Co-option and neofunctionalization of stomatal executors for defense against herbivores in Brassicales

Abstract Co-option of gene regulatory networks leads to the acquisition of new cell types and tissues. Stomata, valves formed by guard cells (GCs), are present in most land plants and regulate CO2 exchange. The transcription factor (TF) FAMA globally regulates GC differentiation but also promotes the development of idioblast myrosin cells (MCs), another type of specialized cell along the vasculature essential for Brassicales-specific chemical defenses. Here, we show that the TF gene WASABI MAKER (WSB), directly induced by FAMA, triggers MC differentiation. WSB and STOMATAL CARPENTER 1 (SCAP1), a stomatal lineage–specific direct FAMA target, synergistically promote GC differentiation. wsb mutants lacked MCs, while the wsb scap1 double mutant lacked normal GCs. Evolutionary analyses revealed that WSB is conserved across stomatous angiosperms. Furthermore, the FAMA–WSB positive feedback loop acts as a cell fate switch. We propose that the conserved and reduced transcriptional FAMA–WSB module was co-opted before evolving to induce MC differentiation.