Endogenous peptide defense signals in Arabidopsis differentially amplify signaling for the innate immune response

Abstract

AtPep1, a 23-aa peptide encoded by Arabidopsis PROPEP1, a member of a small, six-member gene family, activates expression of the defense gene PDF1.2 (encoding defensin) and its own precursor gene, PROPEP1, through the jasmonate/ethylene signaling pathway, mediated by a cell-surface receptor, PEPR1. Overexpression of two family members, PROPEP1 and PROPEP2, enhances resistance of Arabidopsis plants against the pathogen Pythium irregulare, and PROPEP2 and PROPEP3 are expressed at highly elevated levels in Arabidopsis in response to pathogen infections and to several pathogen-associated molecules (general elicitors). Here, we report that PDF1.2, PR-1 (pathogenesis protein), and PROPEP genes were differentially expressed in the leaves of intact plants sprayed with methyl jasmonate and methyl salicylate and in excised leaves supplied through cut petioles with peptides derived from the C terminus of each of the encoded proteins. The expression of PDF1.2 and PR-1 elicited by the peptides was blocked in mutant plants deficient in the jasmonate/ethylene and salicylate pathways, and in wild-type plants by treatment with diphenylene iodonium chloride, an inhibitor of hydrogen peroxide production. PROPEP1, PROPEP 2, and PROPEP3 genes appear to have roles in a feedback loop that amplifies defense signaling pathways initiated by pathogens.

Fig. 1.

Inducibility of PROPEP family genes by MeJA and MeSA. (A) Relative fold change in expression of PROPEP family genes in Arabidopsis plants sprayed with 625 μM MeJA compared with untreated control plants as determined by semiquantitative RT-PCR analysis using the β-tubulin gene as a control. (B) Relative expression of PROPEP genes in plants sprayed with 2 mM MeSA versus control plants. Error bars indicate standard deviation from the mean of three experiments.

Fig. 2.

Expression of PROPEP precursor genes in response to AtPep peptides. Relative fold change in expression of PROPEP family genes in leaves supplied with 20 nM AtPep peptides relative to control leaves supplied with water, determined by semiquantitative RT-PCR analysis.

Fig. 3.

AtPep-induced PDF1.2 and PR-1 expression is blocked in mutant plants. (A) Relative expression of PDF1.2 in leaves of wild-type Arabidopsis, fad triple-mutant, and ein2-1 plants leaves supplied with 20 nM solution of AtPep peptides through their cut petioles, compared with control leaves supplied with water. (B) Expression of PR-1 in leaves from wild-type Arabidopsis, sid2-2, and npr1-1 mutant plants supplied with 20 nM AtPep peptides through their cut petioles relative to leaves supplied with water. Expression of PDF1.2 and PR-1 was determined by semiquantitative RT-PCR.

Fig. 4.

(Left) Basal expression levels of PDF1.2 in untreated wild-type (WT) Arabidopsis plants compared with the basal levels in untreated npr1-1 and sid2-2 mutant plants. (Right) Basal expression levels of PR-1 in wild-type plants compared with basal levels in fad3,7,8 and ein2-1 mutant plants.

Fig. 5.

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.