Trichome structure of Coscinium fenestratum (Gaertn.) Colebr., a critically endangered medicinal liana suggests a role in defense mechanism

Abstract

Coscinium fenestratum (Gaertn.) Colebr. is a dioecious liana of the plant family Menispermaceae. This plant is a good source of isoquinoline alkaloids, particularly berberine. Being uprooted for its medicinal stem and roots, the natural populations of the plant are under the threat of extinction. The aim of the present study is to understand the trichome structure of the plant using light as well as scanning electron microscopy. All trichomes observed were uniseriate in C. fenestratum. On the basis of morphology and structure, these trichomes can be grouped into three. Type (1) long pointed trichomes (ii) short trichomes and (iii) ciliary trichomes. The length of long pointed trichomes ranged from 500-750 microns which consisted of around 25 cells. It is speculated that the successive periclinal divisions might have resulted in such a long uniseriate trichome. All the cells of this trichome possesses a thick cross wall suggestive of strong defensive structures. The middle part of the trichome has maximum width. The terminal cell is with a sharp pointed tip confirming its defensive function. The ciliary trichomes were much smaller and the terminal cells were appeared as elongated cylindrical cells. The short trichomes comprised around 5-6 cells with a bulbous basal and terminal cell. The occurrence of such bulbous cells reiterates the glandular nature of trichomes. The SEM analysis conducted at a low voltage of 15 kV revealed a fine tuft of long pointed trichomes towards the terminal bud at the shoot apex. The trichome biology of C. fenestratum provided some valuable insights on the possible role of secondary metabolites as a defense against pathogen and pests apart from the basic understanding of the ultrastructure.