Structures, components and functions of secretory tissues in Houttuynia cordata

Houttuynia cordata Thunb., traditionally used as a therapeutic plant in folk medicine, has shown antioxidant and anticancer activities. The species, as a core component of paleoherbs, is normally characterized based on the presence of different types of secretory tissue: oil cells, three types of secretory cells and glandular hairs. The aim of this work was to study the structural, componential, and the functional characteristics of the secretory tissues in both the floral and vegetative parts. The results indicate that oil cells and secretory cells are distributed in all organs of the plant, while glandular hairs are situated on the aerial stems and leaves. Both oil cells and glandular hairs initiate from the protoderm, but their developmental processes are different. Although three types of secretory cells initiate from different primary meristems, the developmental patterns of different secretory cells are the same. Also, although the origins of secretory cells are different from oil cells, their early developmental processes are the same. Histochemical results show that oil cells, secretory cells and glandular hairs produce flavonoids, phenolic compounds, tannins, lipids, aldehyde and ketone-compounds. In addition, there are terpenoids and pectic-like substances in oil cells, alkaloids in secretory cells of aerial stems, and terpenoids and alkaloids in glandular hairs. These compounds play very important roles in protecting plants from being eaten by herbivores (herbivory) and infected by microbial pathogens. The oil cell and secretory cell, as unicellular secretory tissues, are intermediates between the primitive surface glandular and secretory cavity and canal during the evolution of secretory structures.