<i>Populus simonii × Populus nigra</i> overexpressing <i>PsnWRKY70</i> recruits phyllosphere bacterial strains that inhibit <i>Alternaria alternata</i>

WRKY transcription factors have been implicated in the regulation of disease resistance associated with plant immune responses, which has crucial implications for defense responses against stress in plants. The role played by the <i>PsnWRKY70</i> gene of <i>Populus</i> (<i>Populus simonii</i> × <i>P. nigra</i>) in triggering the mechanism between the phyllosphere microbiome and plant defense against foliar pathogens remains unclear. Molecular ecological network analysis demonstrated that the stability and complexity of the phyllosphere bacterial community of <i>Populus</i> were influenced by <i>Alternaria alternata</i> infection. Specifically, compared to the wild-type line, the <i>PsnWRKY70</i>-overexpressing (OE) line had a higher average clustering coefficient and modularity. Furthermore, metabolomic analysis revealed that 19 differential metabolites were significantly enriched in the leaves of the OE line. Among these metabolites, coumarin compounds, such as fraxetin-8-O-glucoside (fraxin) and scopoletin-7-O-glucoside (scopolin), significantly promoted the proliferation of the genera <i>Methylobacterium</i> and <i>Achromobacter</i> with resistance to <i>A. alternata</i>. Additionally, these genera also served as connectors in the molecular ecological network of the phyllosphere microbiome of the OE line. Thus, we concluded that the <i>PsnWRKY70</i> gene enhanced the stability, complexity, and core taxa cooperation of the phyllosphere microbial network in <i>Populus</i> and regulated the biosynthesis of fraxin and scopolin to recruit beneficial bacteria controlling <i>A. alternata</i> infection. These findings provide valuable insights into the ability of resistant plant genotypes to drive the assembly of the phyllosphere microbiome, advancing our understanding of defense against pathogens using the biocontrol phyllosphere microbial community.IMPORTANCEPoplar leaf blight caused by <i>Alternaria alternata</i>, a common disease in Northeast China, can cause abnormal abscission of poplar leaves and even lead to plant death in severe cases. WRKY transcription factors have been implicated in the regulation of disease resistance associated with plant immune responses to secondary metabolism via a complicated gene network. However, little is known about how the metabolites regulated by the <i>PsnWRKY70</i> gene trigger changes in the phyllosphere microbiome, leading to increased resistance to foliar pathogens. Here, the <i>PsnWRKY70</i> overexpressing line of <i>Populus</i> (<i>Populus simonii</i> × <i>P. nigra</i>) exhibited increased coumarin synthesis in the leaves, triggering changes in microbial species central in phyllosphere microbial networks and leading to increased resistance to <i>A. alternata</i> infection. This study provides insights into the role of the <i>PsnWRKY70</i> gene in triggering the resistance mechanism to <i>A</i>. <i>alternata</i> in <i>Populus</i>.