In the root apical meristem, which contains the stem cells that feed into root development, the phytohormones auxin and cytokinin play opposing roles, with auxin promoting cell division and cytokinin promoting cell differentiation. Cytokinin acts in the root tip in part by modulating auxin transport through regulation of the level of the PIN auxin efflux carriers. Auxin plays a key role in the specification of the quiescent center (QC), which is essential for maintaining the stem cell fate of the surrounding cells.
We demonstrate that cytokinin promotes cell division in the QC, which is generally mitotically inactive. Cytokinin downregulates the expression of several key regulatory genes in the root tip, including SCARECROW, WOX5, and the auxin influx carriers AUX1 and LAX2. The decrease in LAX2 expression in response to cytokinin requires ARR1 and ARR12, two type B ARRs that mediate the primary transcriptional response to cytokinin. ARR1 was found to bind directly to the LAX2 gene in vivo, which indicates that type B ARRs directly regulate genes that are repressed by cytokinin. Disruption of the LAX2 gene results in a phenotype similar to that observed in response to cytokinin, including increased division of the cells in the QC and decreased expression of WOX5 and the auxin response reporter DR5.
Cytokinin acts to regulate auxin distribution in the root apical meristem by regulating both the PINs and LAX2. This redistribution of auxin, potentially coupled with other auxin-independent effects of cytokinin, regulates the mitotic activity in the QC.