The growth of plants depends on continuous function of the meristems. Shoot meristems are responsible for all the post-embryonic aerial organs, such as leaves, stems and flowers1. It has been assumed that the phytohormone cytokinin has a positive role in shoot meristem function2-4. A severe reduction in the size of meristems in a mutant that is defective in all of its cytokinin receptors has provided compelling evidence that cytokinin is required for meristem activity5,6. Here, we report a novel regulation of meristem activity, which is executed by the meristem-specific activation of cytokinins. TheLONELY GUY (LOG)gene of rice is required to maintain meristem activity and its loss of function causes premature termination of the shoot meristem.LOGencodes a novel cytokinin-activating enzyme that works in the final step of bioactive cytokinin synthesis. Revising the long-held idea of multistep reactions, LOG directly converts inactive cytokinin nucleotides to the free-base forms, which are biologically active, by its cytokinin-specific phosphoribohydrolase activity.LOGmessenger RNA is specifically localized in shoot meristem tips, indicating the activation of cytokinins in a specific developmental domain. We propose the fine-tuning of concentrations and the spatial distribution of bioactive cytokinins by a cytokinin-activating enzyme as a mechanism that regulates meristem activity.