A Shout-Out to Stomatal Development: How the bHLH Proteins SPEECHLESS, MUTE and FAMA Regulate Cell Division and Cell Fate

Arabidopsis guard cell development requires a three step series of asymmetric and symmetric divisions followed by terminal differentiation. We have recently identified three paralogous bHLH transcription factors, SPEECHLESS, MUTE and FAMA, that each function as a master regulator of a specific stage of stomatal development. These findings provide the expected counterbalance to the previously described negative regulatory signaling network and raise intriguing new questions about relationships among the regulators that ultimately enable proper stomatal development and pattern.