Protein storage bodies and vacuoles

Plants store proteins in embryo and vegetative cells to provide carbon, nitrogen, and sulfur resources for subsequent growth and development. The storage and mobilization cycles of amino acids that compose these proteins are critical to the life cycle of plants. Mechanisms for protein storage and mobilization serve many different developmental and physiological functions. For example, stored protein provides building blocks for rapid growth upon seed and pollen germination. Similarly, protein reserves in vegetative cells provide the building blocks for seed and fruit set during reproductive growth and for rapid expansion of vegetative structures after periods of dormancy. In agriculture, proteins stored in seeds and vegetative tissues account for much of the protein consumed directly as food by humans and livestock. Consequently, the biochemistry of storage proteins and the cellular and physiological mechanisms regulating their synthesis are of practical as well as academic interest. In this brief review, we discuss the nature of protein storage bodies and the cellular processes involved in the accumulation of storage proteins. Storage proteins accumulate primarily in the protein storage vacuoles (PSVs) of terminally differentiated cells of the embryo and endosperm and as protein bodies (PBs) directly assembled within the endoplasmic reticulum (ER). The synthesis of storage proteins and the formation of specialized vacuoles occur after cell division is complete, when all further growth occurs only through cell expansion and accumulation of storage substances. In the past, the terms PB and PSV have been used interchangeably, but PSV is now used to differentiate vacuoles containing storage proteins from PBs originating from the ER. Our understanding of the cellular context in which storage proteins accumulate derives from many significant advances in gene structure and regulation, as well as the biochemistry and morphogenesis of storage tissues (reviewed in Chrispeels, 1991; Thomas, 1993; Staswick, 1994; Shewry et al., 1995; Galili and Herman, 1997; Nielsen et al., 1997; Shewry and Tatham, 1998; see also Battey et al., 1999; Marty, 1999; Sanderfoot and Raikhel, 1999; Vitale and Denecke, 1999, in this issue).