Arabidopsis guard cell integrity involves the epigenetic stabilization of the FLP and FAMA transcription factor genes

Arabidopsis guard cell (GC) fate is conferred via a transient pulse of expression of FAMA that encodes a bHLH transcription factor. Stomata often function for years, suggesting that the FAMA expression window stabilizes long-term GC identity or that additional factors operate. Transgenic lines harboring a copy of a FAMA transgene were found to induce the fate resetting of mature GCs to that of lineage-specific stem cells causing new stomata to arise within shells of the old, a Stoma-in-Stoma (SIS) phenotype. These lines disrupt the normal trimethylation on lysine 27 of histone3 (H3K27me3) on stomatal stem cell genes, a phenotype rescued by constitutive expression of the Polycomb Group (PcG) gene CURLY LEAF. Thus the stability of stomatal fate is enforced by a PcG-mediated reduction in the transcriptional accessibility of stem cell genes and by the endogenous FAMA gene itself. Moreover, a transgenic FOUR LIPS gene, which encodes a MYB protein that is not required for GC fate, also induces a SIS phenotype and disrupts H3K27 trimethylation. Thus FLP might indirectly enforce GC fate as well.