SPEECHLESS duplication in grasses expands potential for environmental regulation of stomatal development

Plants regulate stomatal development and function to acquire atmospheric carbon dioxide for photosynthesis while minimizing water loss. The ancestral basic helix-loop-helix transcription factor (TF) gene that drove stomata production in early land plants diversified to become paralogs SPEECHLESS (SPCH), MUTE, and FAMA. Extant grasses exhibit a particularly interesting set of duplications and losses of SPCH. Using phylogenetic methods, we tracked the history of SPCH duplications. Brachypodium distachyon and Oryza sativa plants bearing mutations in either SPCH1 or SPCH2, and B. distachyon plants with SPCH1 or SPCH2 translational reporters were assayed under different environmental conditions for their effects on stomatal development. We identified the Poaceae-specific rho whole genome duplication as the origin of SPCH1 and SPCH2 and demonstrated that both paralogs remain under selection. We found paralog-specific divergence in response to two environmental perturbations in both B. distachyon and O. sativa. Plausible molecular mechanisms underpinning paralog divergence, and cellular mechanisms driving the stomatal phenotypes are supported by analyses of BdSPCH1 and BdSPCH2 RNA and protein expression and by sequence variation among grasses. These studies suggest ways in which a duplication of a key stomatal regulator enables adaptation and could inform genetic strategies to mitigate anticipated stressors in agronomically important plants.