Abstract
Pogostemon cablin possesses two morphologically and ontogenetically different types of glandular trichomes, one type of bristle hair on the surfaces of leaves and stems and one type of internal gland inside the leaves and stems. The internal gland originates from elementary meristem and is associated with the biosynthesis of oils present inside the leaves and stems. However, there is little information on mechanism for the oil biosynthesis and secretion inside the leaves and stems. In this study, we identified three kinds of glandular trichome types and two kinds of internal gland in the Pogostemon cablin. The oil secretions from internal glands of stems and leaves contained lipids, flavones and terpenes. Our results indicated that endoplasmic reticulum and plastids and vacuoles are likely involved in the biosynthesis of oils in the internal glands and the synthesized oils are transported from endoplasmic reticulum to the cell wall via connecting endoplasmic reticulum membranes to the plasma membrane. And the comparative analysis of the development, distribution, histochemistry and ultrastructures of the internal and external glands in Pogostemon cablin leads us to propose that the internal gland may be a novel secretory structure which is different from external glands.
Highlights
The Lamiaceae comprise many commercially important species because of their high content of essential oils, which are widely used in pharmaceutical preparations, perfumery and cosmetics
Various colour reactions (Figure S4) indicated that the secretion stored in the sub-cuticular space (SCS) of mature external trichomes contained hydrophilic and lipophilic components (Table 1)
Development and Histochemistry Observations on transverse section of leaves showed internal glands of Pogostemon cablin distributed among palisade cells, which was similar to the intact sub-dermal secretory cavities of Eucalyptus [21] and elaiophores of Oncidium trulliferum [22]
Summary
The Lamiaceae comprise many commercially important species because of their high content of essential oils, which are widely used in pharmaceutical preparations, perfumery and cosmetics. The development and histochemistry of glandular trichomes occurring in plants of the Lamiaceae was well documented [1,2,3,4,5,6] and they were recognized as the defense-related structures on the aerial epidermis of leaves, stems and floral organs [7,8]. As to the types of trichomes (capitate, non-glandular, peltate and different versions and combinations of these), there was some variability in Lamiaceae genera that can occur in a given species [9,10]. Most published studies on the Lamiaceae had concentrated on the ultrastructure of peltate trichomes or capitate trichomes in relation to the secretory process [11,12,13,14,15,16]. There is much distinction about the ultrastructure and secretory process in different glandular trichomes of different species
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