Melatonin activates plant defense against root-knot nematodes through the jasmonate pathway.

Root-knot nematodes are obligate plant parasites that can infect a broad spectrum of hosts, including both monocotyledons and dicotyledons. Among strategies to control plant-parasitic nematodes, plant defense elicitors present a viable and environmentally friendly management approach. Melatonin is a signaling molecule that contributes to physiological processes, helping plants respond to biotic and abiotic stresses. However, its role in plant defense against parasitic nematodes has not yet been investigated. The expression of melatonin biosynthesis genes in galls formed upon nematode infection on tomato plants was investigated, and their expression profile indicated their involvement in processes. Foliar application of 100 μM melatonin on tomato plants decreased the number of galls and nematodes significantly. Biochemical assays showed that the activity of defense-related enzymes peaked two days after melatonin treatment. Concurrently, the expression of genes linked to the jasmonic acid pathway was significantly upregulated two days after melatonin post-treatment, indicating the contribution of the jasmonic acid pathway in the induction of resistance. Confirming this view, mutants deficient in the jasmonic acid pathway (jasmonic acid-insensitive1-1 (jai1-1) and JA-deficient mutant defenseless-1 (def-1)) failed to exhibit melatonin-mediated resistance.