Timely expression of the A rabidopsis stoma‐fate master regulator MUTE is required for specification of other epidermal cell types

Summary Epidermal differentiation in A rabidopsis thaliana aerial organs involves stomatal lineage development. Lineages derive from meristemoids, which arise from asymmetric divisions of protodermal cells. Each meristemoid divides repeatedly in an inward spiral before it transits to a guard mother cell ( GMC ) that produces the stoma, leaving a trail of surrounding stomatal lineage ground cells ( SLGC s) that eventually differentiate into endoreplicated pavement cells. MUTE is a b HLH transcription factor that is expressed in late meristemoids and drives their transition to GMC s. Loss‐of‐function mute mutants are stomata‐less dwarf plants with arrested lineages, in which stunted putative SLGC s surround a halted meristemoid. We analysed MUTE functions using a chemically inducible system for mute‐3 complementation based on conditional MUTE expression in its normal domain. Continuous induction from germination produced stomata‐bearing, normal‐sized plants with viable mute‐3 seeds. In 2‐week‐old mute‐3 cotyledons, meristemoids appeared to retain their identity and synchronously formed stomata in response to induced MUTE expression. However, arrested SLGC s were not complemented: many produced stomata, leading to stomatal clusters, and others remained unexpanded and diploid. In contrast, non‐lineage pavement cells, which are under‐endoreplicated in mute‐3 , expanded and increased their ploidy level upon induction, showing that the lack of response of SLGC s is specific to this arrested cell type. Leaf phenotypic mosaics include wild‐type lineages and adjacent mute‐3 lineages, whose meristemoids and putative SLGC s remained arrested, indicating that the role of MUTE in SLGC fate is strictly lineage‐autonomous. These results show that timely MUTE expression is essential to prevent stomatal fate in SLGC s and to promote their differentiation as pavement cells.