In vitro morphogenesis and secretion of secondary metabolites of Nicotiana tabacum tall glandular trichomes

The tall glandular trichomes occurring on young expanding leaves of Nicotiana tabacum grown in vitro were investigated using light and electron microscopy. Previous studies reported that the tall glandular trichomes of tobacco produce large quantities of various compounds including diterpenes. To explore the cellular structures required and putatively involved in diterpene biosynthesis and secretion, the present study focused on the development of the tall glandular trichomes, and on histochemical and ultrastructural analysis of their secretory cells under in vitro conditions. Electron microscopy confirmed earlier light microscopy observations of the glandular nature of the tall leaf trichomes. Their head cells, which secrete resinous material, exhibited characteristics common to gland cells: a dense cytoplasm, numerous mitochondria and little vacuolation. They contained structurally well-developed chloroplasts and an elaborate network of endoplasmic reticulum, as well as electron dense inclusions in chloroplasts and in small vacuoles. The dominance of smooth endoplasmic reticulum suggests its involvement in diterpene biosynthesis. Histochemical tests were positive for both lipophilic and hydrophilic secretions, which included terpenoids (essential oils and resins) and acidic/neutral lipids, as well as phenolic compounds (tannins), alkaloids, mucilage/ pectin and polysaccharides.