Leaf glandular trichomes of micropropagated Inula britannica – Effect of sucrose on trichome density, distribution and chemical profile

Abstract

Glandular trichomes are secretory organs that vary greatly in size, shape, location, type, and composition of secondary metabolites that are synthesized in them. They represent a protective chemical barrier and metabolic factories for the synthesis of bioactive compounds. We evaluated the effect of sucrose on the growth and biomass production, as well as on the distribution, density, and chemical composition of leaf glandular trichomes developed in vitro cultured Inula britannica, an important medical and aromatic species. Nodal segments (5 mm) were cultivated on the basal Murashige and Skoog medium with different concentrations (0 M, 0.01 M, 0.06 M, 0.1 M and 0.3 M) of sucrose. The lack of sugar does not retard growth, while a high sugar concentration inhibits biomass accumulation in vitro. The density and number of leaf glandular trichomes are changed with the medium composition variation. Morphoanatomical and histochemical analyses of the trichomes of the cultured I. britannica applying SEM, conventional light, fluorescent and Raman microscopy revealed the presence of non- glandular and biseriate glandular trichomes on the leaf surface. The histochemical analysis proved that glandular trichomes synthesized a complex mixture of biomolecules. The Raman microscopy analysis of glandular trichomes confirmed the presence of terpenes, most probably the bicyclic monoterpenes and sesquiterpenes. The multivariate analysis, based on PCA, was applied to distinguish among the glandular trichomes of three sugar concentrations: sugar-free medium (0 M), optimal (0.1 M) and the highest sucrose concentration (0.3 M). The main differences between the chemical profile of the leaf glandular trichomes of the plants grown on the 0 M and the chemical profile of the trichomes developed on the 0.1 and 0.3 M medium come mainly from phenolic compounds and to a smaller degree from cyclic terpenes, while the chemical profile of the glandular trichomes grown on the 0.1 M and 0.3 M differs considerably from the trichomes grown on the sugar-free medium in the polysaccharide cell wall ingredients. Most differences between the chemical profile of the glandular trichomes developed on 0 M and 0.3 M and the chemical profile of the trichomes grown on the 0.1 M of sucrose, come from phenolic compounds.