application. An alternative method could be applied to LGT regions by employing LDI (without matrix) in negative ion mode. The polymethoxylated flavones were easily ionized by LDI, producing images with higher resolution, but the sesquiterpenes were not observed in spectra. Thus, the application and viability of MALDI imaging for the analyses of protein and secondary metabolites inside trichomes were confirmed, highlighting the importance of optimization parameters.

Capitate glandular trichomes in Aldama discolor (Heliantheae - Asteraceae): morphology, metabolite profile and sesquiterpene biosynthesis

The capitate glandular trichome is the most common type described in Asteraceae species. It is known for its ability to produce various plant metabolites of ecological and economic importance, among which sesquiterpene lactones are predominant. In this paper, we applied microscopy, phytochemical and molecular genetics techniques to characterise the capitate glandular trichome in Aldama discolor, a native Brazilian species of Asteraceae, with pharmacological potential. It was found that formation of trichomes on leaf primordia of germinating seeds starts between 24 h and 48 h after radicle growth indicates germination. The start of metabolic activity of trichomes was indicated by separation of the cuticle from the cell wall of secretory cells at the trichome tip after 72 h. This coincided with the accumulation of budlein A, the major sesquiterpene lactone of A. discolor capitate glandular trichomes, in extracts of leaf primordia after 96 h. In the same timeframe of 72-96 h post-germination, gene expression studies showed up-regulation of the putative germacrene A synthase (pGAS2) and putative germacrene A oxidase (pGAO) of A. discolor in the transcriptome of these samples, indicating the start of sesquiterpene lactone biosynthesis. Sequencing of the two genes revealed high similarity to HaGAS and HaGAO from sunflower, which shows that key steps of this pathway are highly conserved. The processes of trichome differentiation, metabolic activity and genetic regulation in A. discolor and in sunflower appear to be typical for other species of the subtribe Helianthinae.

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%).

Localization of sesquiterpene lactone biosynthesis in cells of capitate glandular trichomes of Helianthus annuus (Asteraceae)

Capitate glandular trichomes (CGT) of sunflower, Helianthus annuus, synthesize bioactive sesquiterpene lactones (STLs) within a short period of only a few days during trichome development. In the current project, the subcellular localization of H. annuus germacrene A monooxygenase (HaGAO), a key enzyme of the STL biosynthesis in sunflower CGT, was investigated. A polyclonal antibody raised against this enzyme was used for immunolabelling. HaGAO was found in secretory and stalk cells of CGT. This correlated with the appearance of smooth endoplasmic reticulum in both cell types. Stalk cells and secretory cells differed in form, size and types of plastids, but both had structures necessary for secretion. No HaGAO-specific immunoreaction was found in sunflower leaf tissue outside of CGT or in developing CGT before the secretory phase had started. Our results indicated that not only secretory cells but also nearly all cells of the CGT were involved in the biosynthesis of STL and that this process was not linked to the presence or absence of a specific type of plastid.

The nonvolatile metabolome of sunflower linear glandular trichomes

Uniseriate linear glandular trichomes occur on stems, leaves and flowering parts of Helianthus species and related taxa. Their metabolic activity and biological function are still poorly understood. A phytochemical study documented the accumulation of bisabolene type sesquiterpenes and flavonoids as the major constituents of the non-volatile metabolome of linear glandular trichomes in the common sunflower, Helianthus annuus. Besides known sesquiterpenes of the glandulone, helibisabonol and heliannuol type, four previously undescribed sesquiterpenes named glandulone D, E, F and helibisabonol C were identified by spectroscopic analysis. In addition, four known nevadensin type flavonoids varying in O-methoxy substitutions were found. None of them has previously been reported from Helianthus annuus.

Trichome differentiation on leaf primordia of Helianthus annuus (Asteraceae): morphology, gene expression and metabolite profile

Sunflower trichomes fully develop on embryonic plumula within 3 days after start of germination. Toxic sesquiterpene lactones are produced immediately thereafter thus protecting the apical bud of the seedling against herbivory. Helianthus annuus harbors non-glandular and two different types of multicellular glandular trichomes, namely the biseriate capitate glandular trichomes and the uniseriate linear glandular trichomes. The development of capitate glandular trichomes is well known from anther tips on sunflower disk florets, but not from leaves and no information is yet available on the development of the linear glandular trichomes. Scanning electron microscopy of sunflower seedlings unravelled that within the first 40 h of seed germination all three types of trichomes started to emerge on primordia of the first true leaves. Within the following 20-30 h trichomes developed from trichoblasts to fully differentiated hairs. Gene expression studies showed that genes involved in the trichome-based sesquiterpene lactone formation were up-regulated between 72 and 96 h after start of germination. Metabolite profiling with HPLC confirmed the synthesis of sesquiterpene lactones which may contribute to protect the germinating seedlings from herbivory. The study has shown that sunflower leaf primordia can serve as a fast and easy to handle model system for the investigation of trichome development in Asteraceae.