Phytosulfokine downregulates defense-related WRKY transcription factors and attenuates pathogen-associated molecular pattern-triggered immunity

Phytosulfokine (PSK) is a plant growth-promoting peptide hormone that is perceived by its cell surface receptors PSKR1 and PSKR2 in Arabidopsis. Plants lacking the PSK receptors show phenotypes consistent with PSK signaling repressing some plant defenses. To gain further insight into the PSK signaling mechanism, comprehensive transcriptional profiling of Arabidopsis treated with PSK was performed, and the effects of PSK treatment on plant defense readouts were monitored. Our study indicates that PSK's major effect is to downregulate defense-related genes; it has a more modest effect on the induction of growth-related genes. WRKY transcription factors (TFs) emerged as key regulators of PSK-responsive genes, sharing commonality with a pathogen-associated molecular pattern (PAMP) responses, flagellin 22 (flg22), but exhibiting opposite regulatory directions. These PSK-induced transcriptional changes were accompanied by biochemical and physiological changes that reduced PAMP responses, notably mitogen-activated protein kinase (MPK) phosphorylation (previously implicated in WRKY activation) and the cell wall modification of callose deposition. Comparison with previous studies using other growth stimuli (the sulfated plant peptide containing sulfated tyrosine PSY and Pseudomonas simiae strain WCS417) also reveals WRKY TFs' overrepresentations in these pathways, suggesting a possible shared mechanism involving WRKY TFs for plant growth–defense trade-off.