# Unraveling the Complexity of Plant Trichomes: Models, Mechanisms, and Bioengineering Strategies
## 4 Glandular Secretory Trichomes (GSTs): Natural Biofactories
### Structure and Morphology Characteristics of GSTs  
GSTs develop in a pattern that is derived from protodermal cells. These cells protrude outward and divide periclinally to form stalk cells and apical head cells. Head cells divide differently depending on the shape of the stalk cells, resulting in varying types of glandular secretory trichomes [15]. The heads of glandular secretory trichomes are the secretory parts, usually spherical, ovoid, cylindrical, or other shapes, where secretions are produced and stored. The stalks support and nourish the head by connecting it to the epidermal cells, which may be short and thick or long and thin [5,15,19]. The diversity of GSTs is a result of dual selection pressures from the environment and pests, which causes them to secrete specific secondary metabolites (Figure 2). _Cannabis_ is an herbaceous plant that has been consumed and used medicinally for centuries. In _Cannabis_, GSTs play a central role in plant defense by producing and storing secondary metabolites such as cannabinoids. These compounds not only deter insect pests and suppress pathogen invasion but also enhance the plant's resilience to abiotic stresses, including drought and ultraviolet radiation [13]. _Cannabis_ contains three types of GSTs: capitate-stalked GTs, capitate-sessile GTs, and bulbous GTs. A capitate-stalked GT contains 12-16 disc cells, while a capitate-sessile GT contains 8 disc cells. Bulbous GT, the smallest type, consists of a bulbous head with a minimal stalk structure [28]. _Cannabis_ is a dioiceous species with marked sexual dimorphism in GST distribution, with female plants exhibiting significantly higher GST density levels, which exhibit a preferential accumulation in reproductive structures, particularly bractal structures and floral calves [29]. _Artemisia annua_ is able to adapt to different environmental conditions due to its secretory function. In drought or high-temperature conditions, volatile oils and waxy substances secreted by the GSTs may reduce water loss and enhance the plant's stress tolerance [16]. _Artemisia annua_ GSTs consist of ten cells arranged in a double row: two basal cells, two stem cells, four subapical cells, and two apical cells. A wax vesicle is surrounded by two pairs of apical secretory cells, and the space between them is known as the subepidermal space [30,31]. In tomato, research has found that the various types of trichomes can repel insect pests, trap them physically, and attract natural enemies for indirect defense [19]. Tomato trichomes are classified into eight types, of which types I, IV, VI, and VII are glandular, containing terminal glands that secrete and store various secondary metabolites, particularly terpen [19]. In the GSTs of _Ocinum basilicum_, the phenylpropanol pathway has been shown to be capable of synthesizing eugenol, chavicol, and their methylated derivatives. _Phillyrea latifolia_ also accumulates flavonoid glycosides within its glandular secretory trichomes as a means of protecting itself against ultraviolet radiation [32]. _Cucurbitaceae_ cucumber plants possess two different types of glandular secretory trichomes: types I and VI. Type I consists of three to four stalk base cells and four to five glandular cells within the head. The stalk base of type VI is longer than that of type I. The variability of stalk length and thickness allows GSTs to adapt to various environments in different ways [33; 34]. Thyme is a perennial herb or half-shrub in the _Lamiaceae_ family. The high levels of active ingredients in thyme give it certain medicinal properties, including antioxidant, antibacterial, and anticancer properties. Two types of GSTs are found on the leaf blades of thyme: peltate glandular trichomes (PGTs), capitate glandular trichomes (CGTs), and non-glandular trichomes (NGTs). Terpenoids produced in PGTs are responsible for their medicinal properties [35]. Honeysuckle refers to the dried flower buds or partially opened flowers of _Lonicera japonica_ Thunb., a plant belonging to the Caprifoliaceae family and the _Lonicera genus_.