Glandular trichomes of medicinal plants: types, separation and purification, biological activities

Trichomes, one of the epidermal structures of medicinal plants, can be divided into two types according to whether they have a secretory function: glandular trichomes (GTs) and nonglandular trichomes (non-GTs) (Yu 2020). This article mainly elaborates on medicinal plant GTs because the tips of non-GTs lack synthesis and storage cells and do not synthesize and accumulate secondary metabolites. GTs, as an important epidermal tissue, are responsible for pollination and protection (Champagne and Boutry 2016). Some plant GTs release volatile compounds in the air. Some of these compounds are attractants for pollinators, some are repellents for herbivores and ineffective pollinators, and some are even attractants for natural enemies of herbivores (Jacek et al. 2018, Giuliani et al. 2020). Environmental conditions and seasonal changes affect the growth and development of GTs, leading to differences in the types, density, morphology, and inclusions of GTs. These differences may be related to environmental stress and the adaptive survival of plants (Li et al. 1949, Soliman et al. 2019). There are differences in the types, density, morphology, and inclusions of GTs in different plant species; thus, GTs can be used as one of the distinguishing characteristics of medicinal plants (Zhang et al. 2016, Guesmi et al. 2019). GTs, as a secretory tissue, have the remarkable characteristics of synthesizing, modifying, and storing a variety of medicinal active ingredients and contain a great complex of secondary metabolites, including terpenes, flavonoids, alkaloids, lignose, polysaccharides, glycosides, fatty acids, proteins, and alkaloids (Balcke et al. 2017, Konarska and Łotocka 2020). These secondary metabolites accumulate in GTs and have various