Stem cell specification in multicellular organisms relies on the precise spatiotemporal control of RNA polymerase II (Pol II)-dependent gene transcription, in which the evolutionarily conserved Mediator coactivator complex plays an essential role. In Arabidopsis thaliana, SHORTROOT (SHR) and SCARECROW (SCR) orchestrate a transcriptional program that determines the fate and asymmetrical divisions of stem cells generating the root ground tissue. The mechanism by which SHR/SCR relays context-specific regulatory signals to the Pol II general transcription machinery is unknown. Here, we report the role of Mediator in controlling the spatiotemporal transcriptional output of SHR/SCR during asymmetrical division of stem cells and ground tissue patterning. The Mediator subunit MED31 interacted with SCR but not SHR. Reduction of MED31 disrupted the spatiotemporal activation of CYCLIND6;1 (CYCD6;1), leading to defective asymmetrical division of stem cells generating ground tissue. MED31 was recruited to the promoter of CYCD6;1 in an SCR-dependent manner. MED31 was involved in the formation of a dynamic MED31/SCR/SHR ternary complex through the interface protein SCR. We demonstrate that the relative protein abundance of MED31 and SHR in different cell types regulates the dynamic formation of the ternary complex, which provides a tunable switch to strictly control the spatiotemporal transcriptional output. This study provides valuable clues to understand the mechanism by which master transcriptional regulators control organ patterning.

多细胞生物中的干细胞规格依赖于RNA聚合酶II（Pol II）依赖性基因转录的精确时空控制，其中进化保守的Mediator共激活复合物起着至关重要的作用。在拟南芥中，SHORTROOT（SHR）和SCARECROW（SCR）编排一个转录程序，该程序确定生成根基组织的干细胞的命运和不对称分裂。SHR / SCR将上下文特定的调节信号中继到Pol II通用转录机制的机制尚不清楚。在这里，我们报告调解员在干细胞的不对称分裂和地面组织模式控制SHR / SCR的时空转录输出中的作用。介体亚基MED31与SCR相互作用，但与SHR不相互作用。MED31的减少破坏了CYCLIND6; 1（CYCD6; 1）的时空激活，导致干细胞产生基底组织的不对称分裂缺陷。MED31被招募为MED31的启动子CYCD6; 1依赖于SCR。MED31通过界面蛋白SCR参与了动态MED31 / SCR / SHR三元复合物的形成。我们证明了在不同细胞类型中MED31和SHR的相对蛋白丰度调节了三元复合物的动态形成，从而提供了可调节的开关来严格控制时空转录输出。这项研究提供了宝贵的线索，以了解主要转录调节因子控制器官模式的机制。