Cell cycle dynamics and window of MUTE action during the terminal division of stomata and ramification of its loss-of-function on the cell cycle of an uncommitted precursor
Plants develop in the absence of cell migration. As such, cell division and differentiation need to be coordinated for functional tissue formation. Cellular valves on the plant epidermis, stomata, are generated through a stereotypical sequence of cell division and differentiation events. In Arabidopsis, three master-regulatory transcription factors, SPEECHLESS (SPCH), MUTE, and FAMA sequentially drive initiation, proliferation, and differentiation of stomata. Among them, MUTE switches the cell cycle mode from proliferative asymmetric division to terminal symmetric division and orchestrates the execution of the single symmetric division event. However, it remains unclear to what extent MUTE regulates the expression of cell cycle genes through the symmetric division and whether MUTE accumulation itself is gated by the cell cycle. Here, we show that MUTE directly upregulates the expression of cell cycle components throughout the terminal cell cycle phases of a stomatal precursor, not only the core cell cycle engines but also checkpoint regulators. Time-lapse live imaging using the multi-color cell cycle indicator PlaCCI revealed that MUTE accumulates up to the early G2 phase, whereas its successor and direct target, FAMA, accumulates at late G2 through terminal mitosis. In the absence of MUTE, meristemoids fail to differentiate, and their G1 phase elongates as they reiterate asymmetric divisions. Combined, our work provides the framework of cell cycle and master regulatory transcription factors to coordinate a single symmetric cell division, and suggests a mechanism for the eventual cell cycle arrest of an uncommitted stem-cell-like precursor at the G1 phase.