Induced resistance to black spot disease through exogenous application of arachidonic acid in Brassica juncea

Black spot disease is an important disease infected by Alternaria brassicicola, which seriously affects the yield and quality of Brassica juncea. Arachidonic acid (ARA), a polyunsaturated fatty acid, induces the defense response of plants against pathogens. Here, we analyzed the induced resistance effect of ARA against black spot disease, and further revealed the underlying mechanism via physiological and transcriptome analyses. Treatment with 10 μM ARA exhibited the strongest resistance induction, dramatically improving the activities of defense enzymes (laccase, phenylalanine ammonia-lyase, polyphenol oxidase, peroxidase, and chitinase), as well as the contents of lignin and glucosinolates. Transcriptome analysis identified 47,529 differentially expressed genes (DEGs) enriched in phenylpropanoid biosynthesis, starch and sucrose metabolism, flavonoid biosynthesis, glutathione metabolism, and plant hormone signal transduction. In addition, several transcription factors (MYB, MYC, WRKY, and bZIP) were involved in ARA-induced resistance to black spot disease. Weighted gene co-expression network analysis (optimal β = 16) identified six co-expression modules, among which MEred and MEblue were positively correlated with pathogen treatment, while MEturquoise was negatively correlated. Hub genes (BjuVA04G24670, BjuVA02G38020, and BjuVA09G09890) participated in plant immunity, cell wall remodeling, and photosynthesis. Collectively, ARA enhances resistance of B. juncea to A. brassicicola by improving the activities of defense enzymes, regulating DEGs, co-expression networks, and key transcription factors. This work provides a theoretical basis for the further exploration of ARA as a potential component of integrated crop disease management strategies.