To determine the site at which the peptide was cleaved, the reaction products were analyzed by MALDI-TOF analysis. The first N-terminal cleavage product that appeared had a molecular mass of 862.25, indicating a preferred cutting site at Abz-RRSLVLflHTDY(NO2)D-OH (Figure 4d ). MS/MS analysis of the 862.25 peak showed a spectrum of ions consistent with Abz-RRSLVL as the initial cleavage product ( Figure S5 ). It should be noted that this cut is three amino acid residues upstream of the N-terminus of the mature peptide as has been described for PSKs in Asparagus officinalis . If the mature Arabidopsis peptide is similar, then further N-terminal proteolytic processing, as well as C-terminal processing, probably occurs to generate the active peptide.