Cytokinin Transporters: GO and STOP in Signaling

Cytokinins are phytohormones essential for cytokinesis and many other physiological and developmental processes in planta. Long-distance transport and intercellular transport have been postulated. For these processes, the existence of cytokinin transporters has been suggested. Recently, a transporter loading the xylem (AtABCG14) and another for cellular import (AtPUP14) have been discovered. AtABCG14 participates in the xylem loading process of cytokinins and contributes to the positive regulation of shoot growth. The cellular importer AtPUP14 is required to suppress cytokinin signaling. A role of a transporter as stop signal is a new paradigm for a hormone transporter. Recently, two types of cytokinins transporters have been identified and shown to play key roles in plant development and many important physiology processes. ABCG14 contributes to the ‘GO’ signal regulating shoot growth by delivering cytokinins via the xylem. By contrast, the cytokinin uptake transporter PUP14 is in charge of ‘stopping’ the signal at the top of intracellular cytokinin signaling, by removing the ligand that activates the receptors.