Stomatal development: three steps for cell-type differentiation.

Stomata are microscopic pores on the plant epidermis that act as a major passage for the gas and water vapor exchange between a plant and the atmosphere. A pair of specialized guard cells work in concert to adjust pore size to maintain gas exchange while minimizing the water loss. The formation of stomata requires a series of cell-fate transitions from an initial meristemoid mother cell (MMC), to a stem-cell-like precursor meristemoid, to a guard mother cell (GMC), and finally to terminally-differentiated guard cells. Three closely-related Arabidopsis basic helix-loop-helix (bHLH) genes SPEECHLESS (SPCH), MUTE, and FAMA act sequentially at each key step to direct cell-fate transitions during stomatal development. In this addendum, we propose that a three-step relay of the three bHLHs establishes the molecular framework for stomatal differentiation. Specific expression patterns as well as protein domain structure and dimerization partners of each stomatal bHLH protein may determine the specific function as a key switch in each regulatory node.