Glandular Trichomes: Bio-cell Factories of Plant Secondary Metabolites

Trichomes are specialised epidermal outgrowth that is present on the aerial parts of plants. On the basis of morphological and cellular variation, they are categorized into non-glandular trichomes (NGTs) and glandular trichomes (GTs). NGTs are known to be involved in the protective and defensive roles that attribute to provide structural and chemical corroboration to form specialized groups of secondary metabolites. GTs are specialized micro-organs that are considered factories for the biosynthesis of a considerable amount of different classes of bioactive metabolites. Conventionally these glandular and non-glandular trichomes are known for their protective roles against different biotic and abiotic stresses. Recently, they have attracted the interest of various researchers as a specialized organ for the production of various bioactive molecules of high pharmaceutical and commercial values. The major groups of secondary metabolites such as terpenoids, flavonoids, phenylpropanes, methyl ketones, acyl sugars and defensive proteins are reported in the trichomes of different plant species. However, the conception of the molecular regulation of their biosynthesis, storage and distribution during the development of trichomes is scattered. This review compiles structural and functional aspects of GTs and NGTs along with the molecular mechanism regulated for the production of secondary metabolite in these specialized organs. In addition, the role of several bio-physical parameters that affect the trichome biochemistry, which either directly or indirectly influence the biosynthesis of secondary metabolite, will also be focussed. The systemized knowledge of trichome biology, secondary metabolite pathway modulation and metabolic engineering at one platform will be helpful to explore recent advances in the field of trichome engineering in many medicinally important plants.