Membrane transport of root-borne <i>trans</i> -zeatin riboside maintains the cytokinin homeostasis in shoots

Ribosylated forms of the plant hormones cytokinins (CKs) are the dominant CK species translocated over long distances. The irreplaceable role of root-to-shoot translocated trans-zeatin riboside in the mediation of shoot development implies the existence of a yet-uncharacterized CK riboside-specific membrane transport system. In this work, we report significant differences in the kinetics of the membrane transport of CK nucleobases and ribosides and the overall affinity of membrane-bound carriers towards the two CK forms. We further characterize the membrane transport of CK nucleobases and ribosides mediated by Arabidopsis EQULIBRATIVE NUCLEOSIDE TRANSPORTER 3 (AtENT3) in tobacco BY-2 cells. Combining experimental data with computational modelling, we show that residues Tyr61 and Asp129, which are conserved among plant ENTs but not among ENTs from other species, are necessary for CK binding and that their mutation abolishes the ability of AtENT3 to transport CKs. Finally, we show that changes in AtENT3 have different effects on the concentrations of trans-zeatin riboside throughout Arabidopsis plants and on the overall CK concentrations in roots, implying that AtENT3 participates in both the long- and the short-distance transport of CKs.