Extracurricular roles and divergent regulation of FAMA in Brachypodium and Arabidopsis stomatal development

Stomata, cellular valves found on the surface of aerial plant tissues, present a paradigm for studying cell fate and patterning in plants. A highly conserved core set of related basic helix-loop-helix (bHLH) transcription factors (TFs) regulate aspects of stomatal cell identity in diverse species. We characterized BdFAMA in the temperate grass, Brachypodium distachyon, and found this late-acting TF was necessary and sufficient for specifying stomatal GC fate, and unexpectedly could also induce the recruitment of subsidiary cells in the absence of its paralogue, BdMUTE. The overlap in function is paralleled by an overlap in expression pattern and by unique regulatory relationships between BdMUTE and BdFAMA. To better appreciate the relationships among the Brachypodium stomatal bHLHs, we characterized the diversity of bHLH complexes through in vivo proteomics in developing leaves and found evidence for multiple shared interaction partners. The ability of BdFAMA to compensate for BdMUTE then prompted us to reexamine the roles of these paralogues in Arabidopsis. By testing genetic sufficiency within and across species, we found that, while BdFAMA and AtFAMA can rescue stomatal production in Arabidopsis fama and mute mutants, only AtFAMA can participate in the specification of Brassica-family-specific myrosin idioblasts. Taken together, our findings further our understanding of how the molecular framework of stomatal development has been reprogrammed across the monocot/dicot divide, as well as within the grasses, to produce distinct stomatal forms and patterns, and compel us to refine the current models of stomatal bHLH function and regulatory feedbacks amongst paralogues.