During development, cells interpret complex and often conflicting signals to make optimal decisions. Plant stomata, the
cellular interface between a plant and the atmosphere, develop according to positional cues, which include a family of
secreted peptides called epidermal patterning factors (EPFs). How these signalling peptides orchestrate pattern
formation at a molecular level remains unclear. Here we report in Arabidopsis that Stomagen (also called EPF-LIKE9)
peptide, which promotes stomatal development, requires ERECTA (ER)-family receptor kinases and interferes with the
inhibition of stomatal development by the EPIDERMAL PATTERNING FACTOR 2 (EPF2)–ER module. Both EPF2 and
Stomagen directly bind to ER and its co-receptor TOO MANY MOUTHS. Stomagen peptide competitively replaced EPF2
binding to ER. Furthermore, application of EPF2, but not Stomagen, elicited rapid phosphorylation of downstream
signalling components in vivo. Our findings demonstrate how a plant receptor agonist and antagonist define
inhibitory and inductive cues to fine-tune tissue patterning on the plant epidermis.