Transport In Plant Cells Research Articles

Plant solute transport

Plant solute transport

Characterization of coat proteins of COPI- and clathrin-coated vesicles in the unicellular green alga Botryococcus braunii

Unicellular green algae are a useful system for studying intracellular protein transport in plant cells because they possess various plant-specific features such as cell wall formation and isolated Golgi bodies with distinct multilayered cisternae. In this study, two genes encoding the β-COP subunit of COPI-coated vesicles and the σ1 subunit of the AP-1 complex of clathrin-coated vesicles were cloned from the green alga Botryococcus braunii. The β-COP gene contains an open reading frame (ORF) that codes for a 947-amino acid protein with a deduced molecular mass of 104 kDa, while the σ1 gene contains an ORF for a 159-amino acid protein with a deduced molecular mass of 19 kDa. Polyclonal antibodies raised against these proteins recognized proteins with predicted molecular masses in cell lysates prepared from B. braunii and the green alga Chlamydomonas reinhardtii. Cell fractionation studies revealed that both β-COP and σ1 were associated with membranous compartments. By immunoelectron microscopy, β-COP was found mainly on Golgi cisternae and to a lesser extent on Golgi-associated vesicles at all levels of the Golgi bodies, consistent with a proposed role of COPI-coated vesicles in inter-compartmental transport between Golgi cisternae.

Arabidopsis Sterol Endocytosis Involves Actin-Mediated Trafficking via ARA6-Positive Early Endosomes

Background: In contrast to the intense attention devoted to research on intracellular sterol trafficking in animal cells, knowledge about sterol transport in plant cells remains limited, and virtually nothing is known about plant endocytic sterol trafficking. Similar to animals, biosynthetic sterol transport occurs from the endoplasmic reticulum (ER) via the Golgi apparatus to the plasma membrane. The vesicle trafficking inhibitor brefeldin A (BFA) has been suggested to disrupt biosynthetic sterol transport at the Golgi level.Results: Here, we report on early endocytic sterol trafficking in Arabidopsis root epidermal cells by introducing filipin as a tool for fluorescent sterol detection. Sterols can be internalized from the plasma membrane and localize to endosomes positive for the early endosomal Rab5 GTPase homolog ARA6 fused to green fluorescent protein (GFP) (ARA6-GFP). Early endocytic sterol transport is actin dependent and highly BFA sensitive. BFA causes coaccumulation of sterols, endocytic markers like ARA6-GFP, and PIN2, a polarly localized presumptive auxin transport protein, in early endosome agglomerations that can be distinguished from ER and Golgi. Sterol accumulation in such aggregates is enhanced in actin2 mutants, and the actin-depolymerizing drug cytochalasin D inhibits sterol redistribution from endosome aggregations.Conclusions: Early endocytic sterol trafficking involves transport via ARA6-positive early endosomes that, in contrast to animal cells, is actin dependent. Our results reveal sterol-enriched early endosomes as targets for BFA interference in plants. Early endocytic sterol trafficking and recycling of polar PIN2 protein share a common pathway, suggesting a connection between plant endocytic sterol transport and polar sorting events.

Protein transport in plant cells: in and out of the Golgi.

In plant cells, the Golgi apparatus is the key organelle for polysaccharide and glycolipid synthesis, protein glycosylation and protein sorting towards various cellular compartments. Protein import from the endoplasmic reticulum (ER) is a highly dynamic process, and new data suggest that transport, at least of soluble proteins, occurs via bulk flow. In this Botanical Briefing, we review the latest data on ER/Golgi inter-relations and the models for transport between the two organelles. Whether vesicles are involved in this transport event or if direct ER-Golgi connections exist are questions that are open to discussion. Whereas the majority of proteins pass through the Golgi on their way to other cell destinations, either by vesicular shuttles or through maturation of cisternae from the cis- to the trans-face, a number of membrane proteins reside in the different Golgi cisternae. Experimental evidence suggests that the length of the transmembrane domain is of crucial importance for the retention of proteins within the Golgi. In non-dividing cells, protein transport out of the Golgi is either directed towards the plasma membrane/cell wall (secretion) or to the vacuolar system. The latter comprises the lytic vacuole and protein storage vacuoles. In general, transport to either of these from the Golgi depends on different sorting signals and receptors and is mediated by clathrin-coated and dense vesicles, respectively. Being at the heart of the secretory pathway, the Golgi (transiently) accommodates regulatory proteins of secretion (e.g. SNAREs and small GTPases), of which many have been cloned in plants over the last decade. In this context, we present a list of regulatory proteins, along with structural and processing proteins, that have been located to the Golgi and the 'trans-Golgi network' by microscopy.

PAA1, a P-type ATPase of Arabidopsis, functions in copper transport in chloroplasts.

Copper (Cu) is an essential trace element with important roles as a cofactor in many plant functions, including photosynthesis. However, free Cu ions can cause toxicity, necessitating precise Cu delivery systems. Relatively little is known about Cu transport in plant cells, and no components of the Cu transport machinery in chloroplasts have been identified previously. Cu transport into chloroplasts provides the cofactor for the stromal enzyme copper/zinc superoxide dismutase (Cu/ZnSOD) and for the thylakoid lumen protein plastocyanin, which functions in photosynthetic electron transport from the cytochrome b(6)f complex to photosystem I. Here, we characterized six Arabidopsis mutants that are defective in the PAA1 gene, which encodes a member of the metal-transporting P-type ATPase family with a functional N-terminal chloroplast transit peptide. paa1 mutants exhibited a high-chlorophyll-fluorescence phenotype as a result of an impairment of photosynthetic electron transport that could be ascribed to decreased levels of holoplastocyanin. The paa1-1 mutant had a lower chloroplast Cu content, despite having wild-type levels in leaves. The electron transport defect of paa1 mutants was evident on medium containing <1 micro M Cu, but it was suppressed by the addition of 10 micro M Cu. Chloroplastic Cu/ZnSOD activity also was reduced in paa1 mutants, suggesting that PAA1 mediates Cu transfer across the plastid envelope. Thus, PAA1 is a critical component of a Cu transport system in chloroplasts responsible for cofactor delivery to plastocyanin and Cu/ZnSOD.

Sir Rutherford Ness Robertson. 29 September 1913 – 5 March 2001 Elected FRS 1961

Sir Rutherford Ness (Bob) Robertson was one of Australia' most distinguished, influential and respected scientists. He was eminent both for his contributions to scientific thought and knowledge and for the remarkable range of activities he undertook in the cause of science. His contributions to our understanding of the bioenergetics of inorganic ion transport in plant cells were widely recognized. In his other life he served in several key administrative–managerial positions, was the prime mover in a variety of major initiatives that had critical and lasting impacts on the development of Australian science and was a trusted friend and adviser to a generation of younger plant scientists. Known universally as Bob and almost universally addressed as such, it would seem odd and almost disrespectful not to refer to him in this way here. In the following account we draw heavily on four sources of information about Bob's life and work. These are a Personal Record submitted to the Royal Society (Robertson 1991), the transcript of an interview conducted for the Australian Academy of Science (Robertson 1993), an autobiographical article written for the Annual Review of Plant Physiology and Plant Molecular Biology (23)* and a personal account of his involvement in the field of charge separation and energy transduction in biological membranes (24).

Brefeldin A: deciphering an enigmatic inhibitor of secretion.

The fungal macrocyclic lactone brefeldin A (BFA) has proved to be of great value as an inhibitor of protein trafficking in the endomembrane system of mammalian cells ([Sciaky et al., 1997][1]). BFA has also often been used as an inhibitor of secretion and vacuolar protein transport in plant cells,

Protein Transport in Plant Cells

The subcellular localization and secretion of proteins synthesized in the cytosol are determined by short amino acid sequences in their molecules. N-terminal transit peptides provide for protein translocation across the membranes of the ER, mitochondria, plastids, and microbodies. Later, these peptides are cleaved off by processing peptidases. C-terminal peptides direct some proteins into microbodies and vacuoles. Transport into the nucleus and insertion in the membranes are determined by the specific sequences that reside in the molecule of the mature protein. Specific receptors associated with the protein-translocating channel recognize transit peptides. Protein unfolding is required for successful protein transport through these channels. Chaperones maintain proteins in such a state. Folded proteins cross the nuclear pore complex and the membrane of microbodies. Protein transport is tightly associated with their processing. During the vesicular protein transport within the endomembrane system (ER, Golgi apparatus, plasma membrane, and vacuoles), correct protein targeting is ensured by protein sorting during vesicle loading, the assembly of corresponding protein coats, vesicle transport to the acceptor membrane, and specific membrane fusion.

Transport and metabolic degradation of hydrogen peroxide in Chara corallina: model calculations and measurements with the pressure probe suggest transport of H(2)O(2) across water channels.

A mathematical model is presented that describes permeation of hydrogen peroxide across a cell membrane and the implications of solute decomposition by catalase inside the cell. The model was checked and analysed by means of a numerical calculation that raised predictions for measured osmotic pressure relaxation curves. Predictions were tested with isolated internodal cells of CHARA: corallina, a model system for investigating interactions between water and solute transport in plant cells. Series of biphasic osmotic pressure relaxation curves with different concentrations of H(2)O(2) of up to 350 mol m(-3) are presented. A detailed description of determination of permeability (P(s)) and reflection coefficients (sigma(s)) for H(2)O(2) is given in the presence of the chemical reaction in the cell. Mean values were P(s)=(3.6+/-1.0) 10(-6) m s(-1) and sigma(s)=(0.33+/-0.12) (+/-SD, N=6 cells). Besides transport properties, coefficients for the catalase reaction following a Michaelis-Menten type of kinetics were determined. Mean values of the Michaelis constant (k(M)) and the maximum rate of decompositon (v(max)) were k(M)=(85+/-55) mol m(-3) and v(max)=(49+/-40) nmol (s cell)(-1), respectively. The absolute values of P:(s) and sigma(s) of H(2)O(2) indicated that hydrogen peroxide, a molecule with chemical properties close to that of water, uses water channels (aquaporins) to cross the cell membrane rapidly. When water channels were inhibited with the blocker mercuric chloride (HgCl(2)), the permeabilities of both water and H(2)O(2) were substantially reduced. In fact, for the latter, it was not measurable. It is suggested that some of the water channels in CHARA: (and, perhaps, in other species) serve as 'peroxoporins' rather than as 'aquaporins'.

A dominant negative mutant of sar1 GTPase inhibits protein transport from the endoplasmic reticulum to the Golgi apparatus in tobacco and Arabidopsis cultured cells.

Protein secretion plays an important role in plant cells as it does in animal and yeast cells, but the tools to study molecular events of plant secretion are very limited. We have focused on the Sar1 GTPase, which is essential for the vesicle formation from the endoplasmic reticulum (ER) in yeast, and have previously shown that tobacco and Arabidopsis SAR1 complement yeast sar1 mutants. In this study, we have established a transient expression system of GFP-fusion proteins in tobacco and Arabidopsis cultured cells. By utilizing confocal laser scanning microscopy, we demonstrate that a dominant negative mutant of Arabidopsis Sar1 inhibits the ER-to-Golgi transport of Golgi membrane proteins, AtErd2 and AtRer1B, and locates them to the ER. The same mutant Sar1 also blocks the exit from the ER of a vacuolar storage protein, sporamin. These results not only provide the first evidence that the Sar1 GTPase functions in the ER-to-Golgi transport in plant cells, but also prove that conditional expression of dominant mutants of secretory machinery can be a useful tool in manipulating vesicular trafficking.

Inhibitors of the carrier-mediated influx of auxin in suspension-cultured tobacco cells.

Active auxin transport in plant cells is catalyzed by two carriers working in opposite directions at the plasma membrane, the influx and efflux carriers. A role for the efflux carrier in polar auxin transport (PAT) in plants has been shown from studies using phytotropins. Phytotropins have been invaluable in demonstrating that PAT is essential to ensure polarized and coordinated growth and to provide plants with the capacity to respond to environmental stimuli. However, the function of the influx carrier at the whole-plant level is unknown. Our work aims to identify new auxin-transport inhibitors which could be employed to investigate its function. Thirty-five aryl and aryloxyalkylcarboxylic acids were assayed for their ability to perturb the accumulation of 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene-1-acetic acid (1-NAA) in suspension-cultured tobacco (Nicotiana tabacum L.) cells. As 2,4-D and 1-NAA are preferentially transported by the influx and efflux carriers, respectively, accumulation experiments utilizing synthetic auxins provide independant information on the activities of both carriers. The majority (60%) of compounds half-inhibited the carrier-mediated influx of [14C]2,4-D at concentrations of less than 10 microM. Most failed to interfere with [3H]NAA efflux, at least in the short term. Even though they increasingly perturbed auxin efflux when given a prolonged treatment, several compounds were much better at discriminating between influx and efflux carrier activities than naphthalene-2-acetic acid which is commonly employed to investigate influx-carrier properties. Structure-activity relationships and factors influencing ligand specificity with regard to auxin carriers are discussed.

Blue light‐dependent monovalent anion uptake

Blue light is one of the most important environmental signals regulating monovalent anion transport in plant cells. In the unicellular freshwater chlorophyte Monoraphidium braunii, blue light is essential for the activation of HCO3−, NO3−, NO22 and Cl− transport systems. These anions are taken up when blue light is present but the uptake ceases when this radiation is suppressed, indicating that blue light is a switch signal for the monovalent anion transport system(s) of this alga. Similar results have been obtained in other green algae and higher plants. The action spectra for the uptake of NO3− and Cl− in M. braunii are very similar and resemble the absorption spectra of flavins or a combination of flavins and pterins. It is proposed that both anions share the same transport system(s). The uptake of monovalent anions consists of a cotransport with H+, thus producing alkalinization of the external medium. The time between the onset of blue light and the beginning of alkalinization can be as short as 2 s. Taken together, the results suggest that the photoreceptor mediating the blue light activation of monovalent anion uptake in this green alga is a plasma membrane‐bound flavoprotein.

Insights into the secretory pathway and vesicular transport in plant cells

The vectorial transport of membrane and macromolecules within the cytoplasm of eukaryotic cells has been the subject of intense investigation over the last decade. In this paper we review some of the recent advances made in our understanding of vesicle transport and the secretory pathway in plant cells.

Vacuolar H(+)-translocating pyrophosphatases: a new category of ion translocase.

The membrane surrounding the central vacuole of plant cells contains an H(+)-translocating ATPase (H(+)-ATPase) and an H(+)-translocating inorganic pyrophosphatase (H(+)-PPase). Both enzymes are abundant and ubiquitous in plants but the H(+)-PPase is unusual in its exclusive use of inorganic pyrophosphate (PPi) as an energy source. The lack of sequence identity between the vacuolar H(+)-PPase and any other characterized ion pump implies a different evolutionary origin for this translocase. The existence of the vacuolar H(+)-PPase, in conjunction with increasing recognition of PPi as a key metabolite in plant systems, necessitates reconsideration of ATP as the primary energy source for membrane transport in plant cells.

Use of the signal peptide of Pisum vicilin to translocate β-glucuronidase in Nicotiana tabacum

A hybrid protein system was used for the study of protein transport in plant cells. A nucleotide sequence ( vic) encoding a putative signal peptide of 15 amino acid residues, derived from the published aa sequence of one Pisum vicilin, was synthesized and fused in frame to the gus gene encoding a bacterial cytosolic β-glucuronidase (GUS). When the hybrid vic::gus gene was expressed in tobacco cells using the cauliflower mosaic virus 35S promoter, the hybrid GUS protein was targeted to, and glycosylated inside the rough endoplasmic reticulum. Glycosylation could be blocked with the antibiotic tunicamycin. The study of transient expression in protoplasts showed that extracellular secretion efficiency was low, which may be due to the nature of the GUS protein.

On polar auxin transport in plant cells

We present here explicit mathematical formulas for calculating the concentration, mass, and velocity of movement of the center of mass of the plant growth regulator auxin during its polar movement through a linear file of cells. The results of numerical computations for two cases, (a) the conservative, in which the mass in the system remains constant and (b) the non-conservative, in which the system acquires mass at one end and loses it at the other, are graphically presented. Our approach differs from that of Mitchison's (Mitchison 1980) in considering both initial effects of loading and end effects of substance leaving the file of cells. We find the velocity varies greatly as mass is entering or leaving the file of cells but remains constant as long as most of the mass is within the cells. This is also the time for which Mitchison's formula for the velocity, which neglects end effects, reflects the true velocity of auxin movement. Finally, the predictions of the model are compared with two sets of experimental data. Movement of a pulse of auxin through corn coleoptiles is well described by the theory. Movement of auxin through zucchini shoots, however, shows the need to take into account immobilization of auxin by this tissue during the course of transport.

Identification of heat shock protein hsp70 homologues in chloroplasts.

Cytoplasmic members of the heat shock protein hsp70 family have recently been implicated in the transport of proteins to the endoplasmic reticulum and mitochondria. In addition, other hsp70 homologues have been found in the endoplasmic reticulum and mitochondria and, at least for the endoplasmic reticulum hsp70 homologue, may be involved in the proper folding and assembly of newly transported proteins. Since chloroplasts are an important site of protein transport in plant cells, we were interested in determining whether hsp70 proteins might be located in this organelle. By using immunoblotting techniques and two antibody preparations against hsp70 proteins, we have identified three chloroplastic proteins of approximately 70 kDa that are related to hsp70 proteins. One of these proteins was tightly associated with the outer envelope membrane and was not exposed at the outer surface of the chloroplasts. The other two were soluble proteins located in the stroma. Steady-state levels of the chloroplastic hsp70 homologues did not change after heat stress nor were any additional hsp70 homologues detected in chloroplasts isolated from heat-stressed plants. We discuss the possible functions of these hsp70 homologues in the transport of proteins into and within chloroplasts.

Solute Transport in Plant Cells and Tissues. D. A. Baker , J. L. Hall , M. Wilkins

Previous articleNext article No AccessNew Biological BooksSolute Transport in Plant Cells and Tissues. D. A. Baker , J. L. Hall , M. Wilkins J. B. HansonJ. B. Hanson Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Quarterly Review of Biology Volume 64, Number 2Jun., 1989 Published in association with Stony Brook University Article DOIhttps://doi.org/10.1086/416279 Views: 1Total views on this site Copyright 1989 The Stony Brook Foundation, Inc.PDF download Crossref reports no articles citing this article.

REVIEWS

Books reviewed in this article: Solute Transport in Plant Cells and Tissues‐ Edited by D. A. Bakkr and J. L, Hall. Cell and Tissue Culture in Forestry, Volume 2. Specific Principles and Methods: Growth and Developments. Ed. by J. M, Bonga and Don J. Durzan. Improving Vegetatively Propagated Crops. Ed. by A. J. Abbot and R. K. Aitkin. Acid Rain and Trees. No. 19 on Focus of Nature Conservation. By K. A. Ling and M. R. Ashmore.

Dual Pattern of Potassium Transport in Plant Cells: A Physical Artifact of a Single Uptake Mechanism

Analysis of potassium transport in plant root cells shows two rate-influencing regions: an unstirred boundary layer adjacent to the cell wall acts as a rate-limiting region and the negatively charged cell wall acts as a rate-enhancing region. The rate-enhancing region gives rise to a pseudo 'dual mechanism'. These two regions act in concert to influence significantly the characteristics of the concentration-dependent potassium uptake process. The anomalous ion uptake behaviour found in the literature is explained on the basis of a single active uptake mechanism operating under the influence of these two regions. The most critical property in support of the concept of a 'dual mechanism' for cation uptake in plant roots is explained on this basis. It is unnecessary to invoke separate groups of enzymes, each one of which is active over different concentration ranges. One mechanism operating over the entire concentration range is sufficient.