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Proteins of the endocytosis machinery in plants, such as clathrin and adaptor proteins, were isolated and characterized using combinations of molecular biological (cloning and tagging) and biochemical methods (gel filtration, pull-down assays, surface plasmon resonance and immunoblotting). Other biochemical methods, such as cell fractionation and sucrose density gradients, were applied in order to isolate and further characterize clathrin-coated vesicles and endosomes in plants. Endocytosis was visualized in plant cells by using both non-fluorescent and fluorescent markers, and by employing antibodies raised against endosomal proteins or green fluorescent protein-tagged endocytic proteins in combination with diverse microscopic techniques, including confocal laser scanning microscopy and electron microscopy. Genetic and cell biological approaches were used together to address the role of a few proteins potentially involved in endocytosis. Additionally, biochemical and/or biophysical/electrophysiological methods were occasionally combined with microscopic methods (including both in situ and in vivo visualization) in plant endocytosis research.

After arrival at the surface of heterotrophic cells, nutrients are taken up by these cells via endocytosis to sustain metabolic processes. Recent advances in plant endocytosis reveal that this is true for their heterotrophic cells, either cultivated in suspension cultures or for intact root apices. Importantly, sucrose appears to act as a specific stimulus for fluid-phase endocytosis. Uptake of extracellular nutrients by endocytosis is not in direct conflict with transport through membrane-bound carriers given that cell homeostasis can be better maintained if both these mechanisms operate in parallel. Besides nutrients, plant cells also accomplish internalization of cell wall molecules, such as xyloglucans and boron/calcium cross-linked pectins. Even large portions of apparently fluidized cell wall together with symbiotic bacteria can be internalized into some plant cells, suggesting that they can perform phagocytosis-like tasks despite their robust cell walls. Internalized cell wall molecules allow effective adaptation to osmotic stress, and also may serve for nutritive purposes. Plant endosomes enriched with the internalized cell wall molecules are used for new cell wall formation during plant cytokinesis. Moreover, rapid remodeling of cell walls through endosomal recycling is likely involved in opening/closing movements of stomata, and perhaps also in the formation of wall papillae during pathogen attacks and in recovery of cells from plasmolysis.

Prevacuolar compartments (PVCs) are membrane-bound organelles mediating protein traffic from both Golgi and plasma membrane to vacuoles in eukaryotic cells. Recent studies demonstrate that PVCs in plant cells are multivesicular bodies (MVBs) that merge secretory and endocytic pathways leading to the lytic vacuole, a compartment thought to be equivalent to the mammalian lysosome or the yeast vacuole. In this review, we discuss recent studies on the identity, molecular components and functional roles of plant PVCs and examine whether the plant PVC can also be claimed to be equivalent to the endosome/MVB of mammalian and yeast cells.

The plant vacuolar system is far more complex than originally expected and multiple sorting pathways leading to various types of vacuoles can be found depending on the cell type and on the stage of development. In addition, the vacuolar system is highly dynamic and can adjust to environmental signals to meet the changing needs of the plant. Some recent advances have been made in the identification of the molecular mechanisms by which such a complex compartmentation develops and evolves over time. In this review, we present an update of the latest results in this exciting field and propose distinct biogenesis models for the formation of vacuoles in vegetative and seed tissues, taking into account some apparently contradictory results.

In the last few years, the endocytic vesicular uptake in plant cells has gained increasing significance in several physiological processes. Therefore, an insight into plant clathrin endocytosis at the molecular level is essential. Plants do contain homologs to several key proteins of the mammalian clathrin-dependent endocytosis machinery, but so far only very few have been functionally characterized. Thus, this chapter deals first with the description of the molecular mechanism of clathrin-dependent endocytosis of non-plant organisms which is followed by the outline of similarities to plant endocytosis with an emphasis on its clathrin-dependency.

Binding of ligands activates cell-surface receptors and triggers a series of signalling events. The activation of the receptors accelerates their internalisation, a process called receptor-mediated endocytosis. Thus, entire receptor--ligand complexes are internalised and processed within the cell. Recent work in a variety of cellular and developmental animal systems further supports the idea that the role of endocytosis extends beyond simply controlling the number of receptors at the cell surface. It has been shown that endocytic transport of the receptor complexes regulates signal transduction and mediates the formation of specialised signalling complexes. Signal transduction events can also modulate specific components of the endocytic machinery. Receptor internalisation in plant cells has recently been demonstrated; however, evidence for receptor-mediated endocytosis in plants is just beginning to emerge. In this review, we highlight the most recent advances in the study of receptor-mediated endocytosis in animals and compare them with what is currently known in plant systems.

Structural sterols are integral components of biological membranes. They regulate membrane permeability and fluidity, and they influence the activity of membrane proteins. In Arabidopsis , their composition is critical for normal plant development. The endocytosis and recycling of plasma membrane sterols display similar pathways as some polarly distributed proteins, and thus sterol-dependent trafficking can be an integral part of the polarity establishment in plants. Here, we summarise recent data about sterol endocytosis and sterol trafficking within endocytic pathways in different aspects of cell development in plants.

Polar transport of the phytohormone auxin is mediated by plasma-membrane and endosome localized carrier proteins. PIN proteins are the best studied auxin efflux components implicated in the establishment of the auxin gradient required for growth and patterning in plants. Emerging models postulate a role for vesicular trafficking and protein phosphorylation and dephosphorylation in the regulation of PIN protein subcellular localization and auxin transport activity, providing a conceptual framework for our understanding of auxin transport and its role in plant development.

Auxin is an essential regulator of plant growth and development. Polarized transport of auxin is responsible for apical dominance, tropic growth, and organ development. Previous studies have demonstrated that the polarized movement of auxin is dependent upon the action of polarly localized, endocytotically cycled PIN auxin efflux facilitator proteins. More recently, plant orthologs of mammalian multidrug-resistance (MDR)/P-glycoprotein (PGP) type ABC transporters have been shown to function in auxin transport. In this review, the PGP nomenclature/numbering system established by Martinoia et al., ( Planta 214:345--355,2002) is used, as there is increasing evidence that in plants MDR/PGPs function as PGPs and not as multiple specificity MDR proteins. Defects in PGP1 and PGP19 ( MDR1 )genes result in decreased auxin transport and reduced growth phenotypes in Arabidopsis ( pgp1 , pgp19 ), maize ( br2 ), and sorghum ( dw3 ). Further, dwarf phenotypes are more severe in Arabidopsis double mutants, indicating that PGPs have overlapping functions. More recently, MDR/PGPs have been shown to function as ATP-activated hydrophobic anion transporters capable of auxin transport. Further, MDR/PGPs have been shown to stabilize PIN1 in detergent-resistant membrane microdomains, and synergistic MDR/PGP-PIN interactions have been shown to increase the rate and specificity of MDR/PGP-mediated auxin transport. Several lines of evidence indicate that, like their mammalian counterparts, Arabidopsis MDR/PGPs are regulated via endocytic cycling. Here we review the evidence for endocytic cycling of MDR/PGPs in planta and provide a model by which this cycling could occur.

The Rab family is part of the Ras superfamily of small GTPases. In eukaryotes Rab GTPases are present as members of gene families, and the different Rab GTPase isoforms are localized specific intracellular membranes, where they function as regulators of distinct steps in membrane traffic pathways. They perform these regulatory functions through the specific recruitment of cytosolic effector proteins onto membranes. This recruitment occurs when the Rab GTPase is in the GTP-bound, or active, form. Through these recruited effector proteins, Rab GTPases regulate many aspects of membrane trafficking including vesicle formation, actin- and tubulin-dependent vesicle movement, and membrane fusion. The recent sequencing of complete genomic sequences from animal, yeast, and plant organisms has revealed that a number of Rab GTPase families are conserved from yeast to animals and plants. The plant model system, Arabidopsis thaliana, contains 57 Rab GTPases, of which 40 distinct Rab GTPase members of four subfamilies RabA (26 members), RabC (three members), RabF (three members), and RabG (eight members) share significant similarity with Rab GTPases implicated in endocytic events in animals and yeast. In this review we will highlight recent observations of the function of some of these plant Rab GTPases during endocytosis in plants, and discuss possible roles of plant endocytic Rab GTPases in relation to what is currently known in animal and yeast systems.