SIAMESE-RELATED1 imposes differentiation of stomatal lineage ground cells into pavement cells.

The leaf epidermis represents a multifunctional tissue consisting of trichomes, pavement cells and stomata, the specialized cellular pores of the leaf. Pavement cells and stomata both originate from regulated divisions of stomatal lineage ground cells (SLGCs), but whereas the ontogeny of the stomata is well characterized, the genetic pathways activating pavement cell differentiation remain relatively unexplored. Here, we reveal that the cell cycle inhibitor SIAMESE-RELATED1 (SMR1) is essential for timely differentiation of SLGCs into pavement cells by terminating SLGC self-renewal potency, which depends on CYCLIN A proteins and CYCLIN-DEPENDENT KINASE B1. By controlling SLGC-to-pavement cell differentiation, SMR1 determines the ratio of pavement cells to stomata and adjusts epidermal development to suit environmental conditions. We therefore propose SMR1 as an attractive target for engineering climate-resilient plants.