Little is known about the proteolytic processing of the PSK propeptide precursors, other than the fact that the precursors have conserved di-basic residues 8-10 amino acids upstream from the mature peptide sequence . Di-basic residues are characteristic of substrate sites for subtilases, subtilisin-like serine proteases (Barr, 1991) ; therefore, we wished to determine whether any of the proteases encoded by the 56 subtilase genes in the Arabidopsis genome (Rautengarten et al., 2005) are responsible for cleavage of the AtPSK4 precursor. In doing so, we identified a subtilase, AtSBT1.1, that is required for cleavage of the fusion protein AtPSK4-myc. Cleavage of AtPSK4-myc is induced in root explants, suggesting that release of the peptide hormone from the precursor protein is controlled, in part, by the proteolytic processing machinery.