Regulation von Biosynthese und Akkumulation der Reserveproteine wahrend der Entwicklung pflanzlicher Samen1)

THE SAP OF PLANT CELLS

Recent advances in the understanding of the functional relevance of sap of mature plant cells are reviewed. The emphasis is placed on roles of vacuoles played in the temporary storage of saccharides and organic acids, in the accumulation of water soluble products of secondary metabolism and in the intracellular digestion of protein. Contents Summary 1 I. Introduction 1 II. Functions of vacuoles 2 III. Vacuoles as pools of saccharides 3 IV. Organic acids 7 V. (Potentially) toxic cell saps 9 VI. Pools of protein 14 VII. Digestive cell saps 15 VIII. Tonoplast, cell sap and cytoplasm 18 IX. Cellular homeostasis 19 Acknowledgement 20 References 20.

Signal recognition particle triggers the translocation of storage globulin polypeptides from field beans (Vicia faba L.) across mammalian endoplasmic reticulum membrane

Hybridization-selected mRNAs coding for individual storage globulin polypeptides of field beans (Vicia faba L.) were translated in a cell-free system. Added mammalian signal recognition particle (SRP) recognizes cleavable signal peptides of the major vicilin and both legumin polypeptide precursors and induces translational arrest. The latter can be released by potassium-washed membranes (K-RM) leading to shortened polypeptides protected against proteases. Thus, SRP and K-RM function in a similar way with plant polypeptides as described for mammalian secretory proteins [(1981) J. Cell Biol. 91, 557-561]. Obviously, the initial steps in the biosynthesis and processing of plant storage globulin polypeptides are principally identical to those of animal secretory proteins.

Secretion of plant storage globulin polypeptides by Xenopus laevis oocytes

Xenopus oocytes injected with poly(A)-containing RNA from developing cotyledons of field beans (Vicia faba L. var. minor) synthesize precursor polypeptides to the major storage globulins legumin and vicilin. These polypeptides are secreted into the medium without proteolytic cleavage of the legumin propolypeptides into the mature disulfide-linked alpha and beta chains. Similarly, storage globulin polypeptides from pea (Pisum sativum L.) and french bean (Phaseolus vulgaris L.) were secreted from oocytes. Inhibition of glycosylation by tunicamycin does not prevent secretion. This first report on the secretion of plant polypeptides by Xenopus oocytes shows that (a) intracellular deposition of storage proteins in membrane-bounded organelles (protein bodies) of plants and extracellular secretion have step(s) in common, and (b) the cell, in addition to the mRNA, determines the final destination of these polypeptides.