Figure 7.

Models of Stomatal Cell-State Transition by Sequential Actions of bHLH Core Regulatory Units.

Schematic diagram of ICE1 and SCRM2 actions during stomatal differentiation. Both SPCH and ICE1·SCRM2 are required for the first asymmetric cell division that initiates stomatal cell lineage. SPCH and ICE1·SCRM2 may influence each other's expression via a positive feedback (gray arrows). ICE1 and SCRM2 heterodimerize with SPCH, MUTE, and FAMA to direct initiation, meristemoid differentiation, and terminal guard cell differentiation, respectively (white arrows). The greater the loss of functional ICE1 and SCRM2 copies, the earlier the step of arrest during sequential, stomatal cell-state transition. +, functional gene copy; −, absence of functional gene copy by knockout mutation.