Jasmonic Acid-Mediated Defense Activation Confers Resistance to <em>Ustilago maydis </em>Infection in Maize

Jasmonic acid (JA) plays a crucial role in plant disease resistance defense. To explore its regulatory mechanism in plant resistance to pathogens, this study focused on Ustilago maydis (U. maydis) using, exogenous application of different concentrations of JA and jasmonic acid inhibitor ibuprofen (IBU), combined with physiological and biochemical analysis, cytological observation, transcriptome analysis and weighted gene co-expression network analysis (WGCNA), to systematically analyze the JA-induced disease resistance mechanism in maize. The results showed that 0.1 mmol/L JA treatment significantly inhibited pathogen mycelial growth (with an inhibition rate of 19.0%) and improved plant disease resistance (disease index decreased by 52.65%), while IBU treatment exacerbated disease susceptibility. Physiological and biochemical analysis showed that JA treatment significantly increased the activity of defense enzymes (POD, PPO, &beta;-1,3-GA), reduced malondialdehyde (MDA) content and alleviated membrane damage. Cytological observation demonstrated that JA maintained the integrity of mesophyll cells and vascular bundle sheath structures, effectively inhibiting cell expansion and division caused by pathogens. Transcriptome analysis indicated that JA induced more differentially expressed genes (DEGs) in the susceptible variety Ye478 compared to the resistant variety Qi319, and these genes were significantly enriched in phenylpropanoid biosynthesis, starch and sucrose metabolism pathways. Through WGCNA analysis, 16 key genes were identified from four modules, further identifying six candidate genes related to disease resistance, including self-resistance-resistant genes (PP2C, WRKY) and JA-induced resistance-related genes (CGT, GLCAT, RING zinc finger protein, CaM). In summary, JA significantly enhances maize resistance to U. maydis by enhancing defense enzyme activity, maintaining cell structure stability and regulating key gene expression. This study provides an important theoretical basis for clarifying the JA-mediated disease resistance mechanism and disease-resistant breeding.