Uptake and apparent digestion of cytoplasmic organelles by protein bodies (protein storage vacuoles) in mung bean cotyledons.

The large protein bodies of the storage parenchyma cells of mung bean (Vigna radiata) cotyledons contain vesicles measuring 0.2 to 2.0 mum in diameter. The vesicles contain ribosomes, ribosomes, membranous elements which may be derived from the endoplasmic reticulum and occasionally Golgi bodies and mitochondria. The vesicles can be seen by transmission electron microscopy in thin sections of plastic embedded specimens and in replicas of freeze-fractured preparations. Serial sections show that the vesicles are completely separated from the protein body membrane and are not invaginations of that membrane. Vesicles with cytoplasmic structures are seen most frequently in 2 to 4 day old seedlings. The vesicles may be formed when undulations of the protein body membrane are so deep as to permit the pinching-off of a portion of the cytoplasm, resulting in its subsequent isolation from the cytoplasm within the protein body. The digestion of the storage protein in the protein body is accompanied by the disappearance of the ribosomes and the membranous elements in the vesicles. We interpret this disappearance of the cytoplasmic structures in the vesicles as being due to their digestion by the protein body hydrolases (ribonuclease, proteinase and lipolytic enzymes). The uptake of cytoplasmic structures by the protein bodies continues after the reverse proteins have been digested. Cytochemical staining shows that the protein bodies and especially the vesicles are rich in acid phosphatase, a known marker of lytic activity in cells. The evidence presented here indicates that the protein bodies are the intracellular sites at which the digestion of cytoplasmic structure occurs. Protein bodies should therefore be considered not only as compartments for the hydrolysis of the stored protein, but also as autophagic organelles involved in the degradation of cytoplasmic macromolecules. The term protein bodies is well established, but the term protein storage vacuoles may describe these organelles more precisely.