Interest in phytomelatonin biosynthesis and activity has increased due to apparent effects on enhancing resistance to biotic and abiotic stress. Activity related to biotic stress has been associated with increased salicylic acid production and interaction of melatonin with reactive oxygen radicals. Activity related to abiotic stress is associated with alterations in auxin responses, providing reduced levels of senescence. The role of melatonin was investigated in potato using weekly exogenous application of a 2 mM melatonin sprayed onto leaves of whole plants. Modification of endogenous melatonin biosynthesis involved alterations in expression of two genes in the phytomelatonin biosynthetic pathway, using overexpression of caffeic acid O-methyltransferase (COMT) and antisense silencing of serotonin N-acetyltransferase (SNAT). Plant leaflets were assessed for disease resistance by inoculation with Alternaria solani (early blight). Exogenous application of melatonin did not increase pathogen resistance in detached leaflet assays. Melatonin applications did increase total tuber weights in the greenhouse by 8 to 30% depending on the cultivar tested. Endogenous modification of the melatonin biosynthetic gene SNAT did not affect disease resistance. Although the SNAT silenced plants had normal phenotypes, the COF overexpression resulted in short, thickened stems and leaves along with undersized, misshaped tubers, indicating that COF overexpression is undesirable for modification of endogenous melatonin levels.
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