LAUSR

Significance SHORTROOT (SHR) and SCARECROW (SCR) orchestrate a transcriptional program that is essential for ground tissue patterning. The regulation of the transcriptional output of SHR/SCR remains unclear. We show that MED31, a subunit of the plant Mediator coactivator complex, bridges the communication between SHR/SCR and the RNA polymerase II general transcriptional machinery. MED31 and SHR bind to the same protein domain of SCR, and the relative abundance of MED31 and SHR determines the dynamic formation of a MED31/SCR/SHR ternary complex. We studied the role of the MED31/SCR/SHR ternary complex in controlling the spatiotemporal expression of CYCLIND6;1 during ground tissue patterning. This study provides insight into the mechanisms by which master transcriptional regulators control organ patterning. 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.