Integration of Transcriptomics and WGCNA to Characterize Trichoderma harzianum-Induced Systemic Resistance in Astragalus mongholicus for Defense against Fusarium solani

Beneficial fungi of the genus <i>Trichoderma</i> are among the most widespread biocontrol agents that induce a plant's defense response against pathogens. <i>Fusarium solani</i> is one of the main pathogens that can negatively affect <i>Astragalus mongholicus</i> production and quality. To investigate the impact of <i>Trichoderma harzianum</i> on <i>Astragalus mongholicus</i> defense responses to <i>Fusarium solani</i>, <i>A. mongholicus</i> roots under <i>T. harzianum</i> + <i>F. solani</i> (T + F) treatment and <i>F. solani</i> (F) treatment were sampled and subjected to transcriptomic analysis. A differential expression analysis revealed that 6361 differentially expressed genes (DEGs) responded to <i>T. harzianum</i> induction. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of the 6361 DEGs revealed that the genes significantly clustered into resistance-related pathways, such as the plant-pathogen interaction pathway, phenylpropanoid biosynthesis pathway, flavonoid biosynthesis pathway, isoflavonoid biosynthesis pathway, mitogen-activated protein kinase (MAPK) signaling pathway, and plant hormone signal transduction pathway. Pathway analysis revealed that the <i>PR1</i>, formononetin biosynthesis, biochanin A biosynthesis, and <i>CHIB</i>, ROS production, and <i>HSP90</i> may be upregulated by <i>T. harzianum</i> and play important roles in disease resistance. Our study further revealed that the H₂O₂ content was significantly increased by <i>T. harzianum</i> induction. Formononetin and biochanin A had the potential to suppress <i>F. solani</i>. Weighted gene coexpression network analysis (WGCNA) revealed one module, including 58 DEGs associated with <i>T. harzianum</i> induction. One core hub gene, <i>RPS25</i>, was found to be upregulated by <i>T. harzianum</i>, SA (salicylic acid) and ETH (ethephon). Overall, our data indicate that <i>T. harzianum</i> can induce induced systemic resistance (ISR) and systemic acquired resistance (SAR) in <i>A. mongholicus</i>. The results of this study lay a foundation for a further understanding of the molecular mechanism by which <i>T. harzianum</i> induces resistance in <i>A. mongholicus</i>.