Nuclear NPR1 Modulates Salicylic Acid/Jasmonic Acid Crosstalk via Transcriptional Control of WRKY Transcription Factors
The plant hormones salicylic acid (SA) and jasmonic acid (JA) are major regulators of plant defense. They can antagonize each other's activity, enabling an optimized defense response. The SA master regulator NPR1 is a key player in this SA/JA crosstalk, although the mechanisms of NPR1-mediated SA/JA crosstalk remain only partly understood, especially regarding the ethylene-coregulated ERF branch of the JA pathway. Here, we investigated the function of NPR1 in SA/JA crosstalk using several Arabidopsis npr1 mutant variants. Assays using the NPR1-HBD and npr1-nls lines, expressing only cytosolic NPR1, revealed that NPR1 has a nuclear role in SA/JA crosstalk. RNA-seq analysis of lines expressing NPR1 mutated in residues Cys82 or Cys216, in which repression of the JA pathway was impaired while SA pathway induction remained mostly intact, identified 32 SA-responsive genes with reduced expression compared to Col-0, including four WRKY transcription factor genes. Of these, WRKY53 and WRKY70 could suppress expression of the JA marker gene PDF1.2 induced by ORA59, the master regulator of the ERF branch of the JA pathway, in a protoplast system. Additional SA-inducible WRKYs (WRKY50, WRKY54, WRKY63, and WRKY67) similarly suppressed ORA59-induced PDF1.2 expression, implicating WRKY63 and WRKY67 for the first time in SA/JA crosstalk and confirming earlier reports on the role of the other WRKY TFs. WRKY53 and WRKY67 were further shown to reduce accumulation of the ORA59 protein in protoplasts. Thus, our study indicated that NPR1 can regulate SA/JA crosstalk in the nucleus via transcriptional activation of genes encoding specific JA-repressing WRKY transcription factors.