ABA-regulated JAZ1 suppresses phytoalexin biosynthesis by binding GmNAC42-1 in soybean

Abstract

Phytoalexins are plant defense metabolites whose biosynthesis remains suppressed until elicited by a pathogen or stress, yet the mechanism of their suppression has remained elusive. The transcription factor GmNAC42–1 directly activates glyceollin phytoalexin biosynthesis in soybean, but its overexpression without elicitation fails to induce glyceollin biosynthetic genes, suggesting suppression by a negative regulator. JAZ1 proteins act as negative regulators in the canonical jasmonic acid (JA) signaling pathway. JAZ protein degradation and JAZ gene transcription comprise antagonistic mechanisms that activate and suppress JA signaling. Here, RNA-seq analysis revealed that abscisic acid (ABA) signaling and GmJAZ1 genes are inversely regulated compared to glyceollin biosynthesis at late timepoints following elicitation, identifying them as potential long-term negative regulators. Long-term ABA treatment increased GmJAZ1 transcript levels, whereas an ABA biosynthesis inhibitor completely suppressed their upregulation by dehydration. Opposite patterns were observed for glyceollin biosynthesis. RNAi silencing of GmJAZ1s prevented the suppression of glyceollin biosynthesis by long-term dehydration stress and derepressed glyceollin synthesis in non-elicited tissues. Overexpressing GmJAZ1–9 in hairy roots elicited with Phytophthora sojae wall glucan elicitor partially suppressed short-term elicitation of glyceollin biosynthesis. The GmJAZ1–9 protein interacted with GmNAC42–1, inhibiting its transactivation and DNA-binding activities. JAZ1 silencing in Arabidopsis and grapevine derepresses phytoalexin biosynthesis. While the short-term response to pathogen elicitation includes a reduction in ABA levels and JA-mediated JAZ protein degradation, our work demonstrates a subsequent long-term response where ABA upregulates JAZ1 transcript levels by an unknown mechanism and JAZ1 proteins bind NAC42-type transcriptional activators to directly inhibit their transactivation of phytoalexin biosynthesis.