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In eukaryotic cells, the transport vesicles carry various cargo proteins from a donor compartment to a target compartment, and discharge the cargo into the target compartment by fusing with the membrane of the target compartment. SNARE molecules have a central role for initiating membrane fusion between transport vesicles and target membranes by forming a specific trans-SNARE complex in each transport step. In higher plants, the numbers of SNARE molecules are greater than those of yeast and mammals, suggesting a higher complexity of membrane traffic in higher plant cells. In this chapter, we will focus on the functions and subcellular localizations of plant SNARE molecules and discuss the complexity and evolution of endocytosis and the post-Golgi traffic in the higher plant cells.

Over the past decade, it has become evident that multiple endocytic pathways operate in eukaryotic cells, and several of these are dependent on dynamins and dynamin-related proteins (DRPs). Many members of the DRP superfamily possess the ability to self-assemble into long spiral polymers that wrap around lipid bilayers and thus facilitate tubulation and vesicle pinching from the plasma membrane and other membrane compartments, a process that is fundamental for endocytosis. Here, we discuss the roles of dynamins and DRPs in plants. DRPs have been shown to be present at different subcellular locations in plant cells including the cell plate, plasma membrane, Golgi apparatus, vesicles, mitochondria, chloroplasts, and peroxisomes. Arabidopsis contains 16 DRP members that are grouped into six functional subfamilies (DRP1--6) on the basis of their phylogeny and the presence of functional motifs. Members of the DRP1 subfamily are closest to soybean phragmoplastin and mediate membrane tubulation at the cell plate. The DRP2 subfamily members represent the bona fide plant dynamins characterized by the presence of a pleckstrin homology (PH) domain in the middle of the molecule and a proline-rich (PR) motif near the C-terminus ; they are involved in membrane recycling at the cell plate and the trans-Golgi region. The DRP3 subfamily does not contain PH or PR motifs; their function has been implicated in the division of mitochondria and peroxisomes, whereas the DRP5 subfamily in Arabidopsis is likely to play a role in plastid division. A DRP5 ortholog from the red alga Cyanidioschyzon has recently been shown to be a component of the chloroplast outer division-ring on the cytoplasmic face of the plastid double membrane. Finally, the DRP4 subfamily contains orthologs of the animal antiviral Mx proteins, but their function has not yet been established, and the role of DRP6 is entirely unknown so far. It is obvious that plant cells employ unique DRP subfamilies to carry out the mechanochemical work required for membrane deformation and segregation in various membranous compartments. However, to understand the function of DRPs in further detail, much is yet to be learned about the proteins that apparently interact with them to regulate their activity and specify their functions.

Mutual interactions between actin and endocytic assembly machineries are essential for successful clathrin-mediated endocytosis in yeast and mammals. The actin cytoskeleton is indispensable for endocytic internalization and for short-range transport of endocytic vesicles. In plants as well, actin seems to be essential for endocytic recycling of plasma membrane proteins and sterols, but surprisingly also for the turnover of cell wall pectins, which have been identified as a major cargo of endocytic vesicles. Endosomes in animal cells perform long-range movements along microtubules, whereas plant endosomes use preferentially an actin polymerization mechanism but also actin tracks for their short- and long-range movements, respectively. Thus, the actin cytoskeleton not only assists endocytic internalization and is in fact inherently associated with endosomal vesicles and endosomes, but also is responsible for their movements at the cell cortex and for their targeted delivery into the cell interior.

Symbioses are widespread in nature and occur between organisms that belong to a large variety of taxonomic divisions (Hentschel et al. 2000). Most often, only two partners are involved and the outcome may be either beneficial to both, i.e. mutualism, or detrimental to one of them, i.e. parasitism. Mutualism varies from simple protection against a hostile environment to an intimate cohabitation with exchange of essential nutrients. Important and well-studied examples are the symbiosis between nitrogen-fixing bacteria and plants of the Leguminosae family (approximately 750 genera and 20 000 species) and the arbuscular mycorrhizal interactions that involve more than 80% of land plants with fungi of the Glomeromycota. In the first case, plants profit through the supply of a nitrogen source, and in the second, through an uptake of phosphate. The microsymbionts benefit through the acquisition of carbon sources in a specific and exclusive ecological niche. In both types of interactions, the microsymbionts invade the plant host and the nutrient exchange takes place inside specialised plant cells. The establishment of the symbiosis is a complex process that requires the coordinated action of both symbionts and most probably the involvement of endocytosis in a number of critical events. In this chapter, we will describe both types of endosymbiosis in view of endocytosis and endocytosis-like processes.

Stomatal movement requires large and repetitive changes to cell volume and consequently surface area. These alterations in surface area are accomplished by addition and removal of plasma membrane material. Recent studies of membrane turnover in guard cell protoplasts using electrophysiology and fluorescence imaging techniques implicate that exocytosis and endocytosis are sensitive to changes in membrane tension. This may provide a regulatory mechanism for the adaptation of surface area to osmotically driven changes in cell volume in guard cell protoplasts as well as turgid guard cells. In addition guard cells also exhibit constitutive membrane turnover. Constitutive and tension-driven membrane turnover were found to be associated with addition and removal of K ^+ channels. This implies that some of the exocytosis and endocytic vesicles carry K^+ channels. Together the results demonstrate that exocytosis and endocytosis are essential for stomatal movement and thus gas exchange in plants.

In plants, tip-growing cells are an ideal system to investigate signal transduction mechanisms and, among these, pollen tubes are one of the favourite models. Many signalling pathways have been identified during germination and tip growth and, not surprisingly, the apical secretory machinery, essential for tip growth, seems to be an intersection point for all these pathways. Here we review previous data on the pollen tube endocytic machinery and its coupling to the exocytic delivery of new cell wall material. Additionally, we discuss new methodologies and how these are shaping our current working hypothesis to explain endocytosis in pollen tubes.

Recent advances in molecular cell biology have provided new insights into different cellular processes that all turn out to contribute to polarized cell growth in a variety of model systems used to analyse growth, differentiation and development. Polarized cell growth, although a general feature of the living cell, can be found in a pronounced fashion during pollen tube outgrowth and root hair development in plants, during neurite outgrowth, and during filamentous hyphal growth. Filamentous fungi represent excellent model systems to analyse polarized cell growth owing to their genetic tractability and the ease of generating and keeping mutant strains. Contributing to this is the fact that already a number of fungal genomes have been sequenced, which allows the rapid analysis and comparison of gene function. This has led to the finding that polarized cell growth can be influenced by perturbations in different cellular pathways. Control of polarity establishment and the maintenance of polarized cell growth are exerted by a number of conserved GTP-binding proteins of the Ras/Rho subfamily and their specific regulators that organize the actin cytoskeleton. Hyphal tip growth requires coordination of vesicle transport using actin and microtubule cytoskeletons. Recent evidence has shown that hyphal growth not only depends on polarized secretion but also requires endocytosis, suggesting that the recycling of the membrane and sorting of vesicles is required for fast elongation of hyphal tubes. Key players on the molecular level that direct tip growth and endocytosis in the fungal hyphae based on differential regulation of the actin cytoskeleton are discussed.