Abstract Cells in a multicellular organism constantly communicate by ligand-receptor-mediated signaling to coordinate their growth, development, and adaptation to the environment. The peptide ligands-triggered signaling is the most abundant in this network in humans, dysregulation of which contributes to many prevalent human diseases. However, knowledge of how the peptide ligand-triggered signaling is regulated by the environment remains incomplete. We study a classical peptide ligand-receptor-mediated signaling pathway that regulates the epidermal stomatal development in Arabidopsis thaliana, a naturally simplified and experimentally accessible system. Stomata are micropores on the aerial surface of plants that facilitate gas exchange with the environment. Stomatal development is flexibly adjusted under variable environmental conditions to optimize plant adaptation, making it an attractive system for studies on environmental regulation of peptide ligand-triggered signaling pathways. Upstream of the stomatal signaling pathway are a group of secretory ligands named Epidermal Patterning Factors (EPFs), many of which are highly responsive to environmental cues. We found that osmotic stress caused a dramatic reduction in stomatal density. Interestingly, the transcript of a putative EPF ligand is preferentially induced under osmotic stress. Compared to wild-type plants, the epf mutants produce more stomatal precursor cells under osmotic stress. With these new findings, we hypothesize that osmotic stress inhibits stomatal development via enhancing the candidate EPF ligand-triggered signaling. Our study and others have previously indicated that distinct EPF-triggered signaling pathways target different steps of the stomatal developmental process. The specificity is, at least partially, due to the precise spatiotemporal expression of a particular EPF ligand and the differential subcellular behavior of the receptor kinases including ERECTA-LIKE 1, that transduce the EPF signaling. This MIRA proposal aims to comprehensively evaluate the osmotic stress- induced EPF ligand-triggered signaling pathway by addressing the following three questions: 1) Elucidate the osmotic regulation of the transcription of the candidate EPF gene; 2) Dissect the osmotic regulation of the stomatal receptor complex; 3) Identify the targets of the osmotic stress-induced EPF signaling pathway. Completion of this work is likely to provide valuable insight into our understanding of osmotic regulation of organism development via peptide ligand-triggered signaling pathways.