Associative transcriptomic analysis of defensive responses of Brassica juncea to Alternaria brassicae

Context Periodic outbreaks of Alternaria leaf blight (ALB) or brown spot caused by Alternaria brassicae result in significant production losses in mustard crops globally. Understanding the genetic basis of resistance to ALB is crucial for developing stable, resistant cultivars. Aims This study is aimed to evaluate defensive responses to a purified isolate of A. brassicae in a diverse germplasm collection of B. juncea and to identify candidate genes associated with resistance. Methods A germplasm collection comprising 261 lines of B. juncea was assessed for resistance to A. brassicae. Phenotypic variation was analysed to determine inheritance patterns, and associative trascriptomics was used to identify candidate genes involved in the defense response. Key results Resistance responses exhibited a near-normal distribution, suggesting quantitative inheritance. Wild Brassicaceae introgression lines, such as IM-152, IM-87, J-8, and the exotic germplasm line ZEM-3356, consistently displayed resistance. Among them, J-8 and ZEM-3356 showed the lowest disease severity. Thirteen candidate genes were identified, with TSD2 emerging as a strong candidate, being specifically linked to resistance against Alternaria brown spot. Other key genes, including ADT5, CBF2, and DUF677, were also implicated in fungal resistance. Conclusions Resistance to ALB is quantitatively inherited, and specific candidate genes were identified as critical for defense. Incorporating these genes into mustard cultivars may improve resistance. Implications Strategies such as pyramiding these genes, marker-assisted recurrent selection, and interspecific hybridization could enhance genetic resistance in mustard and related crops such as canola rapeseed, ultimately reducing losses caused by ALB.