Overexpression of mannitol dehydrogenase in zonal geranium confers increased resistance to the mannitol secreting fungal pathogen Botrytis cinerea

Abstract

The sugar alcohol mannitol is a carbohydrate with well-documented roles in both metabolism and osmoprotection in plants and fungi. In addition, however, mannitol is an antioxidant, and current research suggests that pathogenic fungi can secrete mannitol into the plant’s extracellular spaces during infection to suppress reactive oxygen-mediated host defenses. In response to pathogen attack, plants have been shown to secrete the normally symplastic enzyme, mannitol dehydrogenase (MTD). Given that MTD converts mannitol to the sugar mannose, extracellular MTD may be an important defense against mannitol-secreting fungal pathogens. Previous work demonstrated that overexpression of MTD in tobacco did, in fact, provide increased resistance to the mannitol-secreting fungal pathogen Alternaria alternata. In the present work we demonstrate that the fungal pathogen Botrytis cinerea also can secrete mannitol, and that overexpression of MTD in zonal geranium (Pelargonium × hortorum) in turn provides increased resistance to B. cinerea. These results are not only an important validation of previous work, but support the idea that MTD-overexpression might be used to engineer a broad variety of plants for resistance to mannitol-secreting fungal pathogens like B. cinerea for which specific resistance is lacking.