574 / 2019-04-07 10:11:28

The Mediator subunit MED31 regulates stem cell asymmetric division and radial patterning of Arabidopsis roots

Mediator, stem cells, transcriptional regulation, root patterning, MED31

Growth and Development > Growth and Development

Stem cell specification and organ patterning in multicellular organisms relies on the precise spatiotemporal control of RNA polymerase II (Pol II)-dependent gene transcription, in which the evolutionarily conserved Mediator co-activator complex plays an essential role. The GRAS family proteins SHR and SCR orchestrate a transcriptional program that determines the fate and asymmetric cell divisions (ACDs) 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 stem cell ACD and ground tissue patterning. The Mediator subunit MED31 interacted with SCR but not SHR. Reduction of MED31 disrupted the spatiotemporal activation of CYCD6;1, a cell-cycle gene, leading to defective ACD of stem cells generating ground tissue. MED31 was recruited to the promoter of CYCD6;1 in a SCR-dependent manner, and MED31 and SHR competed each other for binding to SCR in a dose-dependent manner. We show that MED31 is involved in the formation of a 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 MED31-SCR-SHR ternary complex, which provides a tunable switch to strictly control the spatiotemporal transcriptional output.