Plastid Degeneration in Tillandsia albida (Bromeliaceae) and Lobivia rauschii (Cactaceae) Provides Evidence about the Origin and Destiny of Multilamellar Bodies in Plants.

Vesicle-like structures containing several to numerous concentric membranes, called multilamellar bodies (MLBs), are present both in animal and plant cells. The origin of MLBs in animal cells has been partially elucidated, while that of plant MLBs is unknown. MLBs in plant cells are present in the cytoplasm, at the interface of cytoplasm and vacuole, and inside vacuoles. This suggests that they become transported from the cytoplasm to the vacuole. This investigation, aims to reveal the origin and destiny of MLBs in plants. The method of choice was the ultrastructural observation of plastid degeneration in anthers of Tillandsia albida and in Lobivia rauschii (respectively one monocot and one dicot species). The function of plant MLBs seems to be the transfer of cellular membranes to the vacuole. Although it is often impossible to discern whether they have a single or a double outer membrane, in some examples a double outer membrane is present. This might suggest autophagic/mitochondrial/plastidial origin. Membrane structures similar to those in MLBs apparently have not been described in mitochondria. By contrast, structures similar to MLBs are found in autophagous structures and in degenerating chloroplasts and other plastids. The data might suggest the hypothesis that plant MLBs derive from autophagous structures and/or from plastids. A mechanism of formation of lipid droplets inside the vacuole, starting from MLB is proposed.