1Plant and Environmental Protection Sciences, University of Hawaii at Manoa, 3190 Maile Way, St. John 317, Honolulu, HI 96822, U.S.A.; 2Korea Bio-Safety Institute, Co., LTD., 362-20 Seongju-ro, Gamgok, Eumseong, Chungbuk 369-850, South Korea; 3Korea Research Institute of Bioscience and Biotechnology, 30 Yeongudanji-ro, Ochang, Cheongwon, Chungbuk 363-883, South Korea
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Accepted 8 August 2013.
Brassinin is an antifungal compound induced in Brassica plants after microbial infection. Molecular evidence is incomplete, however, in supporting the importance of brassinin in plant resistance to pathogens. To test the importance of brassinin in plant defense, we studied the functions of the gene Bdtf1 in the necrotrophic fungus Alternaria brassicicola. Several strains of mutants of this gene were weakly virulent on Brassica species, causing lesions 70% smaller in diameter than the wild type on three Brassica species. These mutants, however, were as virulent as the wild type on Arabidopsis thaliana. They were similar to the wild type in spore germination, colony morphology, and mycelial growth in nutrient-rich media, both with and without stress-inducing chemicals. Unlike wild-type A. brassicicola, however, the mutants failed to germinate and their hyphal growth was arrested in the presence of 200 μM brassinin. When grown in a medium containing 100 μM brassinin, wild-type mycelium entirely converted the brassinin into a nontoxic derivative, of which the precise chemical nature was not established. Mutants of the Bdtf1 gene were unable to perform this conversion. Our results support the hypothesis that the ability of A. brassicicola to detoxify brassinin is necessary for successful infection of Brassica species.
© 2013 The American Phytopathological Society