An updated history of TDIF-PXY signalling: a study in cell fate and tissue patterning

Non-cell autonomous signalling is a mechanism by which stem cells are maintained across the tree of life. In plants, many stem cell populations are regulated by peptide ligands related to CLAVATA3, which are secreted from one cell type and bind to the leucine-rich repeat domain of a plasma-membrane-localised receptor kinase related to CLAVATA1 in an adjacent cell type. Activation of CLAVATA1-like receptors then leads to a suite of events regulating stem cell fate. The vascular cambium, the meristem from which xylem and phloem are derived, is no different in this respect. Here, TRACHEARY ELEMENT DIFFERENTIATION FACTOR (TDIF) peptide is excreted from phloem cells and binds to the PHLOEM INTERCALATED WITH XYLEM (PXY) receptor kinase, which in turn leads to activation of a suite of factors that influence xylem and phloem production. In 2016, we reviewed the literature that described signalling components, phytohormones and transcription factors that interacted with TDIF and PXY. In the intervening period, our understanding of these interactions has significantly progressed and here we provide an update, describing how TDIF and PXY promote growth while maintaining pattern in the stem cell populations that they regulate.