Mobile MUTE specifies subsidiary cells to build physiologically improved grass stomata

Making more of your stomata Stomata on grasses are made up of two guard cells and two subsidiary cells, and they perform better than stomata on broad-leaved plants, which are made up only of two guard cells. Raissig et al. found that the MUTE transcription factor in the wheat-like grass Brachypodium is a little bigger than the equivalent protein in the model broad-leaved plant Arabidopsis. The extension in the grass protein promotes its movement into adjacent cells, prompting them to become subsidiary cells. Mutant Brachypodium whose MUTE protein could not move between cells lacked stomatal subsidiary cells and grew poorly. Science, this issue p. 1215 Mobile transcription factor builds better stomata in grasses. Plants optimize carbon assimilation while limiting water loss by adjusting stomatal aperture. In grasses, a developmental innovation—the addition of subsidiary cells (SCs) flanking two dumbbell-shaped guard cells (GCs)—is linked to improved stomatal physiology. Here, we identify a transcription factor necessary and sufficient for SC formation in the wheat relative Brachypodium distachyon. Unexpectedly, the transcription factor is an ortholog of the stomatal regulator AtMUTE, which defines GC precursor fate in Arabidopsis. The novel role of BdMUTE in specifying lateral SCs appears linked to its acquisition of cell-to-cell mobility in Brachypodium. Physiological analyses on SC-less plants experimentally support classic hypotheses that SCs permit greater stomatal responsiveness and larger range of pore apertures. Manipulation of SC formation and function in crops, therefore, may be an effective approach to enhance plant performance.