Targets of SPEECHLESS and FAMA control guard cell division and expansion in the late stomatal lineage

These later undergo differentiation, losing division potential and acquiring specific identities and functions. In the stomatal lineage, protodermal cells develop into guard cells (GCs) through the action of bHLH transcription factors (TFs) SPEECHLESS (SPCH), MUTE and FAMA.

Existing models support that these regulators act sequentially, but recent evidence indicates that SPCH expression and function are retained in late stomatal cells. Here, we combine transcriptomic and genetic approaches to define SPCH's function during the late stomatal lineage. We show that relative levels and activities of SPCH and FAMA control GC division and expansion.