Stomata consist of two specialised epidermal cells termed guard cells (GCs) surrounding a pore, through which gas exchange can occur. In Arabidopsis thaliana, a dedicated cell lineage is initiated and undergoes a series of cell divisions to produce a stoma. A set of basic helix-loop-helix (bHLH) transcription factors regulates the differentiation events through the lineage. The initiation and proliferation of stomatal lineage cells is controlled by the transcription factor SPEECHLESS (SPCH), which drive the asymmetric division of meristemoid mother cell to originate a small triangular cell termed meristemoid. Which transcriptional events precede stomata lineage specification and are required for correct GC patterning, however, remain unclear. The DOF transcription factor STOMATAL CARPENTER 1 (SCAP1) has previously shown to be expressed in GC and involved in stomata function, by activating a set of GC\u2013specific genes required for GC maturation and activity. We show that SCAP1 expression can also be observed in young leaf primordia, before any GC differentiation occurs. The study of transgenic plants carrying a proSCAP1:GUS-GFP transcriptional fusion coupled with quantitative PCR analyses indicate that SCAP1 expression is maximal in a stomatal lineage competence domain, coincident with SPCH expression. We found that SCAP1 modulates stomata development; independent scap1 loss-of- function mutants show a reduced number of GCs whilst SCAP1 over expression lines have an increased number of GCs in addition to altered GC distribution and spacing patterns. Confocal imaging of SPCH-GFP protein in a background carrying inducible SCAP1 shows that SCAP1 activation results in an increased number of nuclei expressing SPCH-GFP. Our results suggest an early role for SCAP1 in GC differentiation through SPCH protein stabilization. SCAP1 may thus link different aspects of GC biology including specification, maturation and function
