Glandular trichomes are epidermal outgrowths that are the site of biosynthesis and storage of large quantities of specialized metabolites. Besides their role in the protection of plants against biotic and abiotic stresses, they have attracted interest owing to the importance of the compounds they produce for human use; for example, as pharmaceuticals, flavor and fragrance ingredients, or pesticides. Here, we review what novel concepts investigations on glandular trichomes have brought to the field of specialized metabolism, particularly with respect to chemical and enzymatic diversity. Furthermore, the next challenges in the field are understanding the metabolic network underlying the high productivity of glandular trichomes and the transport and storage of metabolites. Another emerging area is the development of glandular trichomes. Studies in some model species, essentially tomato, tobacco, and Artemisia, are now providing the first molecular clues, but many open questions remain: How is the distribution and density of different trichome types on the leaf surface controlled? When is the decision for an epidermal cell to differentiate into one type of trichome or another taken? Recent advances in gene editing make it now possible to address these questions and promise exciting discoveries in the near future.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.16283

Glandular trichomes: micro-organs with model status?

Plant glandular trichomes (GTs) are epidermal outgrowths with the capacity to biosynthesize and secrete specialized metabolites, that are of great scientific and practical significance. Our understanding of the developmental process of GTs is limited, and no single plant species serves as a unique model. Here, we review the genetic mechanisms of GT initiation and development and provide a summary of the biosynthetic pathways of GT-specialized metabolites in nonmodel plant species, especially horticultural crops. We discuss the morphology and classification of GT types. Moreover, we highlight technological advancements in methods employed for investigating GTs. Understanding the molecular basis of GT development and specialized metabolites not only offers useful avenues for research in plant breeding that will lead to the improved production of desirable metabolites, but also provides insights for plant epidermal development research.

/pdf/glandular-trichomes-new-focus-on-horticultural-crops-13npqg3bhx.pdf

Glandular trichomes: new focus on horticultural crops.

Plant trichomes are outgrowths developed from an epidermal pavement cells of leaves and other organs. Trichomes (also called ‘hairs’) play well-recognized roles in defense against insect herbivores, forming a physical barrier that obstructs insect movement and mediating chemical defenses. In addition, trichomes can act as a mechanosensory switch, transducing mechanical stimuli (e.g., insect movement) into physiological signals, helping the plant to respond to insect attacks. Hairs can also modulate plant responses to abiotic stresses, such as water loss, an excess of light and temperature, and reflect light to protect plants against UV radiation. The structure of trichomes is species-specific and this trait is generally related to their function. These outgrowths are easily analyzed and their origin represents an outstanding subject to study epidermal cell fate and patterning in plant organs. In leaves, the developmental control of the trichomatous complement has highlighted a regulatory network based on four fundamental elements: (i) genes that activate and/or modify the normal cell cycle of epidermal pavement cells (i.e., endoreduplication cycles); (ii) transcription factors that create an activator/repressor complex with a central role in determining cell fate, initiation, and differentiation of an epidermal cell in trichomes; (iii) evidence that underlines the interplay of the aforesaid complex with different classes of phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we reviewed the role of genes in the development of trichomes, as well as the interaction between genes and hormones. Furthermore, we reported basic studies about the regulation of the cell cycle and the complexity of trichomes. Finally, this review focused on the epigenetic factors involved in the initiation and development of hairs, mainly on leaves.

https://www.mdpi.com/2223-7747/8/8/253/pdf

The Dynamic Genetic-Hormonal Regulatory Network Controlling the Trichome Development in Leaves.

Thyme has medicinal and aromatic value because of its potent antimicrobial and antioxidant properties. However, the absence of a fully sequenced thyme genome limits functional genomic studies of Chinese native thymes. Thymus quinquecostatus Čelak., which contains large amounts of bioactive monoterpenes such as thymol and carvacrol, is an important wild medicinal and aromatic plant in China. Monoterpenoids are abundant in glandular secretory trichomes. Here, high-fidelity and chromatin conformation capture technologies were used to assemble and annotate the T. quinquecostatus genome at the chromosome level. The 13 chromosomes of T. quinquecostatus had a total length of 528.66 Mb, a contig N50 of 8.06 Mb, and a BUSCO score of 97.34%. We found that T. quinquecostatus had experienced two whole-genome duplications, with the most recent event occurring ∼4.34 million years ago. Deep analyses of the genome, in conjunction with comparative genomic, phylogenetic, transcriptomic, and metabonomic studies, uncovered many regulatory factors and genes related to monoterpenoids and glandular secretory trichome development. Genes encoding terpene synthase (TPS), cytochrome P450 monooxygenases (CYPs), short-chain dehydrogenase/reductase (SDR), R2R3-MYB, and homeodomain-leucine zipper (HD-ZIP) IV were among those present in the T. quinquecostatus genome. Notably, Tq02G002290.1 (TqTPS1) was shown to encode the terpene synthase responsible for catalyzing production of the main monoterpene product γ-terpinene from geranyl diphosphate (GPP). Our study provides significant insight into the mechanisms of glandular secretory trichome formation and monoterpenoid biosynthesis in thyme. This work will facilitate the development of molecular breeding tools to enhance the production of bioactive secondary metabolites in Lamiaceae. 

https://doi.org/10.1016/j.xplc.2022.100413

Chromosome-level assembly and analysis of the Thymus genome provide insights into glandular secretory trichome formation and monoterpenoid biosynthesis in thyme

Plant trichomes, protrusions formed from specialized aboveground epidermal cells, provide protection against various biotic and abiotic stresses. Trichomes can be unicellular, bicellular or multicellular, with multiple branches or no branches at all. Unicellular trichomes are generally not secretory, whereas multicellular trichomes include both secretory and non-secretory hairs. The secretory trichomes release secondary metabolites such as artemisinin, which is valuable as an antimalarial agent. Cotton trichomes, also known as cotton fibers, are an important natural product for the textile industry. In recent years, much progress has been made in unraveling the molecular mechanisms of trichome formation in Arabidopsis thaliana, Gossypium hirsutum, Oryza sativa, Cucumis sativus, Solanum lycopersicum, Nicotiana tabacum, and Artemisia annua. Here, we review current knowledge of the molecular mechanisms underlying fate determination and initiation, elongation, and maturation of unicellular, bicellular and multicellular trichomes in several representative plants. We emphasize the regulatory roles of plant hormones, transcription factors, the cell cycle and epigenetic modifications in different stages of trichome development. Finally, we identify the obstacles and key points for future research on plant trichome development, and speculated the development relationship between the salt glands of halophytes and the trichomes of non-halophytes, which provides a reference for future studying the development of plant epidermal cells.

/pdf/molecular-mechanisms-of-plant-trichome-development-1g1gp022.pdf

Molecular Mechanisms of Plant Trichome Development

A novel gene NbGIS positively regulates glandular trichome initiation through GA Signaling in tobacco. NbMYB123-like regulates glandular trichome initiation by acting downstream of NbGIS in tobacco. Glandular trichome is a specialized multicellular structure which has capability to synthesize and secrete secondary metabolites and protects plants from biotic and abiotic stresses. Our previous results revealed that a C2H2 zinc-finger transcription factor GIS and its sub-family genes act upstream of GL3/EGL3-GL1-TTG1 transcriptional activator complex to regulate trichome initiation in Arabidopsis. In this present study, we found that NbGIS could positively regulate glandular trichome development in Nicotiana benthamiana (tobacco). Our result demonstrated that 35S:NbGIS lines exhibited much higher densities of trichome on leaves, main stems, lateral branches and sepals than WT plants, while NbGIS:RNAi lines had the opposite phenotypes. Furthermore, our results also showed that NbGIS was required in response to GA signal to control glandular trichome initiation in Nicotiana benthamiana. In addition, our results also showed that NbGIS significantly influenced GA accumulation and expressions of marker genes of the GA biosynthesis, might result in the changes of growth and maturation in tobacco. Lastly, our results also showed that NbMYB123-like regulated glandular trichome initiation in tobacco by acting downstream of NbGIS. These findings provide new insights to discover the molecular mechanism by which C2H2 transcriptional factors regulates glandular trichome initiation through GA signaling pathway in tobacco.

NbGIS regulates glandular trichome initiation through GA signaling in tobacco

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NbGIS regulates glandular trichome initiation through GA signaling in tobacco