Traffic jams affect plant development and signal transduction

Analysis of the Arabidopsis thaliana endomembrane system has shown that plant cell viability depends on a properly functioning vacuole and intact vesicular trafficking. The endomembrane system is also essential for various aspects of plant development and signal transduction. In this review, we discuss examples of these newly discovered roles for the endomembrane system in plants, and new experimental approaches and technologies that are based on high-throughput screens, which combine chemical genetics and automated confocal microscopy.     

Mobile reticular organization of plastids and mitochondria in plant cells

Results of confocal, fluorescent and video microscopy of plant cell organelles and of stromule network uniting them are reviewed. The vast information on the structure of stromules, their mobility, proposed functions and development has been analyzed, in addition to factors stimulating and suppressing this development. Structural similarity between the network of stromules in living cells, observed by confocal fluorescence microscopy, and the endoplasmic reticulum, seen on micrographs of preparations fixed for electron microscopy is discussed. As a result of this discussion, a conclusion is made with regard to the identity of these endomembranous networks. The intercellular symplastic organization is shown for both networks in plant tissues. The existence of a common transport and trophic compartment is proposed that includes organelles, intercellular endoplasmic reticulum and its derivatives, phloem and xylem. The trophic system development might have been induced in the course of endosymbiogenesis with some bacterial precursors of organelles.     

Endosperm Development in Barley: Microtubule Involvement in the Morphogenetic Pathway

An immunofluorescence study of sectioned barley endosperm imaged by confocal laser scanning microscopy provided three-dimensional data on the relationship of microtubules to the cytoplasm, nuclei, and cell walls during development from 4 to 21 days after pollination (DAP). Microtubules play an important role throughout endosperm ontogeny. The syncytium is organized into units of nuclear-cytoplasmic domains by nuclear-based radial microtubule systems that appear to control the pattern of the first anticlinal walls at 5 to 6 DAP. After 7 DAP, phragmoplasts of two origins (interzonal and cytoplasmic) guide wall formation. Large compartments formed by the "free growing" walls in association with cytoplasmic phragmoplasts formed adventitiously at interfaces of opposing microtubule systems are subsequently subdivided by interzonal phragmoplast/cell plates to give rise to the starchy endosperm. During development of the aleurone layer from 8 to 21 DAP, the microtubule cycle is typical of plant histogenesis; cortical microtubules are hooplike, and preprophase bands of microtubules predict the division plane.     

Distribution of polycyclic aromatic hydrocarbons in subcellular root tissues of ryegrass (Lolium multiflorum Lam.)

Background

Because of the increasing quantity and high toxicity to humans of polycyclic aromatic hydrocarbons (PAHs) in the environment, several bioremediation mechanisms and protocols have been investigated to restore PAH-contaminated sites. The transport of organic contaminants among plant cells via tissues and their partition in roots, stalks, and leaves resulting from transpiration and lipid content have been extensively investigated. However, information about PAH distributions in intracellular tissues is lacking, thus limiting the further development of a mechanism-based phytoremediation strategy to improve treatment efficiency.

Results

Pyrene exhibited higher uptake and was more recalcitrant to metabolism in ryegrass roots than was phenanthrene. The kinetic processes of uptake from ryegrass culture medium revealed that these two PAHs were first adsorbed onto root cell walls, and they then penetrated cell membranes and were distributed in intracellular organelle fractions. At the beginning of uptake (< 50 h), adsorption to cell walls dominated the subcellular partitioning of the PAHs. After 96 h of uptake, the subcellular partition of PAHs approached a stable state in the plant water system, with the proportion of PAH distributed in subcellular fractions being controlled by the lipid contents of each component. Phenanthrene and pyrene primarily accumulated in plant root cell walls and organelles, with about 45% of PAHs in each of these two fractions, and the remainder was retained in the dissolved fraction of the cells. Because of its higher lipophilicity, pyrene displayed greater accumulation factors in subcellular walls and organelle fractions than did phenanthrene.

Conclusions

Transpiration and the lipid content of root cell fractions are the main drivers of the subcellular partition of PAHs in roots. Initially, PAHs adsorb to plant cell walls, and they then gradually diffuse into subcellular fractions of tissues. The lipid content of intracellular components determines the accumulation of lipophilic compounds, and the diffusion rate is related to the concentration gradient established between cell walls and cell organelles. Our results offer insights into the transport mechanisms of PAHs in ryegrass roots and their diffusion in root cells.     

Image-based analysis of cell-specific productivity for plant cell suspension cultures

More and more plant cell suspension cultures are regarded as an attractive alternative to mammalian cells as host organism for production of complex recombinant proteins. The most important advantages of the production platform are low costs, easy scalability and enhanced safety by complete lack of animal components in the cultivation media. In order to characterize, understand and control such systems accurately, it is important to determine the cell-specific productivity (Qp) of plant cell-based production platforms. Compared to many microbial and mammalian cells the morphology of plant cells is nonhomogeneous and the cells tend to form aggregates, therefore commercial cell counting systems are too unreliable to determine cell numbers in plant suspension cultures. We addressed this limitation by developing a novel cell counting method based on a combination of cell-staining and automated confocal fluorescence microscopy. This method allowed us, for the first time, to determine the cell-specific productivity of transgenic tobacco (Nicotiana tabacum cv. Bright Yellow-2) cell suspension cultures producing the human antibody M12. In the future this method will be a useful tool in the development of optimized plant cell-based production processes.     

Physiological role of nickel and its toxic effects on higher plants

The focus of the review is on the specific aspects of nickel effect on plants as compared to other heavy metals; their specificity is derived from different physical and chemical properties. The various facets of the physiological role of nickel and its toxic activity in higher plants, its intracellular partition and transport in plant tissues and organ are discussed. The putative mechanisms of nickel hyperaccumulation are considered in several representatives of angiosperm plant families. The existing evidence was used to outline the metabolic changes in plants affected by nickel. The comparison with other heavy metals is used to disclose the general mechanisms that disturb plant mineral nutrition, water regime, photosynthesis, and morphogenesis as well as the common cell responses aimed at detoxification of heavy metals. The numerous nonspecific effects of heavy metals depend on their direct and indirect action; in addition, some effects of nickel are specific. To illustrate, high Ni content in endoderm and pericycle cells blocks cell divisions in the pericycle and results in the inhibition of root branching.     

Thigmo responses in plants and fungi

Thigmo mechanisms are adaptations that permit a plant to alter growth rates, change morphology, produce tropisms, avoid barriers, control germination, cling to supporting structures, infect a host plant, facilitate pollination, expedite the movement of pollen, spores, or seeds, and capture prey. Through these varied functions, plant thigmo systems have evolved impressive controls of cell differentiation, localized growth rates, regulated synthesis of novel products, and some elegant traps and projectile systems. For most thigmo events, there will be a dependence upon transmission of a signal from the cell wall through the plasmalemma and into the cytoplasm. We propose the possible involvement of integrin-like proteins, Hechtian strands, and cytoskeletal structures as possible transduction components. Many thigmo mechanisms may use some modification of the calcium/calmodulin signal transduction system, though the details of transduction systems are still poorly understood. While transmission of thigmo signals to remote parts of a plant is associated with the development of action potentials, hormones may also play a role. Thigmo mechanisms have facilitated an enormous array of plant and fungal adaptations that make major contributions to their success despite their relatively sessile or immobile states.     

Regulatory roles of phosphoinositides in membrane trafficking and their potential impact on cell-wall synthesis and re-modelling

Background

Plant cell walls are complex matrices of carbohydrates and proteins that control cell morphology and provide protection and rigidity for the plant body. The construction and maintenance of this intricate system involves the delivery and recycling of its components through a precise balance of endomembrane trafficking, which is controlled by a plethora of cell signalling factors. Phosphoinositides (PIs) are one class of signalling molecules with diverse roles in vesicle trafficking and cytoskeleton structure across different kingdoms. Therefore, PIs may also play an important role in the assembly of plant cell walls.

Scope

The eukaryotic PI pathway is an intricate network of different lipids, which appear to be divided in different pools that can partake in vesicle trafficking or signalling. Most of our current understanding of how PIs function in cell metabolism comes from yeast and mammalian systems; however, in recent years significant progress has been made towards a better understanding of the plant PI system. This review examines the current state of knowledge of how PIs regulate vesicle trafficking and their potential influence on plant cell-wall architecture. It considers first how PIs are formed in plants and then examines their role in the control of vesicle trafficking. Interactions between PIs and the actin cytoskeleton and small GTPases are also discussed. Future challenges for research are suggested.     

Deciphering the Golgi apparatus: from imaging to genes

The Golgi apparatus is a vital organelle in eukaryotic cells. It grabs and processes secretory materials synthesized by the endoplasmic reticulum (ER) before sorting them to their destination. The Golgi also receives materials from vacuoles/lysosomes and the plasma membrane for further recycling to other compartments within the cell (1) (Figure 1). Given the vital role of the Golgi in a cell, it is important to understand how this organelle attains and maintains its structural and functional integrity during the intense processes of membrane traffic. Despite an equally central role of the Golgi in membrane traffic in eukaryotes, the organization of this organelle has some unique features in each cell system. Therefore, the wealth of information available on the structure and activity of the Golgi in one system is not always directly transferable to others. However, certain morphological and functional aspects are common among cell systems. Therefore, studying the factors that regulate organelle biogenesis and organization of the Golgi apparatus is important in basic cell biology of eukaryotes and may also contribute to a better understanding of how different cell systems have evolved. In this study, we report on the identification of Golgi mutants in plant cells. We have developed a screen that is a promising strategy not only for the identification of genes responsible for the morphological and functional integrity of the plant Golgi but could also provide fundamental information on other multicellular systems for which the power of forward genetics cannot be exploited as easily as in Arabidopsis.     

Real-time tracking of root hair nucleus morphodynamics using a microfluidic approach

Root hairs (RHs) are tubular extensions of root epidermal cells that favour nutrient uptake and microbe interactions. RHs show a fast apical growth, constituting a unique single cell model system for analysing cellular morphodynamics. In this context, live cell imaging using microfluidics recently developed to analyze root development is appealing, although high-resolution imaging is still lacking to enable an investigation of the accurate spatiotemporal morphodynamics of organelles. Here, we provide a powerful coverslip based microfluidic device (CMD) that enables us to capture high resolution confocal imaging of Arabidopsis RH development with real-time monitoring of nuclear movement and shape changes. To validate the setup, we confirmed the typical RH growth rates and the mean nuclear positioning previously reported with classical methods. Moreover, to illustrate the possibilities offered by the CMD, we have compared the real-time variations in the circularity, area and aspect ratio of nuclei moving in growing and mature RHs. Interestingly, we observed higher aspect ratios in the nuclei of mature RHs, correlating with higher speeds of nuclear migration. This observation opens the way for further investigations of the effect of mechanical constraints on nuclear shape changes during RH growth and nuclear migration and its role in RH and plant development.     

The parAB gene products of Pseudomonas putida exhibit partition activity in both P. putida and Escherichia coli

The bacteria for which there is evidence that proteins of the ParAB family act in chromosome segregation also undergo developmental transitions that involve the ParAB homologues, raising the question of whether the partition activity is equivalent to that of plasmid partition systems. We have investigated the role in partition of the parAB locus of a free-living bacterium, Pseudomonas putida, not known to pass through developmental phases. A parAB deletion mutant, compared with wild type, showed slightly higher frequencies of anucleate cells in exponentially growing cultures but much higher frequencies in deceleration phase. This increase was growth medium dependent. Oversupply of ParA and ParB proteins also raised anucleate cell levels, specifically in the deceleration phase, in wild-type and mutant strains and regardless of medium, as well as generating abnormal cell morphologies. Absence or oversupply of ParAB function had either slight or considerable effects on growth rate, depending on temperature and medium. The need for the Par proteins in chromosome partition thus appears to be subject to the cell's physiological state. Three sequences similar to cis-acting stabilization sites of Bacillus subtilis are present in the P. putida oriC-parAB region. One was inserted into an unstable mini-F and shown to stabilize it in E. coli in a ParAB-dependent manner.     

Partition-mediated plasmid pairing

Plasmid partition systems are essential for the stability and thus the survival of low-copy-number plasmids in growing bacterial populations. The partition reaction is responsible for proper intracellular distribution of plasmids in the bacterial cell cycle. One common step in most partition models is the pairing of plasmids to each other by partition components. Here, evidence that supports the pairing of plasmids via their partition complexes is reviewed, and discussed in light of recent observations that many plasmids, including those without active partition systems are clustered in limited groups inside bacterial cells.     

EMB2473/MIRO1, an Arabidopsis Miro GTPase, is required for embryogenesis and influences mitochondrial morphology in pollen

The regulation of mitochondrial biogenesis, subcellular distribution, morphology, and metabolism are essential for all aspects of plant growth and development. However, the molecular mechanisms involved are still unclear. Here, we describe an analysis of the three Arabidopsis thaliana orthologs of the evolutionarily conserved Miro GTPases. Two of the genes, MIRO1 and MIRO2, are transcribed ubiquitously throughout the plant tissues, and their gene products localize to mitochondria via their C-terminal transmembrane domains. While insertional mutations in the MIRO2 gene do not have any visible impact on plant development, an insertional mutation in the MIRO1 gene is lethal during embryogenesis at the zygote to four-terminal-cell embryo stage. It also substantially impairs pollen germination and tube growth. Laser confocal and transmission electron microscopy revealed that the miro1 mutant pollen exhibits abnormally enlarged or tube-like mitochondrial morphology, leading to the disruption of continuous streaming of mitochondria in the growing pollen tube. Our findings suggest that mitochondrial morphology is influenced by MIRO1 and plays a vital role during embryogenesis and pollen tube growth.     

Asymmetric cell division in plant development

Asymmetric cell division(ACD) is a fundamental process that generates new cell types during development in eukaryotic species.In plant development,post-embryonic organogenesis driven by ACD is universal and more important than in animals,in which organ pattern is preset during embryogenesis.Thus,plant development provides a powerful system to study molecular mechanisms underlying ACD.During the past decade,tremendous progress has been made in our understanding of the key components and mechanism...     

An interpretation of some whole plant water transport phenomena

A treatment of water flow into and through plants to the evaporating surface of the leaves is presented. The model is driven by evaporation from the cell wall matrix of the leaves. The adsorptive and pressure components of the cell wall matric potential are analyzed and the continuity between the pressure component and the liquid tension in the xylem established. Continuity of these potential components allows linking of a root transport function, driven by the tension in the xylem, to the leaf water potential. The root component of the overall model allows for the solvent-solute interactions characteristic of a membrane-bound system and discussion of the interactions of environmental variables such as root temperature and soil water potentials. A partition function is developed from data in the literature which describes how water absorbed by the plant might be divided between transpiration and leaf growth over a range of leaf water potentials.

Relationships between the overall system conductance and the conductance coefficients of the various plant parts (roots, xylem, leaf matrix) are established and the influence of each of these discussed.

The whole plant flow model coupled to the partition function is used to simulate several possible relationships between leaf water potential and transpiration rate. The effects of changing some of the partition function coefficients, as well as the root medium water potential on these simulations is illustrated.

In addition to the general usefulness of the model and its ability to describe a wide range of situations, we conclude that the relationships used, dealing with bulk fluid flow, diffusion, and solute transport, are adequate to describe the system and that analogically based theoretical systems, such as the Ohm's law analogy, probably ought to be abandoned for this purpose.

     

A burst of plant NADPH oxidases

Reactive oxygen species (ROS) are highly reactive molecules able to damage cellular components but they also act as cell signalling elements. ROS are produced by many different enzymatic systems. Plant NADPH oxidases, also known as respiratory burst oxidase homologues (RBOHs), are the most thoroughly studied enzymatic ROS-generating systems and our understanding of their involvement in various plant processes has increased considerably in recent years. In this review we discuss their roles as ROS producers during cell growth, plant development and plant response to abiotic environmental constraints and biotic interactions, both pathogenic and symbiotic. This broad range of functions suggests that RBOHs may serve as important molecular 'hubs' during ROS-mediated signalling in plants.     

The effect of the lipid composition on the partition of liposomes in aqueous two-phase systems

Liposomes have been partitioned in aqueous two-phase systems consisting of water, dextran, poly(ethylene glycol), salt and buffer. Liposomes were used as a model system in order to determine the contribution of the lipids on the partition of membrane particles. The liposomes were composed of phospholipids with different polar head groups and different degrees of unsaturation. The role of cholesterol was also investigated.The polar head group of the phospholipid plays a dominant role in determining the partition of liposomes, while the degree of unsaturation is of less importance, thereby indicating that partition in two-phase systems is a surface dependent method. Incorporation of cholesterol in liposomes reduces differences in partition between liposomes of various composition.