Phloem -- Contents -- Contributors -- Section A Introduction -- 1 Phloem, the Integrative Avenue -- 2 Cell Biology of Sieve Element-Companion Cell Complexes -- Development of the Sieve Element-Companion Cell Complex -- Symplastic Connections to and within the Phloem -- Origins and Development of Plasmodesmata -- Connections between SE-CCs and Surrounding Cells -- Sieve Pores-Connecting Sieve Elements into Sieve Tubes -- Modified Plasmodesmata between Sieve Elements and Companion Cells: The Pore-Plasmodesma Units -- Other Specialized Features of Sieve Elements -- The Sieve Element Reticulum -- Cytoskeletal Proteins in Mature Sieve Elements -- Transport between Sieve Element and Companion Cell -- Cytosolic Transport through PPUs -- Membrane-Associated Transport through PPUs -- Future Directions -- References -- 3 Fundamentals of Phloem Transport Physiology -- What's In?-Constituents and Their Concentrations in Sieve-Tube Sap -- Propulsion of a Nutrient Enriched Sap through Sieve Tubes-Pressure Flow and Its Physiological Implications -- Phloem Loading in Collection Phloem -- Phloem Loading Mechanisms and Pathways -- Pre-SE-CC Loading Transport Processes Are Common to All Phloem Loading Mechanisms and Pathways -- Apoplasmic Loaders-Probable Cellular Site of Apoplasmic Exchange and Membrane Transport Mechanisms -- Phloem Loading of Nutrients Other Than Sugars -- Phloem Loading of Water and SE-CC Water Relations -- Transport Phloem-the Multitasker -- Transport Phloem Functions -- Sugar and Amino N Leak and Retrieval Functions -- Net Resource Efflux from Transport Phloem -- Net Resource Influx into Transport Phloem -- Xylem-to-Phloem Exchange -- Release Phloem-Letting Go of the Translocation Stream -- Mapping Cellular Pathways of Phloem Unloading -- Some General Physiological Principles of Phloem Unloading.

Pressure Flow-Integrating Collection, Transport, and Release Phloem Functions -- Future Directions -- Acknowledgments -- References -- Section B Functional Aspects of Structural SE Components -- 4 Roles of Plasma Membrane Transporters in Phloem Functions -- Membrane Transporters and Loading of Resources in Collection Phloem -- Membrane Transporters Regulate Cytosolic Pool Sizes of Sugars in Mesophyll Cells Destined for Phloem Loading -- Sugars Are Retrieved into Mesophyll Cells during Their Symplasmic Transit to Collection Phloem -- Sugar Release in Apoplasmic Phloem Loading Species -- H+-ATPases: Membrane Potential Difference, ΔpH, and Alkalinization of Phloem Sap -- Membrane Transporters Loading Sucrose, the Ubiquitous Phloem Sugar, into SE-CCs -- Polyol and Inositol Transporters and Their Role(s) in Phloem Loading -- Potassium Channels: Working in Harmony with Sugar Transporters to Drive Phloem Translocation -- Phloem Loading of Water and Aquaporins -- Nitrogen (and in Some Circumstances Sulfur) Are Primarily Accumulated into the Phloem by Amino Acid Transporters -- Phloem Loading of Macroelements Other Than Potassium -- Microelement Phloem Loading: An Emerging Key Field in Biofortification -- Transport Phloem: A Plurality of Membrane Transport Functions -- Transporters Detected in the Transport Phloem -- Membrane Transporters Act in Retrieval -- Net Resource Efflux from Transport Phloem -- Net Resource Influx into Transport Phloem -- Xylem-to-Phloem Exchange of Solutes -- Letting Go: Membrane Transporters Participate in Phloem Unloading of Resources -- Membrane Transporters Contributing to Phloem Unloading in Growth Sinks -- Phloem Unloading in Storage Sinks: Fleshy Fruits and Tubers -- Resource Unloading in Developing Seeds: A Cacophony of Membrane Transporters in a Sink System Committed to an Apoplasmic Step in the Postphloem Unloading Pathway.

Aquaporins and Phloem Unloading of Water in Growth and Storage Sinks -- The Dark Side: Ferrymen Transporting Toxic Elements or Compounds -- Phloem Transporters Confer Adaptive Responses to Salt Stress -- Transporter Infidelity Opens Gates to Heavy Metals -- Phloem Transporters Engaged in Signal Transmission -- Phloem Loading/Unloading of Plant Hormones -- Sieve Tube Ion Channels Involved in Electrical Signaling -- Phloem Loading of Plant Defense Compounds -- Future Directions -- Acknowledgments -- References -- 5 Regulation of Sucrose Carrier Activities -- Functional Characteristics of Sucrose Transporters -- Cellular and Subcellular Localization of Sucrose Transporters Involved in Sucrose Uptake by Sieve Tubes -- Subcellular Localization of Sucrose Transporters -- A Model for the Endocytosis of Sucrose Transporters -- Vacuolar Sucrose Transporters -- Regulation of Sucrose Transporters -- Transcriptional Control of Sucrose Transporter Expression -- Posttranscriptional Regulation of Sucrose Transporters -- Posttranslational Control of Sucrose Transporters Expression -- Sucrose Sensing and Signaling -- Sucrose Transport and Gibberellins -- Sucrose Transport and Ethylene -- The Role of Sucrose Transporters Outside the Phloem: Sucrose Transport and Pollen Development -- Future Directions -- References -- 6 Cellular Basis of Electrical Potential Waves along the Phloem and Impact of Coincident Ca2+ Fluxes -- Electrical Long-Distance Signals in Higher Plants -- Action Potentials in Plants -- Variation Potentials in Higher Plants -- Stimulus-Specific Generation of APs and VPs? -- EPWs Trigger Ca2+ Influx via Diverse Ca2+ Permeable Channels into Sieve Elements -- Ca2+ Activities in Sieve Elements before, during, and after EPW Passage -- Ca2+-Permeable Channels of the Sieve Element Plasma Membrane.

Passage of EPWs, Ca2+ Influx, and Sieve Element Occlusion Mechanisms -- Ca2+ Hotspots as Result of a Limited Diffusion Range of Ca2+ Ions Released by Clustered Ca2+ Permeable Channels in SEs -- Ca2+ Channels and the Presence of Cytoskeleton in Sieve Elements? -- Electropotential Waves, Sieve-Element Cell Biology and Whole-Plant Responses -- Differential Effects of APs and VPs in Intact Plants -- Impact of Ca2+ Channels on Reversible Sieve-Element Occlusion in Intact Plants -- Collective Effects of Ca2+ Fluxes Generated by Sieve-Element Ca2+ Channels on Whole-Plant Function -- Future Directions -- References -- 7 Sieve Element Occlusion -- Callose -- Callose Biosynthesis -- Callose Deposition in Sieve Tubes -- P-Proteins -- Dispersive P-Protein Bodies -- Nondispersive P-Proteins -- Forisomes -- Sieve Plate Pore Occlusion: A Concerted Effort? -- Future Directions -- References -- Section C Long-Distance Signaling -- 8 Sieve Elements: Puzzling Activities Deciphered through Proteomics Studies -- Methods Used to Collect Sieve-Tube Sap -- Overview of Sieve-Tube Sap Composition -- Sieve-Tube Sap Proteome: An Integral Component of the Plant Communication System -- Noncell-Autonomous Proteins and Large Protein Complexes in Sieve Elements -- The Ubiquitin-Proteasome System: A Role in Protein Degradation or Signaling? -- Phytohormones and Signaling -- Calcium, Kinases, and Signal Transduction Pathways -- Metabolic Activities Reveal Oxygen Status of Phloem Tissues -- Low Oxygen Tensions and Adaptation to Primary Metabolism -- Production of Metabolites Acting on Defense and Detoxification -- Oxidative Metabolism, Antioxidant Defense, and Control of Redox State -- The Phloem Plays a Key Role in Plant Defense -- Defense Proteins and Response to Biotic and Abiotic Stresses -- Lectins and Storage Proteins -- Protease Inhibitors and Protein Turnover.

Structural Components of Sieve Elements -- SE Structural Proteins -- Sites of Sieve-Tube Sap Protein Synthesis -- Presence of the Translational Machinery in Sieve Tubes: An Enigmatic Puzzle -- Future Directions -- Acknowledgments -- References -- 9 Endogenous RNA Constituents of the Phloem and Their Possible Roles in Long-Distance Signaling -- Methods for Identifying Phloem RNAs -- Methods to Localize Specific RNAs in Tissue Sections -- Methods to Analyze RNAs in Sieve-Tube Sap Samples -- RNAs Identified in Sieve-Tube Sap -- mRNAs in Sieve-Tube Sap -- siRNAs in Sieve-Tube Sap -- miRNAs in Sieve-Tube Sap -- tRNAs and Other RNAs in Sieve-Tube Sap -- Methods for Studying RNA Transport in Sieve Tubes -- Demonstrating RNA Transport by Grafting Experiments -- Demonstrating RNA Phloem Transport by Particle Bombardment, Microinjection, or Transient Expression of Molecules -- Demonstrating RNA Transport in Natural "Grafts" Formed by Parasitic Plants -- Evidence for RNA Movement and Possible Functions of Phloem-Mobile RNAs -- Phloem-Mobile mRNAs -- Phloem-Mobile siRNAs -- Phloem-Mobile miRNAs -- Phloem-Mobile tRNAs and Other RNAs -- Import and Transport Mechanism of Phloem-Mobile RNAs -- Future Directions -- Acknowledgments -- References -- 10 The Effect of Long-Distance Signaling on Development -- The Protein of FLOWERING LOCUS T Functions as the Mobile Floral Signal -- Full-Length mRNA as a Long-Distance Signal -- Mobile GAI RNA -- The Long-Distance Transport of a BEL1-like mRNA Regulates Tuber Development -- Evidence for Movement of BEL5 RNA through the Phloem -- The Role of Untranslated Regions in RNA Transport -- Untranslated Regions of StBEL5 Mediate Transcript Mobility -- Future Directions -- Acknowledgments -- References -- 11 Plant Defense and Long-Distance Signaling in the Phloem -- Responses to Long-Distance (Systemic) Signaling.

Systemic Wound Response.