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Differential roles of the plant secondary metabolite melatonin in plant-host resistance and pathogen suppression

Mihir Mandal: ORISE participant, US Vegetable Laboratory, USDA, ARS

<div>Since the 1950s, research on the animal neurohormone melatonin, has focused on its multi-regulatory effect on patients suffering from insomnia, cancer, and Alzheimer’s. Previous studies on melatonin in plants have focused primarily on plant growth and development. However, studies on the physiological function of melatonin in host-pathogen defense mechanism are lacking. This study provides insight on how application of melatonin, an environmental-friendly immune inducer, can boost plant immunity and suppress pathogen growth in a field situation where fungicide resistance and lack of genetic resistance are major problems. We evaluated the effect of spray-applied melatonin and also transformed watermelon plants with the melatonin biosynthetic gene <em>SNAT</em> to determine the role of melatonin in plant defense. Increased melatonin levels in plants were found to boost resistance against the foliar pathogen <em>Podosphaera xanthii</em> (powdery mildew), and the soil-borne oomycete <em>Phythophthora capsici </em>in watermelon and other cucurbits. Transcriptomic data suggests that melatonin alters the expression of genes involved in both PAMP and ETI mediated defenses. Twenty seven upregulated genes were associated with constitutive defense as well as initial priming of the melatonin induced plant resistance response. Our results indicate that developing strategies to increase melatonin levels in specialty crops such as watermelon can lead to resistance against diverse filamentous pathogens.</div>