The Transcriptional MEDIATOR Complex: Linking Root Development, Hormonal Responses, and Nutrient Stress

The MEDIATOR complex is a large multiprotein assembly ubiquitous to eukaryotic organisms that acts as a transcriptional coactivator. Although its identification in plants is relatively recent, accumulating information points to critical functions played by each of its subunits to orchestrate growth, development, and stress adaptation. Here, we review recent advances in understanding the functions of MEDIATOR in root biology, including root architecture configuration, hormonal responses, nutrient stress adaptation, and meristem cell survival. Biochemical studies and protein sequencing strategies have identified 28 subunits as well as accessory histone acetyltransferases as part of MEDIATOR, which may contribute to DNA relaxing and the opening of promoters for gene expression. The mutation of specific components of MEDIATOR in most cases did not compromise plant survival under standard growth conditions, but rather changes functional processes in a highly specific fashion. Interactions between MEDIATOR, its dissociable kinase module and the TOPLESS repressor underlie the auxin and jasmonic acid response and these phytohormones not only affect MEDIATOR composition, but also its affinity to key transcriptional regulators such as AUXIN RESPONSE FACTORS and MYC2 to cis-acting elements within gene promoters. MEDIATOR also emerges as a master component of the strategies to improve phosphate and iron acquisition, opening new possibilities to manage nutrient stress and plant productivity.