Fine Structure of Protein-Storing Plastids in Bean Root Tips

Abstract

The fine structure of leucoplasts in root tip cells of Phaseolus vulgaris L. has been studied in material fixed in glutaraldehyde followed by osmium tetroxide and poststained in uranyl acetate and lead citrate. Plastid development has been followed from the young stages in and near the meristematic region, through an ameboid stage, to the larger forms with more abundant storage products in the outermost cells. The plastids contain a dense stroma penetrated by tubules and cisternae arising from the inner membrane of the plastid envelope. Also located in the stroma are lamellae, ribosome-like particles, phytoferritin granules, and fine fibrils in less dense regions. In some elongate plastids microfilaments run lengthwise in the stroma near the surface. The same plastids store both starch and protein, but in a strikingly different manner. The starch is deposited in the stroma, while the protein always is accumulated within membrane-bounded sacs. These sacs arise as outgrowths from a complex of interconnected tubules which in turn appears to originate by coalescence and proliferation of tubules and cisternae arising from the inner plastid membrane. This "tubular complex" bears a strong resemblance to the prolamellar body of etiolated chloroplasts, but is smaller and ordinarily less regularly organized, and is apparently light-insensitive. Crystallization of the protein commonly occurs in the sacs and occasionally takes place within the tubules of the complex as well. The fine structure of the leucoplasts is discussed in relation to that of etiolated chloroplasts. Suggestions are made concerning the function of the tubular complex, role of the ameboid plastid forms, and manner of accumulation of the storage protein in the plastids.

Formation of Elaborate Networks of T-System Tubules in Cultured Skeletal Muscle With Special Reference to the T-System Formation

“…Elaborate membrane networks somewhat similar to the T-system networks seen here have been reported in the chloride cells of the fish gill (Copeland and Dalton, 1959;Doyle and Gorecki, 1961), in the "clear" cells of the dendritic organs in some marine catfishes (Van Lennep and Lanzing, 1967), in mitochondria (Pappas and Brandt, 1959) and in the plastids of some plants (Gunning, 1965;Newcomb, 1967). A study of extracted proteolipids in vitro (Revel and Ito, 1967) strongly suggests a basic role for proteins in producing and/or stabilizing such membranous networks.…”

Microbodies (Glyoxysomes and Peroxisomes) in Cucumber Cotyledons

“…Similar invaginations and inclusions were frequently observed in the microbodies of the etiolated cotyledons and persisted until shortly after exposure to light. There appears to be no precedent for synthesis of enzymes or other proteins on ribosomes within invaginations into organelles, but invaginations of this sort are not uncommon, and have been reported at certain stages in plastids (41) and mitochondria (2). Moreover, in the epidermal cells of spadix appendices in Sauroinatluin, microbodies increased in numbers during flowering, at which time invaginations into them were commonly observed (4), although no attempt was made to correlate the phenomenon with any enzyme activity.…”

Do plastids inDendrobiumcv. Lucky Duan petals function similar to autophagosomes and autolysosomes?

“…We were unable to show such completely sealed-off intraplastidial spaces yet, as the plastids were too large for complete 3D analysis. As shown in detail by Newcomb 18 the plastids in roottip cells of Phaseolus vulgaris also seem to contain portions of the cytoplasm. Such plastids were found to predominate in the cells located between the meristem and the outermost root layers.…”

“…Such plastids were found to predominate in the cells located between the meristem and the outermost root layers. Newcomb 18 did not report on the further fate of the cytoplasmlike material inside the plastids. Nagl 19 showed effects in plastids of Phaseolus coccineus suspensor cells.…”

“…Newcomb 18 made a detailed study of plastids in root cells of Phaseolus vulgaris. His micrographs indicate that at least some of the plastids contained portions of the cytoplasm.…”