Progeny of Selfed Plants from Tomato Breeding Line ‘NC1 Grape’ Overexpressing Mannitol Dehydrogenase (MTD) Have Increased Resistance to the Early Blight Fungus, Alternaria solani

Reactive oxygen species (ROS) made by plants in response to pathogen infection not only initiate local and systemic defenses, they are also antimicrobial. A number of fungi are hypothesized to secrete the antioxidant mannitol to protect against this antimicrobial ROS during infection. This hypothesis is supported by reports that overexpression of the mannitol catabolic enzyme mannitol dehydrogenase (MTD) in plants increases resistance to mannitol-secreting pathogens like Botrytis cinerea and Alternaria alternata. To extend this hypothesis and test the general utility of this approach, we overexpressed celery MTD in a tomato breeding line (NC1 Grape) currently used in our program. Although we reported earlier that MTD overexpression provides resistance to Botrytis gray mold in a greenhouse tomato, this is the first report of overexpression in an elite breeding variety providing heritable, whole-plant resistance to A. solani (tomato early blight). In this study, progeny from a high-MTD-expressing line had infection rates <65% those of nontransformed plants, and transformants outgrew infection by 7 days post-inoculation. Finally, our results suggest that screening for higher innate MTD expression in plants, rather than screening solely for the presence of the Mtd gene, might be a more effective way to identify parental lines for use in conventional breeding of early blight resistance. Accepted for publication 7 August 2015. Published 14 August 2015.