Mediator CKM and MUTE-ICE1/SCRM2 reinforce auxin responsiveness to sustain MUTE expression during stomatal lineage differentiation

Abstract MUTE functions as a pivotal transcription factor that governs the transition from self-renewal to differentiation in meristemoids during stomatal development. This transition requires MUTE expression to reach a critical threshold, yet the mechanisms that maintain its expression homeostasis remain unclear. Here, we show that the Mediator CDK8 kinase module (CKM) acts as a transcriptional coactivator of the MUTE-ICE1/SCRM2 complex to regulate meristemoid fate. Specifically, CKM cooperates with MUTE-ICE1/SCRM2 to promote the transcription of the auxin-responsive genes, such as SAUR52 and SAUR53, thereby enhancing local auxin response in meristemoids. AUXIN RESPONSE FACTORs (ARFs), including ARF6, ARF7, and ARF19, then bind to MUTE promoter and contribute to its transcription, establishing a positive feedback loop. Disruption of CKM or MUTE-ICE1/SCRM2 components reduces SAUR52/53 expression and auxin responses, leading to decreased MUTE transcription, excessive meristemoid self-renewal divisions and arrested differentiation. These defects can be partially rescued by exogenous auxin application or by overexpression of SAUR53 in the mutants. Together, our results uncover a self-reinforcing transcriptional-auxin circuit in which CKM-assisted MUTE-ICE1/SCRM2 amplifies auxin response, and auxin reciprocally sustains MUTE expression. This feedback mechanism ensures that MUTE reaches the threshold required to terminate meristemoid self-renewal and initiate differentiation, revealing how transcriptional regulation and hormone signaling are integrated to control stem cell-like behaviors during tissue patterning.