The Plant Vascular System: Evolution, Development and Functions

Contents: I. Introduction 295 II. Evolution of the Plant Vascular System 295 III. Phloem Development & Differentiation 300 IV. Molecular Mechanisms Underlying Xylem Cell Differentiation 307 V. Spatial & Temporal Regulation of Vascular Patterning 311 VI. Secondary Vascular Development 318 VII. Physical and Physiological Constraints on Phloem Transport Function 321 VIII. Physical & Physiological Constraints on Xylem Function 328 IX. Long-distance Signaling Through the Phloem 339 X. Root-to-shoot Signaling 347 XI. Vascular Transport of Microelement Minerals 351 XII. Systemic Signaling: Pathogen Resistance 356 XIII. Future Perspectives 361 XIV. Acknowledgements 362 XV. References 362 [William J. Lucas (Corresponding author)] The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.