Diplotaxis erucoides-derived SynCom suppresses Alternaria blight in mustard by regulating ROS homeostasis and defense response.

Alternaria blight, caused by Alternaria brassicae, is a major constraint in Indian rapeseed-mustard cultivation, causing substantial yield losses. Diplotaxis erucoides, an oligotrophic wild crucifer exhibiting natural tolerance to Alternaria blight was explored as a source of beneficial endophytic bacteria. In this study, endophytic bacteria isolated from various tissues of D. erucoides were screened for plant growth-promoting and antagonistic traits against A. brassicae. Functionally efficient isolates showing growth promotion, phytohormone production, siderophore production and pathogen inhibition were selected using a Bonitur-based trait-guided approach to synthesize root-derived, leaf-derived and combined synthetic microbial communities (SynComs) comprising 12, 14 and 26 isolates, respectively. In pot trials, the Combined SynCom (CSC) significantly improved plant growth and yield-related parameters in Brassica juncea and provided the greatest disease suppression, reducing disease severity by 30.75% and conferring 47.08% disease protection at 21 days post-inoculation. Histochemical and biochemical analysis revealed reduced Hydrogen peroxide (H2O2) accumulation and necrotic symptoms, along with enhanced antioxidative enzyme activity in CSC-treated Alternaria-challenged plants. These responses were accompanied by increased activities of phenylalanine ammonia lyase, peroxidase, and polyphenol oxidase, together with greater accumulation of callose and phenolics compounds. These findings indicate that a SynCom assembled from functionally complementary endophytes associated with a wild Brassica relative can enhance plant growth and promote disease resistance through both microbial antagonism and activation of host defense responses. The results demonstrate the utility of trait-guided SynCom assembly for developing microbial inoculants and highlights CSC as a promising SynCom-based biocontrol and biofertilizer candidate for sustainable mustard production.