Figure 10.

Putative model explaining regulation of root system architecture by Mediator subunit MED17. In the presence of sucrose, TOR phosphorylates and activates TFs E2FA and E2FB (Xiong et al., 2013). The active E2FA/B TFs bind to the promoter of MED17 and induce its transcription. In addition to inducing the transcription of MED17, the E2FA/B TFs physically interact with MED17 protein. Thus, it seems that the E2FA/B TFs recruit the Mediator complex (through their interaction with MED17 subunit) on the promoters of the cell cycle genes MCM5, ETG1, and ORC6 and the auxin-responsive ARF7 TF gene and activate their transcription. In the subsequent step, ARF7 interacts with MED17 and regulates the expression of auxin-responsive LBD genes LBD16, LBD18, LBD29, and LBD33. As these genes (MCM5, ETG1, ORC6, and LBDs) collectively regulate the overall root architecture, MED17 seems to play a critical role in integrating the sucrose and auxin signaling to regulate the root system architecture in Arabidopsis.