A phylogenetic study of the Plantaginaceae

In a study based on morphological, embryological and chemical data of the Plantaginaceae, within the subclass Sympetalae or Asteridae, the superorder Lamianae is shown to be monophyletic. However, it was not possible to reconstruct the phylogeny within Lamianae or to find a sister-group for the monophyletic Plantaginaceae – Hydrostachyaceae for the latter is rejected. Three or rarely four genera have previously been recognized within Plantaginaceae, but in both cases Plantago appears as polyphyletic, which is considered unacceptable. Six clades are recognized as subgenera within Plantago: subgen. Plantago (c. 131 species), subgen. Coronopus (c. 11 species), subgen. Littorella (three species), subgen. Psyllium (c. 16 species), subgen. Bougueria (one species), and subgen. Albicans (51 species). Within P. subgen. Plantago, the paraphyletic sect. Plantago (c. 42 species) is found in all parts of the world except South America, New Zealand, Australia, and New Guinea, where sect. Oliganthos and sect. Mesembryniae vicariate. It is not possible to infer the phylogeny within sect. Plantago, for example between the endemic species from distant Pacific Islands; their common ancestor might be a species that once had a very wide distribution. Keys to genera, subgenera, sections and series are given. Only one genus, Plantago L., is recognized. The following proposals are made in the Appendix: P. ser. Oliganthos Rahn, ser. nov.; P. ser. Carpophorae (Rahn) Rahn, stat. nov. (= sect. C.); P. ser. Microcalyx (Pilg.) Rahn, stat. nov. (= sect. M.); P. unibrackteata Rahn, nom. nov. (=P. uniflora Hook.f. non L.); P. subgen. Littorella (P. J. Bergius) Rahn, stat. nov. (=Littorella P. J. Bergius); P. araucana Rahn, nom. nov. (=Littorella australis Griseb. non Plantago australis Lam.); P. americana (Fernald) Rahn, comb. nov.(=Littorella a.); P. subgen. Bougueria (Decne.) Rahn, stat. nov. (=Bougueria Decne.); P. nubicola (Decne.) Rahn, comb. nov. (=Bougueria n. Decne.); P. subgen. Albicans Rahn, subgen. nov. Ten lectotypes are selected.     

Glandular Trichomes in Satureja thymbra Leaves

The leaves of the aromatic plant Satureja thymbra have numerousglandular trichomes of two morphologically distinct types glandularhairs and glandular scales Investigations of the anatomy ofthese glandular trichomes with serial thick sections revealedthat the glandular hairs consist of three cells a foot, stalkand head cell Glandular scales also have a unicellular footand stalk Their heads, however, are composed of 12 cells Fourof these cells are small, occupying the central region of thehead, whereas the remainder are large and peripherally arrangedMorphometric analysis showed that, in leaf surface view, glandularscales are about 17-fold larger than glandular hairs In addition,glandular scales were found to occupy 5 7 % of the entire leafsurface area In each glandular scale the total amount of essentialoil, contained within both the subcuticular space and the interiorof the secretory cells, was calculated to be 2 51 x 10^–4mm³ The volume of the essential oil produced by all glandularscales on a single mature leaf was correspondingly determinedto be 0.059 mm³ Finally, the theoretical essential oil yieldof 100 g dry leaves of S thymbra was estimated to be 3 54 %(secretory activity of glandular scales only) Satureja thymbra, glandular trichomes, morphology, morphometry     

The Ultrastructure of the Glandular Trichomes of Solanum tuberosum

The potato plant has two types of glandular trichomes whichwere investigated by electron microscopy. One type has a eight celled globular head on a neck cell anda stalk cell Each glandular cell has many rather large vacuoles,a large nucleus, many ribosomes and mitochondria, a few Golgibodies, and darkly coloured, often irregular plastids (chloroplasts).The plastids are mostly located near the axial cell wall borderinga large central intercellular space filled with secretion materialThe plastids are assumed to participate in the formation ofthe secretion material, which reacts positively to esterasetests. The outer wall is covered by a thin cuticle. The other type has a club-shaped multicellular head on a singlestalk cell. The cytoplasmic features in the cells are similarto those of the globular-headed trichome, except that they possesslarge central vacuoles and randomly distributed plastids. Centricendoplasmic reticulum has been observed in young cells. Intercellularspaces develop between the cells and into the outer wall, whichis thus split into two. Whereas the older glandular cells reactpositively to tests for esterase, the secretion material itselfis pectinaceous and reacts negatively. The outer wall is cutinizedand covered by a cuticle. Solanum tuberosum L., potato, glandular trichomes, ultrastructure     

Chemotaxonomic markers in Digitalideae (Plantaginaceae)

In a chemosystematic investigation of Digitalideae (Plantaginaceae), the water-soluble part of extracts of two species of Digitalis, two species of Isoplexis, as well as Erinus alpinus and Lafuentea rotundifolia were studied with regard to their content of main carbohydrates, iridoids and caffeoyl phenylethanoid glycosides (CPGs). Digitalis and Isoplexis contained sorbitol, cornoside and a number of other phenylethanoid glycosides including the new tyrosol beta-D-mannopyranoside, sceptroside but were found to lack iridoid glucosides. Erinus contained mainly glucose, the new 8,9-double bond iridoid, erinoside, and a number of known iridoid glucosides including two esters of 6-rhamnopyranosylcatalpol, as well as the CPG poliumoside. Finally, Lafuentea was characterized by the presence of glucose, aucubin and cryptamygin B but apparently lacked CPGs. The chemosystematic significance of the isolated compounds is discussed.     

Chemotaxonomy of Veroniceae and its allies in the Plantaginaceae

In a chemosystematic investigation of tribe Veroniceae (Plantaginaceae), representatives of Camptoloma, Sibthorpia, Veronica subg. Pentasepalae and subg. Hebe, Veronicastrum, Wulfenia, and the related Ellisiophyllum and Globularia were examined for non-flavonoid glycosides. From the 14 species studied, 28 different iridoid glucosides and 10 caffeoyl phenylethanoid glucosides (CPGs), as well as salidroside and arbutin were isolated and characterized by NMR; of these, five compounds were previously unknown. It was found that the representatives of Veroniceae, as well as Globularia, were characterized by mannitol, aucubin, catalpol and catalpol esters. Each of the three studied species of Veronica subg. Hebe contained at least one of the 6-O-catalpol esters typical for Veronica s. str. (verminoside), supporting the inclusion of Hebe in Veronica. However, their main constituents were esters of 6-O-rhamnopyranosylcatalpol; a CPG, hebeoside (2'-beta-xylopyranosyl-verbascoside) was isolated from V. (Hebe) salicifolia. The two species of Veronicastrum also contained 6-O-rhamnopyranosylcatalpol esters, including the previously unknown 2',3'- and 3',4'-dicinnamoyl derivatives and, in contrast to the earlier reports, they lacked 6-O-catalpol esters. The main iridoid constituents in the three investigated species of Wulfenia were 10-O-aucubin and 10-O-catalpol esters (isoscrophularioside or globularin) while baldaccioside (10-O-cinnamoyl asystasioside E) was isolated from W. baldaccii. Globularia vulgaris contained 10-O-catalpol esters (e.g., globularin) and, in addition, asperuloside together with its benzoyl analogue named besperuloside. The representatives of Sibthorpia and Ellisiophyllum were almost completely devoid of iridoids; this, however, together with the CPGs present implied a close relationship between the two genera. Camptoloma lyperiiflorum lacked hexitols but contained esters of 6-O-rhamnopyranosylcatalpol different from those found in Veroniceae but known from Buddleja, Scrophularia and Verbascum (Scrophulariaceae s. str.).     

Trichomes as sensors: Detecting activity on the leaf surface

The dramatic movements of some carnivorous plants species are triggered by sensory structures derived from trichomes. While unusual plant species such as the Venus fly trap and sundews may be expected to have elaborate sensors to capture their insect prey, more modest plant species might not be expected to have similar sensory capabilities. Our recent work, however, has revealed that glandular trichomes on tomato (Solanum lycopersicum) appear to have a function similar to trigger hairs of carnivorous species, acting as “early warning” sensors. Using a combination of behavioral, molecular, and biochemical techniques, we determined that caterpillars, moths and mechanical disruption upregulate signaling molecules and defensive genes found in glandular trichomes. Importantly, we discovered that plants whose trichomes have been broken respond more vigorously when their defenses were induced. Taken together, our results suggest that glandular trichomes can act as sensors that detect activity on the leaf surface, and ready plants for herbivore attack.Key words: glandular trichome, induced responses, jasmonic acid, plant-insect interactions, sensor, Solanum lycopersicum, tomatoCertain plant species are renowned for their ability to respond to contact. The Venus fly trap (Dionaea muscipula) and sundew (Drosera) species come to mind quickly as obviously thigmotropic species. When an insect lands on these carnivorous plant species, dramatic movements ensue once the prey is detected. Some Drosera species respond to contact by bending their “tentacles” toward their trapped prey to further ensnare the victim and begin the process of digestion. These dramatic plant species have captured the attention of many scientists, including Darwin, who remarked on the “extraordinary sensitiveness of [their] glands to slight pressure” and surmised that the tentacles of sundew plants “existed primordially as glandular hairs.”¹ As is often the case, Darwin appears to have been quite right. Indeed, morphological and molecular work supports the notion that sundew tentacles and the trigger hairs of the Venus fly trap are homologous sensory structures likely derived from trichomes.^2,3Given Darwin’s appreciation of these trichome-derived sensory organs, he perhaps would not have been surprised by mounting evidence that suggests that trichomes may play even a broader sensory role for plants. We have recently found evidence that glandular trichomes can act as early detection sensors for some plant species.⁴ These trichomes can be disrupted by the footsteps of walking moths and caterpillars (and other forms of light touching), and this apparently minor plant damage leads to a state of defensive readiness that allows plants to respond to herbivory more quickly than undamaged plants. While this level of trichome-mediated detection does not result in the conspicuous responses of some carnivorous plant species, it still results in significant physiological changes that prepare plants for attack.In our recent effort, we worked with tomato (Solanum lycopersicum), using a combination of behavioral, molecular, and biochemical techniques to understand the role of trichomes in detecting activity on the leaf surface.⁴ Defense signaling has been well studied in tomato and there exists a variety of mutants whose defensive responses have been compromised. Moreover, it has been known that tomatoes have a variety of trichome types, including two types of glandular trichomes that burst upon contact with insects, releasing their cellular contents and physically impeding insects (Fig. 1).^5,6Open in a separate windowFigure 1Surface of a tomato leaf showing (A) intact rounded heads of glandular trichomes (black arrows) and (B) trichomes disrupted with a gloved hand (absence of rounded heads except for a few in the upper left corner [black arrows]). Images were captured at 36x magnification and were taken from different parts of the same leaf.To determine if plant defense pathways were induced by insect contact, we allowed three species of caterpillar (Manduca sexta, Heliothis virescens and Helicoverpa zea) and one species of moth (H. zea) to crawl over tomato leaves for ten minutes. As a positive control, we also lightly rubbed leaves with a gloved hand or a metal rod. Within time frames ranging from three to twenty-four hours all treatments, insect and otherwise, significantly induced defensive genes as measured by qRT-PCR. Using a combination of RT-PCR and in situ hybridization, we confirmed that JA-signaling and defensive genes are expressed in trichomes. A GC-MS-based technique then confirmed that JA was present in trichomes of undamaged plants and DAB staining, in combination with catalase treatment, provided evidence that hydrogren peroxide and JA are key signals mediating defensegene induction. These conclusions were further reinforced by experiments with def1 mutants, a line of tomato impaired in JA signaling, and accession LA3610, a tomato variety with reduced numbers of trichomes. Lastly, we conducted a factorial experiment both disrupting trichomes and treating tomato plants with methyl jasmonate (MeJA), which induces plant defenses and increases densities of trichomes.⁷ Results of this final experiment indicated that plants that received both treatments (i.e., MeJA and disruption) had greater defensive gene induction than plants that were only treated with MeJA or plants whose trichomes remained intact, suggesting that increases in trichomes may contribute to greater sensitivity to touch-induced responses.Taken together, our results are highly suggestive that trichomes can act as “early warning” detectors for plants. Moths seeking to lay eggs on tomato are likely to break trichomes as they explore leaves, upregulating plant defenses in anticipation of egg hatch and feeding by neonate caterpillars. Similarly, herbivores colonizing a new host plant and breaking trichomes on their way across a leaf also appear to “tip the plant off” to impending attack. Considering the drastic response of carnivorous plants to touch, perhaps it should not be surprising that trichomes can function more broadly as sensors. In an evolutionary context, it seems logical that trichomes took on this role. For many plant species, “hairy” varieties receive less herbivory,⁸ so within a population there could have been a fitness advantage in having more trichomes. Once established, this hairy phenotype could then have been refined via mutation and selection for trichome varieties that had functions adaptive for the plant, perhaps driving the evolution of glandular trichomes and their role as sensors.Granted, the generalized nature of our results would appear to indicate that plants could be “primed” by nearly any arthropod species that crosses one of their leaves. This would, of course, include natural enemies, which are capable of decreasing herbivore pressure and improving plant fitness.^9,10 However, it has been hypothesized that priming evolved due to high fitness costs associated with defensive induction following threats of only minor severity.¹¹ Priming provides an advantage by settling plants into an intermediate “ready” state that allows them to deploy strong defense responses more quickly and the fitness cost associated with being “primed” are lower than full defensive induction.¹² Presumably, fitness costs following priming due to natural enemyinduced trichome disruption would also be less than the cost incurred from a bout of unanticipated herbivory and, over the life of the plant, it would be worth the effort to prepare for attack even if the perceived risk is from a natural enemy and not a foe.Our results build on previously reported priming mechanisms that prepare plants for attack.^13,14 And they reveal an additional level of sophistication in the sensory capabilities of plants, which have already been shown to be able to detect nearby threats of herbivory and increase their defenses in response.^15,16 It seems that trichomes may have played a much wider role in shaping the nature of plant-animal interaction than previously recognized and we look forward to further work elaborating their function.     

The Identification of the Seeds and Achenes of the British Plantaginaceae

The morphology of seeds and achenes of the British members ofthe Plantaginaceae has been examined. Keys to the identificationof seeds in the genera Plantago and Littorella are presented.Quantitative data on size and weight, as well as informationabout shape, colour and surface features of the seeds are givenfor: Plantago major L., P. media L., P. lanceolata L., P. martimaL., P. coronopus L., P. indica L., P. sempervirens Crantz, andLittorella uniflora (L.) Aschers. A representative sample ofeach seed is illustrated.     

Hair morphology inPlantago sect.Coronopus (Plantaginaceae)

The presence of bottle-like hairs has been described in four hitherto uninvestigated taxa ofPlantago sect.Coronopus sensuDietrich. This hair type is a feature characteristic of sect.Coronopus.     

Hair anatomy inPlantago subg.Psyllium (Plantaginaceae)

The hairs of seven representatives ofPlantago L. subg.Psyllium were studied. Three types of headless hairs and three types of headed hairs were observed. Clavate hairs and hairs with several-celled stalk and unicellular head are characteristic of subg.Psyllium and provide evidence for the distinctness of this taxon. Based on our studies it can be assumed thatRahn's idea of enlarging subg.Psyllium by six sections from subg.Plantago sensuPilger is incorrect.     

Molecular evidence of reticulate evolution in the subgenus Plantago (Plantaginaceae)

Polyploidization is a frequent evolutionary event in plants that has a large influence on speciation and evolution of the genome. Molecular phylogenetic analyses of the taxonomically complex subgenus Plantago were conducted to elucidate intrasubgeneric phylogenetic relationships. A nuclear-encoding single-copy gene, SUC1 (1.0-1.8 kb), was sequenced in 24 taxa representing all five sections of the subgenus Plantago and two taxa from subgenus Coronopus as the outgroup. Fifteen known polyploids and one putative polyploid were sampled to examine polyploid origins and occurrence of reticulate evolution by cloning and sequence analysis of SUC1. Phylogenetic relationships were estimated using maximum parsimony, neighbor-joining, and Bayesian analyses. For the first time, our analysis provides a highly resolved phylogenetic tree. Subgenus Plantago formed a well-supported monophyletic clade. In contrast, alleles from polyploid species were scattered across the whole SUC1 phylogenetic tree, and some independent allopolyploids originated from hybridization between distant lineages. One reason for this taxonomic complexity can be attributed to reticulate evolution within the subgenus Plantago. Our results also suggest the possibility of two independent long-distance dispersals between the northern and southern hemispheres.     

Ultrastructure of Protein Crystals in Potato and Tomato Trichomes

Large protein crystals were located in the leaf and stem trichomesof Solanum tuberosum L. and Lycopersicon esculentum Mill. Inpotato the crystals ranged from 1.05 to 4.5 µm (average2.3 µm) on a side and in tomato they ranged from 1.16to 3.5 µm (average 2.7 µm) on a side. The proteinnature of the crystals was determined by histochemical stainingwith Coumassie brilliant blue R250 and aniline blue black. Thecrystalline structure of the inclusions was observed in ultrathinsections using electron microscopy. In potato, in cleared areasof the cytoplasm, ribosomes were observed scattered among proteinfilaments. The filaments were approximately 7 nm in diameter.Morphologically similar crystals were observed in the tomatotrichomes but the protein filaments were smaller (approximately4 nm in diameter). Protein crystals were also observed in palisadeand spongy parenchyma and epidermal leaf cells in tomato. Protein crystals, trichomes, potato, Solanum tuberosum L., tomato, Lycopersicon esculentum Mill., ultrastructure     

Glandular Trichomes on Alfalfa Impede Searching Behavior of the Potato Leafhopper Parasitoid

New cultivars of alfalfa, Medicago sativa L., have been released with glandular trichomes for resistance to potato leafhopper, Empoasca fabae (Harris). Yet, the impact of the glandular trichomes on the primary natural enemy of the leafhopper, Anagrus nigriventris Girault, is unknown. We compared the host searching behavior of the egg parasitoid on four alfalfa clones varying in trichome characters. Female wasps were videotaped on Ranger, a susceptible clone with relatively sparse trichomes, B14, a resistant clone with dense but nonglandular trichomes, and FG12 and FG18, two resistant clones with glandular trichomes. Although the number of leafhopper eggs per stem exposed to wasps did not significantly differ among the four clones, the frequency of foraging and total foraging time were less on the two clones with glandular trichomes than on the two clones with nonglandular trichomes. In addition, an analysis of covariance demonstrated that, although the number of ovipositional probes increased with egg density on a stem, the number of probes on stems with glandular trichomes was significantly less than that on stems without glandular trichomes. The allocation of time by wasps among drumming, probing, and grooming behaviors was similar among the clones. Wasps tended to fly off of clones with glandular trichomes more often than off of clones with nonglandular trichomes. This study suggests that cultivars with glandular trichomes may interfere with host searching by A. nigriventris.     

Genetic systems of six species ofPlantago (Plantaginaceae)

Investigation of the genetic system of six species ofPlantago has revealed striking differences in their breeding and meiotic systems.Plantago patagonica is an inbreeder on account of preanthesis cleistogamy, whereasP. lanceolata is an obligate outbreeder, as it is self-incompatible.Plantago drummondii, P. lagopus, P. ovata, andP. major show mixed mating but in varying proportions. In terms of their energy budgets, outbreeding species invest more in floral advertisement and male function, while inbreeders invest more in female function. The contribution of the meiotic system to genetic variability, as revealed through recombination index, is more important in the inbreeding species.     

Interaction between oil-collecting bees and seven species of Plantaginaceae

Oil-bee/oil-flower mutualism evolved through multiple gains and losses of the ability to produce floral oil in plants and to collect it in bees. Around 2000 plant species are known to produce floral oils that are collected by roughly 450 bee species, which use them for the construction of nests and for the larval food. The Plantaginaceae contain several Neotropical species that produce floral oils, the main reward offered by these plants. In the genera Angelonia, Basistemon, Monopera and Monttea, mainly associated with Centris bees, the floral oil is produced in trichomes that are located in the inner corolla. The pollinators of a few species in this neotropical clade of Plantaginaceae are known, and the role of flower morphology as well as the requirements from pollinators and the role of other groups of bees in the pollination of these flowers remains unclear. In this paper we provide a list of the flower visitors of seven Plantaginaceae species (six Angelonia species and Basistemon silvaticus) analyzing their behavior to highlight the legitimate pollinators and illustrating little known aspects of flower morphology and oil-collecting apparatuses of the bees. Two general morphological patterns were observed in the Angelonia flowers: deep corolla tube with short lobes, and short corolla tube with long lobes. Corolla tubes of different length result in pollen adherence to different parts of the insect body. The six Angelonia species and B. silvaticus flowers were visited by 25 oil-collecting bee species (10 Centris, 11 Tapinotaspidini and 4 Tetrapedia species), the majority acting as legitimate visitors. The flowers were also visited by illegitimate bee pollinators, which collected pollen but do not transfer it to the female organ. Specialized collectors of Plantaginaceae floral oils present modifications on the first pair of legs, mainly in the basitarsi but also extended to the tarsomeres. The new records of Tapinotaspidini and Centridini species acting as specialized pollinators of Plantaginaceae suggest that there is a geographic variation in the pollinators of the same plant species, and that the evolutionary scenario of the historical relationships between oil-collecting bees and floral oil producing plants is more complex than previously considered.     

TrichOME: A Comparative Omics Database for Plant Trichomes

Plant secretory trichomes have a unique capacity for chemical synthesis and secretion and have been described as biofactories for the production of natural products. However, until recently, most trichome-specific metabolic pathways and genes involved in various trichome developmental stages have remained unknown. Furthermore, only a very limited amount of plant trichome genomics information is available in scattered databases. We present an integrated “omics” database, TrichOME, to facilitate the study of plant trichomes. The database hosts a large volume of functional omics data, including expressed sequence tag/unigene sequences, microarray hybridizations from both trichome and control tissues, mass spectrometry-based trichome metabolite profiles, and trichome-related genes curated from published literature. The expressed sequence tag/unigene sequences have been annotated based upon sequence similarity with popular databases (e.g. Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, and Transporter Classification Database). The unigenes, metabolites, curated genes, and probe sets have been mapped against each other to enable comparative analysis. The database also integrates bioinformatics tools with a focus on the mining of trichome-specific genes in unigenes and microarray-based gene expression profiles. TrichOME is a valuable and unique resource for plant trichome research, since the genes and metabolites expressed in trichomes are often underrepresented in regular non-tissue-targeted cDNA libraries. TrichOME is freely available at http://www.planttrichome.org/.Plant trichomes are epidermal tissues located on the surfaces of leaves, petals, stems, petioles, peduncles, and seed coats depending on species. By virtue of their physical properties (size, density), trichome hairs can directly serve to protect buds of plants from insect damage, reduce leaf temperature, increase light reflectance, prevent loss of water, and reduce leaf abrasion (Wagner, 1991; Wagner et al., 2004).Although the morphology of trichomes varies greatly, they can be generally classified into two types: simple trichomes (STs) and glandular secreting trichomes (GSTs; Wagner et al., 2004). STs of Arabidopsis (Arabidopsis thaliana) have been chosen as models for studying cell fate and differentiation (Wagner, 1991; Breuer et al., 2009; Marks et al., 2009). In Arabidopsis, STs on leaves consist of a unicellular structure with a stalk and three to four branches (Fig. 1B). Although the STs are referred to as “nonglandular” (presumably nonsecreting), expression of genes involved in anthocyanin, flavonoid, and glucosinolate pathways can nevertheless be detected in STs, indicating the roles of STs in the biosynthesis of secondary compounds and defense (Wang et al., 2002; Jakoby et al., 2008). GSTs are found on about one-third of vascular plants. GSTs have a multicellular structure with a stalk terminating in a glandular head (Fig. 1, A and C–G). GSTs are initiated from a single protodermal cell that undergoes vertical enlargement and multiple divisions to give rise to fully developed trichomes. GSTs often produce and accumulate terpenoid and phenylpropanoid oils (Wagner et al., 2004). However, alkaloids, the third major class of plant secondary compounds, are not common in GST exudates (Laue et al., 2000). The amount of exudates produced by GSTs may reach 30% of mature leaf dry weight, as found in certain Australian desert plants (Dell and McComb, 1978). Plant GSTs can impact pathogen defense, pest resistance, pollinator attraction, and water retention based on the phytochemicals they secrete.Open in a separate windowFigure 1.Representative scanning electron microscopy images of trichomes on plants. A, Erect glandular trichome on the stem of M. sativa. B, Nonglandular trichome on a rosette leaf of Arabidopsis. C, Procumbent trichome on the petiole of M. truncatula. D, Field of glandular trichomes on a female bract of Cannabis sativa. E, Glandular trichomes on a bract of H. lupulus. F, Nonglandular trichome on a leaf of M. truncatula. G, Types VI (small arrow) and I (large arrow) trichomes on a leaf of S. lycopersicum. All the scanning electron microscopy images were generated as described previously (Ahlstrand, 1996; Esch et al., 2004) using an Emitech Technologies (www.emitech.co.uk) K1150 cyropreparation system and a Hitachi High Technologies (www.hitachi-hhta.com) S3500N scanning electron microscope. Bars = 100 μm.GSTs on the aerial organ surfaces have a unique capacity for synthesis and secretion of chemicals (largely plant secondary metabolites), and they have been described as “chemical factories” for the production of high-value natural products (Mahmoud and Croteau, 2002; Wagner et al., 2004; Schilmiller et al., 2008). Secondary metabolites play important roles in protecting the plant against insect predation and other biotic challenges (Peter and Shanower, 1998), and they are potential sources for pharmaceutical and nutraceutical product development. For example, the trichome-borne artemisinin from Artemisia annua is still the most effective drug against malaria, and the early steps of its biosynthetic pathway have been extensively studied (Duke et al., 1994; Arsenault et al., 2008). Recently, the mechanisms by which plant glandular trichomes make, transport, store, and secrete a great variety of unique compounds, especially terpenoids and flavoniods, have received extended research interest because of the potential use of these compounds in pharmaceutical and nutraceutical applications. Seminal studies have reported the assignment of gene functions to specific metabolic pathways in glandular trichomes of several plant species, including mint (Mentha × piperita; Alonso et al., 1992; Rajaonarivony et al., 1992; Lange et al., 2000), basil (Ocimum basilicum; Gang et al., 2002; Iijima et al., 2004; Xie et al., 2008), Artemisia (Teoh et al., 2006; Zhang et al., 2008), tomato (Solanum lycopersicum; Fridman et al., 2005; Besser et al., 2009; Schilmiller et al., 2009), and hop (Humulus lupulus; Nagel et al., 2008; Wang et al., 2008).

Studies on Plasmodesmata in the Trichomes and Leaf Epidermis of some Asclepiadaceae

The occurrence of plasmodesmata has been studied in the trichomesand leaf epidermis of 12 species of the Asclepiadaceae. Plasmodesmataare observed in the walls of hair cells, epidermal cells, andstomatal apparatus. Plasmodesmata are present between adjacenthair cells from base to the apex; epidermal and basal hair cells;epidermal cells; epidermal and subsidiary cells; subsidiarycells, and subsidiary and guard cells, thus establishing mutualsymplasmatic contacts.