Stomagen, also called EPIDERMAL PATTERNING FACTOR-like protein 9 (EPFL9 or STOMAGEN), or EPF-like protein 9, is plant peptide hormone expressed in the mesophyll, that acts as a positive regulator of stomatal density and patterning. It competes with EPIDERMAL PATTERNING FACTORS 2 (EPF2) for binding to the ERECTA receptor, while it also competes with EPF1 and EPF2 for binding to TOO MANY MOUTHS (TMM), both receptors for the EPF peptides to initiate a signal cascade for stomatal development. EPFL9 cannot be cleaved by the protease CO2 RESPONSE SECRETED PROTEASE (CRSP). Stomagen is composed of a loop and a scaffold containing three disulphide bonds. It belongs to the fold-family of small globular knottins.
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