Exocytosis and cell polarity in plants - exocyst and recycling domains

Contents Summary 255 Introduction 256 Donor compartments for plant exocytotic vesicles: where do they come from? 256 Mechanisms of exocytotic vesicle formation: is the truth naked? 257 Building the tracks for exocytotic vesicles: cytoskeleton in exocytosis and cell polarization 259 Manysidedness of a plant cell: delimiting functional plasmalemma domains within a cell 260 Vesicle tethering, docking and fusion 262 Polarity by recycling: exocytosis, endocytosis and recycling domains in plant cells 264 Conclusions: constitutive or regulated secretion in plant cells? 266 Acknowledgements 267 References 267 Summary In plants, exocytosis is a central mechanism of cell morphogenesis. We still know surprisingly little about some aspects of this process, starting with exocytotic vesicle formation, which may take place at the trans-Golgi network even without coat assistance, facilitated by the local regulation of membrane lipid organization. The RabA4b guanosine triphosphatase (GTPase), recruiting phosphatidylinositol-4-kinase to the trans-Golgi network, is a candidate vesicle formation organizer. However, in plant cells, there are obviously additional endosomal source compartments for secretory vesicles. The Rho/Rop GTPase regulatory module is central for the initiation of exocytotically active domains in plant cell cortex (activated cortical domains). Most plant cells exhibit several distinct plasma membrane domains, established and maintained by endocytosis-driven membrane recycling. We propose the concept of a 'recycling domain', uniting the activated cortical domain and the connected endosomal compartments, as a dynamic spatiotemporal entity. We have recently described the exocyst tethering complex in plant cells. As a result of the multiplicity of its putative Exo70 subunits, this complex may belong to core regulators of recycling domain organization, including the generation of multiple recycling domains within a single cell. The conventional textbook concept that the plant secretory pathway is largely constitutive is misleading. New Phytologist (2009) 183: 255-272doi: 10.1111/j.1469-8137.2009.02880.x.