Cell-surface receptors enable perception of extracellular cytokinins
Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience 1,2. Although cytokinin receptors are substantially localised to the endoplasmic reticulum 3-5, the cellular sites of cytokinin perception continue to be debated 1,6,7. Several cytokinin types display bioactivity 8,9 and recently a cell-specific cytokinin gradient was reported in roots 10. Yet, the importance of spatially heterogeneous cytokinin distribution and the specific cytokinin(s) that account for the different responses remain unclear. Here we show that cytokinin perception by plasma membrane receptors is an effective path for cytokinin response in root cells. Readout from a Two Component Signalling cytokinin-specific reporter (TCSn::GFP;11) is closely matched to intracellular cytokinin content, yet a proportion of bioactive cytokinins are detected in the extracellular fluid. Using cytokinins covalently linked to beads that could not pass the plasma membrane, we demonstrate that strong TCSn activation still occurs and that this response is greatly diminished in cytokinin receptor mutants. Although intracellular receptors play significant roles, we argue for a revision of concepts of cytokinin perception to include the spatial dimensions. In particular, selective ligand-receptor affinities, cellular localisation and tissue distribution of bioactive cytokinins, their receptors, transporters and inactivation enzymes appear all to be components of the signalling regulatory mechanisms.