Laticifer development and its growth mode in Allamanda blanchetii A. DC. (Apocynaceae)

Abstract

Recent studies have shown the laticifers of Apocynaceae, previously classified as nonarticulated, indeed are articulated, anastomosing laticifers whose transverse walls dissolve rapidly and entirely, although doubts about their growth mode still persist. To better understand the mode of laticifer growth and differentiation in this family, we studied its development in Allamanda blanchetii using anatomical and ultrastructural analyses. Our results showed that laticifers are formed by a row of cells that join each other through dissolution of the transverse walls from the center to the periphery. The laticifers originate from ground meristem and procambium; the laticifers in the different tissues connect through lateral fusion, generating a laticifer network. The laticifers occur in the cortex, pith, and vascular system of the shoot, mesophyll, and vascular bundles of the leaves. There is no apical growth, and all the organelles observed in the apices of the laticifers play a role in the production of latex or in the dissolution of the terminal walls between the cells that compose the laticifer. The latex is composed of many metabolites produced mainly in the endoplasmic reticulum and plastids. Mitochondria are abundant, and dictyosomes are scarce. The vacuome is prominent from the start of laticifer differentiation, and many small vacuoles and vesicles transport the secretion from cytosol into the central vacuole, where an emulsion of substances is stored. The articulated, anastomosing laticifers of A. blanchetii have no subcellular mechanism for production of a cell wall in a polarized manner or dissolution of middle lamella of the cells which surround the laticifer tip, proving that there is no intrusive growth in this secretory structure.