The peptide EPFL8 represses embryonic stomatal precursor formation independently of TOO MANY MOUTHS in Arabidopsis

Plants regulate gas exchange through stomata, which are specialized valves on the aerial epidermis. Stomatal development involves tightly controlled transitions orchestrated by basic helix-loop-helix transcription factors, including SPEECHLESS (SPCH). These factors coordinate cell division and differentiation during stomatal development. Recent studies have underscored the importance of cell-cell communication in this process, mediated by secreted peptide ligands of the EPIDERMAL PATTERNING FACTOR (EPF) family and a Leucine-Rich-Repeat Receptor complex that includes ERECTA family receptor kinases and the receptor TOO MANY MOUTHS (TMM). Although mature stomata do not form during embryogenesis, stomatal cell fate is determined at these early stages. In this study, we discovered that EPFL8 is highly expressed during early embryogenesis and inhibits stomatal development by negatively regulating SPCH. Furthermore, TMM, a SPCH target, interferes with EPFL8-mediated signaling. In the absence of TMM, EPFL8 expression extends into later embryogenesis, which aligns with the reduced number of stomatal precursors observed in tmm mutants. These findings reveal that EPFL8 functions as an embryonic peptide ligand that suppresses stomatal formation via SPCH regulation, highlighting the diversification of EPF family members in fine-tuning plant epidermal patterning.