A model for the amplification of signaling pathways for PAMPs and AtPep peptides in Arabidopsis. The PAMPs flg22 and elf18 are perceived by their respective receptors, FLS2 and EFR (25, 26), to initiate signaling through the JA/Et and SA pathways to express the defense protein genes PDF1.2, PR-1, and PROPEP. Peptides derived from the PROPEP genes are transported to the apoplast, where they can interact with the cell-surface receptor PEPR1 to further amplify signaling.