Identification of secretory vesicles in homogenates of pea stem segments

A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

Protein secretion in plants

Protein secretion is an ubiquitous but poorly understood process in plants. Secreted proteins are synthesized on the membranes of the rough endoplasmic reticulum and transported to the cell surface by secretary vesicles formed at the Golgi apparatus. Whereas many of the structural details of this process are known the mechanisms underlying secretion are just beginning to be understood, in this article we review some of the recent developments in this field, and we compare the progress made with animal and plant cells. CONTENTS Summary 567 I. Introduction 568 II. Proteins secreted by plants 568 III. Synthesis and post-translational modification of secreted proteins 571 IV. Molecular requirements for secretion 576 V. Vehicles of secretory transport 581 VI. Regulation of secretion 585 VII. Conclusions and Perspective 587 Acknowledgements 588 References 588.

The tonoplast proton-translocating ATPase of higher plants as a third class of proton-pumps

Taken together, all the data reported recently in the literature suggest that tonoplast ATPase belongs to a new class of proton pumps. To date, the most studied system is the proton-pumping ATPase from the tonoplast of Hevea latex. Its main characteristics are presented. It resembles the mitochondrial ATPase in its specificity, its substrate affinity, and its sensitivity to different inhibitors. However, for some aspects, it resembles the plasma membrane system in its response to other inhibitors tested (quercetin for example). It differs from both ATPases in its sensitivity to nitrate as well as by its molecular structure, i.e. a complex exhibiting a least 4 or 5 polypeptides. These results favor the existence of a third class of proton pumps, intermediate between the F1F0-class and the E1E2-class.

Cell-fractionation analysis of glucan synthase I and II distribution and polysaccharide secretion in soybean protoplasts : Evidence for the involvement of coated vesicles in wall biogenesis

The organelles of soybean (Glycine max (L.) Merr.) protoplasts were separated using a recently developed procedure which allows rapid (3-h) recovery of a fraction enriched for coated vesicles (CVs). As determined by marker-enzyme enrichment and ultrastructural analysis of isolated membrane fractions, endoplasmic reticulum, Golgi membranes, glucan-synthase-II (EC 2.4.1.34)-containing membranes (putative plasma membrane), mitochondria, and CVs were enriched in separate fractions in a sucrose density gradient. Glucan synthase I (EC 2.4.1.12) had the highest specific activity in the Golgi-enriched and CV-enriched fractions and was found to comigrate with CVs upon rate-zonal centrifugation of a CV-enriched fraction. For further elucidation of the role of these latter organelles in cell-wall regeneration, freshly isolated protoplasts were pulsed with [(3)H]glucose for 20 min, and the disappearance of label from the organelles was followed for the ensuing 1 h. Although a CV-enriched fraction contained glucan synthase I, it contained very small amounts of labelled polysaccharide during the period of study. Pulse-chase experiments with [(3)H]glucose helped to confirm the role of the Golgi apparatus in secretion of matrix polysaccharides by protoplasts.

Calmodulin dependent NAD-kinase is associated with both the outer and inner mitochondrial membranes in maize roots

Maize root mitochondrial have been subfractionated after osmotic rupture. A calcium-calmodulin-dependent NAD-kinase activity has been shown to be present in both inner and outer membrane fractions. Cytochrome c-reductase activities are also associated with outer and inner membrane fractions but whereas the former is entirely insensitive to 50 μmol·1(-1) antimycin A the latter is reduced by 60% in its presence. This residual antimycin A-insensitive cytochrome c-reductase activity cosediments with the major portion of NAD-kinase activity and equilibrates in sucrose gradients at densities around 1.146 g·cm(-3). Rate zonal centrifugation with renografin allows an excellent separation of both cytochrome c-reductase and NAD-kinase activities. We have no evidence for allocating NAD-kinase activity to endo- or plasma membranes.