Single-Cell Resolution of Lineage Trajectories in the Arabidopsis Stomatal Lineage and Developing Leaf

SUMMARY Dynamic cell states underlie flexible developmental programs, such as with the stomatal lineage of the Arabidopsis epidermis. Initial stages of the lineage feature asynchronous and indeterminate divisions modulated by environmental cues, enabling cell fate flexibility to generate the requisite density and pattern of stomata for a given environment. It remains unclear, however, how flexibility of cell fates is controlled. Here, we uncovered distinct models of cell state differentiation within Arabidopsis leaf tissue by leveraging single-cell transcriptomics and molecular genetics. Our findings resolved underlying heterogeneity within cell states of the flexible epidermal stomatal lineage, which appear to exist along a continuum, with progressive cell specification. Beyond the early stages of the lineage, we discovered that the core transcriptional regulator SPEECHLESS is required for cell fate commitment to yield stomatal guard cells. Overall, our work has refined the stomatal lineage paradigm and uncovered progressive cell state decisions along lineage trajectories in developing leaves.