The <scp>bZIP</scp> Transcription Factor <scp>PjbZIP1</scp> Enhances Resistance to Powdery Mildew in <i>Paeonia jishanensis</i> by Directly Activating <i>PjICS</i> Involved in the <scp>SA</scp> Pathway

Paeonia jishanensis is a wild tree peony germplasm resource in China that has strong resistance to powdery mildew. Domesticated peony varieties exhibit compromised immunity to powdery mildew pathogens. The present study investigated the function of PjbZIP1 through expression analysis, virus-induced gene silencing (VIGS), and overexpression in Arabidopsis. Structural analysis identified a canonical basic region/leucine zipper (bZIP) motif at the C-terminal region of PjbZIP1. In P. jishanensis plants, PjbZIP1 expression was upregulated under powdery mildew infection, exogenous salicylic acid (SA) treatment, and methyl jasmonate exposure. Gene-silenced PjbZIP1 plants exhibited increased susceptibility to powdery mildew, whereas transgenic PjbZIP1 overexpression enhanced resistance. In addition, VIGS and overexpression of PjbZIP1 affected SA synthesis and the induction of genes associated with the plant defense signaling pathway. Among the candidate genes involved in SA-regulated defense responses, PjbZIP1 significantly upregulated the expression of PjICS, which encodes a key enzyme for SA biosynthesis. Yeast one-hybrid, firefly luciferase complementation imaging, and electrophoretic mobility shift assays demonstrated in vitro and in vivo that PjbZIP1 interacts with PjICS. Transcription of PjICS was induced in P. jishanensis infected with powdery mildew and was significantly enhanced in overexpressed PjbZIP1 plants. These findings establish that PjbZIP1 enhances powdery mildew resistance in tree peony by regulating the PjICS-mediated SA signaling defense pathway. These findings establish PjbZIP1 as a critical regulator of SA-mediated resistance to powdery mildew in P. jishanensis, providing genetic tools for breeding disease-resistant cultivars.