Plant GSK3 proteins regulate xylem cell differentiation downstream of TDIF–TDR signalling

During plant radial growth typically seen in trees, procambial and cambial cells act as meristematic cells in the vascular system to self-proliferate and differentiate into xylem cells. These two processes are regulated by a signalling pathway composed of a peptide ligand and its receptor; tracheary element differentiation inhibitory factor (TDIF) and TDIF RECEPTOR (TDR). Here we show that glycogen synthase kinase 3 proteins (GSK3s) are crucial downstream components of the TDIF signalling pathway suppressing xylem differentiation from procambial cells. TDR interacts with GSK3s at the plasma membrane and activates GSK3s in a TDIF-dependent fashion. Consistently, a specific inhibitor of plant GSK3s strongly induces xylem cell differentiation through BRI1-EMS SUPPRESSOR 1 (BES1), a well-known target transcription factor of GSK3s. Our findings provide insight into the regulation of cell fate determination in meristem maintenance. The differentiation of procambial cells into xylem cells during plant radial growth is regulated by the TDIF–TDR signalling pathway. Here, the authors show that GSK3 protein kinases and their target transcription factor BES1 act downstream of TDIF–TDR signalling during xylem cell differentiation in Arabidopsis.