PAMP-INDUCED SECRETED PEPTIDE 3 modulates immunity in Arabidopsis.

Small post-translationally modified peptides are important signalling components of plant defence responses against phytopathogens, acting as both positive and negative modulators. PAMP-INDUCED SECRETED PEPTIDE (PIP) 1 and 2 have been shown to amplify plant immunity. Here we investigate the role of the related peptide PIP3 in the regulation of immune response in Arabidopsis. Treatment with synthetic PIP peptides led to similar transcriptome reprogramming, indicating an effect on innate immunity-related processes and phytohormones, including jasmonic acid (JA) biosynthesis and signalling. PIP3 overexpressing (OX) plants showed enhanced growth inhibition in response to flg22 exposure. In addition, flg22-induced production of reactive oxygen species and callose deposition was significantly reduced in PIP3-OX plants. Interestingly, PIP3-OX plants showed increased susceptibility toward both Botrytis cinerea and the biotrophic pathogen Pseudomonas syringae. Expression of both JA and salicylic acid (SA) biosynthesis and signalling genes was more induced during B. cinerea infection in PIP3-OX plants compared with wild-type plants. Promoter and ChIP-seq analyses indicated that the transcription factors WRKY18, WRKY33, and WRKY40 cooperatively act as repressors for PIP3. The results point to a fine-tuning role for PIP3 in modulation of immunity through the regulation of SA and JA biosynthesis and signalling pathways in Arabidopsis.