Slow and not so furious: de novo stomatal pattern formation during plant embryogenesis

The fates of cells on the leaf surface depend on positional cues and environmental signals. New stomata are formed away from existing ones to prevent disadvantageous clustering, but how are the initial precursors positioned? Mature embryos do not have stomata, but we provide transcriptomic, imaging, and genetic evidence that Arabidopsis embryos engage known stomatal fate and patterning factors to create regularly spaced stomatal precursor cells. Analysis of embryos from 35 plant species indicates this trait is conserved across angiosperm clades. Embryonic stomatal patterning in Arabidopsis is established in three stages: first broad SPEECHLESS (SPCH) expression, then coalescence of SPCH and its targets into discrete domains, and finally one round of symmetric division to create stomatal precursors. Lineage progression is then halted until after germination. We show that embryonic stomatal pattern enables quick stomatal differentiation and photosynthetic activity upon germination but also guides the formation of additional stomata as the leaf expands. In addition, key stomatal regulators are prevented from driving the fate transitions they can induce after germination, revealing stage-specific layers of regulation that control lineage progression during embryogenesis.