Unexpected Localization of AGP Epitopes in the Plastids of Aldrovanda vesiculosa: New Questions About the Distribution of These Epitopes in Angiosperms

Arabinogalactan proteins (AGPs) are the glycoproteins that are characteristic of plant cells (except for some algae) and are essential for cell function (cell–cell communication, cell adhesion, cell expansion, cell-wall organization, and signaling) but also for the development of roots, shoots, leaves, flowers, and reproductive structures. They occur inside and on the surface of plant cells, but mainly at the interface between the plasma membrane and the cell wall. Reports on the occurrence of arabinogalactan proteins in plastids are very rare and unclear. Our goal was to determine whether AGP-reactive carbohydrate epitopes are associated with plastids and starch in turion cells of Aldrovanda vesiculosa L. To do this, the localization of the AGP-reactive carbohydrate epitopes was assessed using immunolabeling and confocal and electron microscopy. Abundant signals for the antibodies LM2 and JIM8 against the AGP epitopes were detected in plastids. The LM2 signals were particularly frequent in starch grains. Also, electron microscopy confirmed the presence of AGP-reactive carbohydrate epitopes in plastids. These findings provide further evidence for the unusual intracellular localization of AGP-reactive carbohydrate epitopes in plastids and highlight starch grains as a target for future studies aimed at determining the molecular identity and biological significance of these epitopes.