Structural characterization of the stigma-style complex of Cynara cardunculus (Asteraceae) and immunolocalization of cardosins A and B during floral development

Abstract

Studies were carried out on the structure of the stigma and style of Cynara cardunculus L. (cardoon) during flower development. The stigma is of the dry type with a papillate cuticularized epidermis. During development, the unicellular papillae become match-stick shaped, cuticularize, and show an increase in vacuolar volume. In mature papillae, two morphologically different vacuoles were observed, one electron-dense and the other electron-transparent, putatively corresponding to distinct vacuolar populations. These vacuoles label differently for cardosin A, specifically detected in the electron-dense compartments. The style is solid with a cuticularized epidermis and a central core of transmitting tissue (TT) several cell layers thick. The TT cells show abundant rough endoplasmic reticulum and Golgi bodies, associated with active secretion. During maturation, TT cells become increasingly separated by a polysaccharide-rich extracellular matrix. Communication between TT cells is maintained via plasmodesmata in longitudinal walls. Distribution of cardosins A and B in developing C. cardunculus flowers was also characterized. The presence of aspartic proteinases (APs) in flowers is unusual, generally occurring at low levels. Cardosins A and B are always present in cardoon florets and localize at distinct pistil levels: stigma (papillae) and style (TT), respectively. This differential localization suggests distinct biological functions for cardosins, most likely essential for reproduction in this species.