A brief history of the TDIF‐PXY signalling module: balancing meristem identity and differentiation during vascular development

Summary A significant proportion of terrestrial biomass is constituted of xylem cells that make up woody plant tissue. Xylem is required for water transport, and is present in the vascular tissue with a second conductive tissue, phloem, required primarily for nutrient transport. Both xylem and phloem are derived from cell divisions in vascular meristems known as the cambium and procambium. One major component that influences several aspects of plant vascular development, including cell division in the vascular meristem, vascular organization and differentiation of vascular cell types, is a signalling module characterized by a peptide ligand called TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR ( TDIF ) and its cognate receptor, PHLOEM INTERCALATED WITH XYLEM ( PXY ). In this review, we explore the literature that describes signalling components, phytohormones and transcription factors that interact with these two central factors, to control the varying outputs required in vascular tissues for normal organization and elaboration of plant vascular tissue. Contents Summary 474 I. Introduction 474 II. Peptides that regulate stem cell fate 475 III. A receptor for TDIF ligand 475 IV. TDIF, PXY and promotion of vascular cell division 477 V. Repression of xylem differentiation by PXY signalling 477 VI. Vascular organization defects in plants with altered TDIF‐PXY signalling: cause or effect? 477 VII. Interactions between PXY signalling and phytohormones 479 VIII. Known unknowns 481 Acknowledgements 482 References 482