Cytokinins determine Arabidopsis root meristem size by controlling cell differentiation

Plant postembryonic development takes place in the meristems, where stem cells self-renew and produce daughter cells that differentiate and give rise to dif- ferent organ structures. For the maintenance of meri- stems, the rate of differentiation of daughter cells must equal the generation of new cells: How this is achieved is a central question in plant development. In the Arabidopsis root meristem, stem cells surround a small group of organizing cells, the quiescent center. Together they form a stem cell niche [1, 2], whose position and activity depends on the combinatorial action of two sets of genes—PLETHORA1 (PLT1) and PLETHORA2 (PLT2) [3, 4] and SCARECROW (SCR) and SHORTROOT (SHR) [2]—as well as on polar auxin transport. In contrast, the mechanisms controlling meristematic cell differentiation remain unclear. Here, we report that cytokinins control the rate of mer- istematic cell differentiation and thus determine root- meristem size via a two-component receptor histidine kinase-transcription factor signaling pathway. Analy- sis of the root meristems of cytokinin mutants, spatial cytokinin depletion, and exogenous cytokinin applica- tion indicates that cytokinins act in a restricted region of the root meristem, where they antagonize a non- cell-autonomous cell-division signal, and we provide evidence that this signal is auxin.