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.

Background Perilla frutescens (L.) Britt is a medicinal and edible plant widely cultivated in Asia. Terpenoids, flavonoids and phenolic acids are the primary source of medicinal ingredients. Glandular trichomes with multicellular structures are known as biochemical cell factories which synthesized specialized metabolites. However, there is currently limited information regarding the site and mechanism of biosynthesis of these constituents in P. frutescens. Herein, we studied morphological features of glandular trichomes, metabolic profiling and transcriptomes through different tissues. Results Observation of light microscopy and scanning electron microscopy indicated the presence of three distinct glandular trichome types based on their morphological features: peltate, capitate, and digitiform glandular trichomes. The oil of peltate glandular trichomes, collected by custom-made micropipettes and analyzed by LC–MS and GC–MS, contained perillaketone, isoegomaketone, and egomaketone as the major constituents which are consistent with the components of leaves. Metabolomics and transcriptomics were applied to explore the bioactive constituent biosynthesis in the leaves, stem, and root of P. frutescens. Transcriptome sequencing profiles revealed differential regulation of genes related to terpenoids, flavonoids, and phenylpropanoid biosynthesis, respectively with most genes expressed highly in leaves. The genes affecting the development of trichomes were preliminarily predicted and discussed. Conclusions The current study established the morphological and chemical characteristics of glandular trichome types of P. frutescens implying the bioactive constituents were mainly synthesized in peltate glandular trichomes. The genes related to bioactive constituents biosynthesis were explored via transcriptomes, which provided the basis for unraveling the biosynthesis of bioactive constituents in this popular medicinal plant.

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