Ultrastructural studies on the process of aloin production and accumulation in Aloe arborescens (Asphodelaceae) leaves

Aloin, a kind of anthraquinone, is a chemical component in Aloe leaves used in medicine. The processes of aloin production, transport and storage were studied with a transmission electron microscope using the lead acetate precipitate method for ultracytochemical localization of aloin in the leaf of Aloe arborescens Mill. Results showed that aloin was produced in the plastids of the assimilating tissue, transported through the plastid membrane to the surrounding endoplasmic reticulum and enveloped in the vesicles by the endoplasmic reticulum elements. The vesicles approached, and later fused with, the plasmalemma, released their contents into the apoplast through exocytosis and finally, reached the vascular bundle sheath by apoplastic translocation. Aloin was transported to the internal tangential wall of the vascular bundle sheath cell through endoplasmic reticulum vesicles, and reached the cytoplasm of the aloin cell by means of plasmodesmata. Finally, aloin was stored in the vacuole of the cell in which it was produced.