A tonoplast cytokinin riboside transporter gates intracellular hormone availability at the root–microbe interface

ABSTRACT Cytokinin ribosides are major mobile and precursor forms of cytokinins, plant hormones whose transport and subcellular distribution shape developmental and stress responses. Here, we identify Arabidopsis thaliana EQUILIBRATIVE NUCLEOSIDE TRANSPORTER1 (ENT1) as a tonoplast-localized cytokinin riboside transporter. Tissue-specific subcellular analysis under native regulatory elements localized ENT1 predominantly to the tonoplast of root epidermal and lateral root cap cells, where it gates intracellular cytokinin riboside availability. Accordingly, ENT1 overexpression enhanced cytokinin riboside sensitivity and signalling, whereas loss of ENT1 altered adenosine metabolism and disrupted cytokinin homeostasis, leading to the accumulation of multiple zeatin-type cytokinins. ENT1-dependent cytokinin riboside compartmentalization was required for beneficial microbe-induced protection, as ent1 mutants failed to acquire protection against the fungal pathogen Botrytis cinerea and the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. These findings reveal a vacuolar gatekeeping mechanism that controls intracellular cytokinin riboside availability and links hormone compartmentalization to beneficial microbe-dependent plant defence.