Contrasting effects of melatonin on Brassica juncea and Alternaria brassicae reduce pathogenicity of the fungus and alleviate damage to plants

• Melatonin alleviates negative effect of Alternaria brassicae in Brassica juncea by reducing membrane damage and enhancing antioxidant machinery. • A 200 µM dosage of exogenous melatonin decreased disease severity against A. brassicae by up to 75 %. • Melatonin reduces the pathogenicity by altering fungal metabolism depicted by HRMS metabolite profiling. • Melatonin behave as an antifungal agent which could be a potential eco-friendly and sustainable alternative to chemical fungicides for agricultural improvement in B. juncea . Brassica juncea (Indian mustard) is a vital oil seed crop in India and is grown worldwide for oil and food requirements. Alternaria brassicae is a major threat to B. juncea quality and productivity causing worldwide crop loss. In this work, it was observed that exogenous melatonin (MT) at 200 µM concentration was proven to be well effective against A. brassicae, which reduced disease consequences up to 75 %, attributed to significant reduction of necrotic lesions, delayed onset infection, and improved plant defence by enhancing chlorophyll and proline content, maintaining water retention and reducing oxidative stress. Additionally, biochemical assays, such as reduced oxidative damage, as evidenced by diminished levels of malondialdehyde (MDA), hydrogen peroxide (H 2 O 2 ) and enhanced antioxidant enzyme activities, clearly showed the antifungal effect of MT on Alternaria. This study represents a new insight into the potential role as sustainable antifungal compound. Furthermore, HRMS metabolite profiling of A. brassicae cultured with MT revealed altered fungal metabolite profiles, including increased production of known antifungal compounds, suggesting that MT disrupts fungal metabolism and reduces its pathogenicity. In contrast, a decrease in the levels of antioxidant and indole derivatives in fungal extracts indicated weaker fungal defenses. The major aim of the study to provide evidence for the curative potential of MT as an effective antifungal molecule against A. brassicae in B. juncea that would be beneficial to agricultural improvement and an eco-friendly alternative to chemical fungicides.