Taxonomic studies based on leaf epidermal microanatomy using high-resolution microscopy in Lamiaceous species and their antimicrobial effects

The knowledge of essential oil antimicrobial activity of Lamiaceous species is assessed to describe its effects. The comprehensive foliar trichomes and stomatal morphology of the leaves of essential oil-bearing plants from the family Lamiaceae revealed diverse antimicrobial properties. The aim of this study was to investigate the foliar anatomical traits of 19 Lamiaceous taxa belonging to different tribes using light and scanning electron microscopy to correctly diagnose the species. The microanatomy of the foliar epidermis, trichomes diversity, and the stomatal apertural complex was visualized. Quantitative measurements were noted to describe the variations and the qualitative aspects for example, polygonal shape epidermal cells were examined. The stomatal aperture of four types and trichomes appendages both non-glandular and glandular was identified. Significant variation was found in both quantitative and qualitative traits, including unique ornamentation on the trichomes. The taxonomic key was constructed for accurate identification using qualitative morpho-structural traits. The outcomes of this research explored taxonomically to accurately identify the Lamiaceous species using anatomical characters. This study will provide provides the ecological adaptation linked to evolutionary traits of leaf surfaces that evolve with time to adapt the harsh environmental conditions. RESEARCH HIGHLIGHTS: Investigated foliar anatomical traits of 19 Lamiaceous species The anatomy and antimicrobial activity of essential oil yielding Lamiaceae species. SEM revealed diverse aspects including peculiar sculptured trichomes Microscopic identification of different stomatal complex.

Morphological, phytochemical, and transcriptome analyses provide insights into the biosynthesis of monoterpenes in Monarda citriodora

MAIN CONCLUSION

Morphological, phytochemical, and transcriptome analyses revealed candidate genes involved in the biosynthesis of volatile monoterpenes and development of glandular trichomes in Monarda citriodora. Monarda citriodora Cerv. ex Lag. is a valuable aromatic plant due to the presence of monoterpenes as major constituents in its essential oil (EO). Thus, it is of sheer importance to gain knowledge about the site of the biosynthesis of these terpenoid compounds in M. citriodora, as well as the genes involved in their biosynthesis. In this study, we studied different types of trichomes and their relative densities in three different developmental stages of leaves, early stage of leaf development (L1), mid-stage of leaf development (L2), and later stage of leaf development (L3) and the histochemistry of trichomes for the presence of lipid and terpenoid compounds. Further, the phytochemical analysis of this plant through GC-MS indicated a higher content of monoterpenes (thymol, thymoquinone, γ-terpinene, p-cymene, and carvacrol) in the L1 stage with a substantial decrease in the L3 stage of leaf development. This considerable decrease in the content of monoterpenes was attributed to the decrease in the trichome density from L1 to L3. Further, we developed a de novo transcriptome assembly by carrying out RNA sequencing of different plant parts of M. citriodora. The transcriptome data revealed several putative unigenes involved in the biosynthesis of specialized terpenoid compounds, as well as regulatory genes involved in glandular trichome development. The data generated in the present study build a strong foundation for further improvement of M. citriodora, in terms of quantity and quality of its essential oil, through genetic engineering.

The Foliar Anatomy and Micromorphology of Cyphostemma hypoleucum (Vitaceae)

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

Histochemical Analysis and Ultrastructure of Trichomes and Laticifers of Croton gratissimus Burch. var. gratissimus (Euphorbiaceae)

Croton gratissimus (Lavender croton) possesses three distinct secretory structures. These include lepidote and glandular trichomes and non-articulated unbranched laticifers. The lepidote trichomes form a dense indumentum on the abaxial surface of the leaves and canopy the glandular trichomes. Although assumed to be non-glandular, transmission electron microscopy (TEM) indicated high metabolic activity within the stalk and radial cells. Glandular trichomes are embedded in the epidermal layer and consist of a single cell which forms a prominent stalk and dilated head. Laticifers occur on the mid-vein of leaves and are predominantly associated with vascular tissue. In the stems, laticifers are associated with the phloem and pith. Both trichome types and laticifers stained positive for alkaloids, phenolic compounds, and lipids. Positive staining for these compounds in lepidote trichomes suggests their involvement in the production and accumulation of secondary metabolites. These metabolites could provide chemical defense for the plant and potentially be useful for traditional medicine.

Evaluation of the Effect of Leaf Development in Plectranthus amboinicus L. on Antimicrobial Activity and Virulence Factors of Pseudomonas aeruginosa PAO1 and Staphylococcus aureus NCTC8325

Plectranthus amboinicus is widely recognized as a potential source of antimicrobial compounds due to the presence of bioactive components (essential oils) secreted by the glandular trichomes borne on the leaves. As such, an understanding of the effect of leaf development on the production of these essential oils (EOs) is of crucial importance to its medicinal applications. The current study represents the first comparative investigation of the effect of different stages of leaf development (lag, log, and stationary phase) upon the yield and bioactivity of phytochemicals produced. The effects of leaf extracts on the antimicrobial activity, cell surface hydrophobicity, biofilm formation, and motility of P. aeruginosa and Staphylococcus aureus were evaluated. Cryo-scanning electron microscopy was used to record the abundance and distribution of both glandular and non-glandular trichomes during leaf development. Gas chromatography-mass spectrometry analysis revealed that the potent phytochemical thymol is present primarily in log (30.28%) and stationary phase (20.89%) extracts. Log phase extracts showed the lowest minimum inhibitory concentration (25 mg/ml) when compared to other phases of development. Stationary phase extracts were shown to exhibit the highest biofilm dispersal activity against P. aeruginosa (80%), and log phase extracts against biofilms of S. aureus (59%). Log phase extracts showed the highest biofilm inhibitory activity against P. aeruginosa (66%) and S. aureus (63%). In conclusion, log phase leaf extracts of P. amboinicus exhibited a multimodal mechanism of action by displaying antimicrobial, antibiofilm activities and reducing the motility and hydrophobicity, which are important virulence factors in P. aeruginosa and S. aureus pathogenesis.

Foliar microstructure and histochemical analysis of the lavender tree (Heteropyxis natalensis Harv.)

Heteropyxis natalensis Harv. is a native South African tree used in traditional medicine among Venda and Zulu communities. In this study, micromorphological investigations using light and scanning electron microscopy identified long, tapered non-glandular trichomes on the abaxial and adaxial surfaces of the leaves. The total number of trichomes appeared to be greater on emergent leaves and decreased as leaves matured. In addition, schizolysigenous secretory cavities and druse crystals were found within the leaves. Swollen subdermal secretory cavities were also distributed in the midrib of the leaves. Transmission electron microscopy confirmed the presence of nuclei, plastids, mitochondria, vesicles, rough endoplasmic reticulum cisternae and Golgi bodies in the secretory epithelia of these cavities. Various histochemical tests revealed the presence of alkaloids, phenolics, lipids, proteins, essential oils, resin acids and trace amounts of unesterified pectins and polysaccharides in the leaves. This study provides new findings and contributes to the existing research regarding H. natalensis with respect to the micromorphology, ultrastructure and histochemical composition of this species.

Foliar micromorphology, ultrastructure and histochemical analyses of Tagetes minuta L. leaves

Many Tagetes species are known for producing essential oils and commercially useful bioactive compounds. This study investigated the micromorphological features of the internal and external foliar structures of Tagetes minuta that produce and store these compounds. Stereomicroscopy, light microscopy, scanning electron microscopy, transmission electron microscopy, and histochemical analyses were used to examine T. minuta leaves at three developmental stages. The development of the subdermal secretory cavities revealed that the cells undergo autolysis to form a schizolysigenous cavity in the mature leaves. The ultrastructure of the parenchymal sheath and secretory epithelium within the secretory cavity revealed that plastids change to contain lipid and osmiophilic molecules. The histochemical analyses showed that trichomes on the surface of T. minuta leaves appear to be linear and non-glandular but maintain the ability to store bioactive phytocompounds. These are new findings for T. minuta and provide a better understanding of the exudation process, which can help to optimise essential oil production for industrial applications.

Micromorphology, Ultrastructure and Histochemistry of Commelina benghalensis L. Leaves and Stems

Commelina benghalensis L. is used as a traditional medicine in treating numerous ailments and diseases such as infertility in women, conjunctivitis, gonorrhea, and jaundice. This study used light and electron microscopy coupled with histochemistry to investigate the micromorphology, ultrastructure and histochemical properties of C. benghalensis leaves and stems. Stereo and scanning electron microscopy revealed dense non-glandular trichomes on the leaves and stems and trichome density was greater in emergent leaves than in the young and mature. Three morphologically different non-glandular trichomes were observed including simple multicellular, simple bicellular and simple multicellular hooked. The simple bicellular trichomes were less common than the multicellular and hooked. Transmission electron micrographs showed mitochondria, vesicles and vacuoles in the trichome. The leaf section contained chloroplasts with plastoglobuli and starch grains. Histochemical analysis revealed various pharmacologically important compounds such as phenols, alkaloids, proteins and polysaccharides. The micromorphological and ultrastructural investigations suggest that Commelina benghalensis L. is an economically important medicinal plant due to bioactive compounds present in the leaves and stems.