Abstract
Brassica species produce the antifungal indolyl compounds brassinin and its derivatives, during microbial infection. The fungal pathogen Alternaria brassicicola detoxifies brassinin and possibly its derivatives. This ability is an important property for the successful infection of brassicaceous plants. Previously, we identified a transcription factor, Bdtf1, essential for the detoxification of brassinin and full virulence. To discover genes that encode putative brassinin-digesting enzymes, we compared gene expression profiles between a mutant strain of the transcription factor and wild-type A. brassicicola under two different experimental conditions. A total of 170 and 388 genes were expressed at higher levels in the mutants than the wild type during the infection of host plants and saprophytic growth in the presence of brassinin, respectively. In contrast, 93 and 560 genes were expressed, respectively, at lower levels in the mutant than the wild type under the two conditions. Fifteen of these genes were expressed at lower levels in the mutant than in the wild type under both conditions. These genes were assumed to be important for the detoxification of brassinin and included Bdtf1 and 10 putative enzymes. This list of genes provides a resource for the discovery of enzyme-coding genes important in the chemical modification of brassinin.
Highlights
Plants protect themselves from the attack of potential pathogens
Each strain of the ∆bdtf1 mutants was indistinguishable from wild-type A. brassicicola in mycelial growth on nutrient-rich potato dextrose agar (PDA) (Figure 1A) or glucose-yeast-extract-broth medium (GYEB)
Active mycelial growth of the mutants stopped when transferred to PDA or glucose yeast extract broth (GYEB) containing 0.2 mM brassinin (Figure 1B)
Summary
Plants protect themselves from the attack of potential pathogens. Plant resistance mechanisms include the production of antimicrobial compounds called phytoalexins. Brassinin and its derivatives are phytoalexins produced by plants of the genus Brassica. They are induced during the infection process by microbes, including pathogenic fungi [1,2,3]. Brassinin has antimicrobial activity in vitro [4]. Mutant strains of A. brassicicola with cell wall integrity defects are more sensitive to brassinin [5,6]. It is possible that brassinin affects the cell integrity of pathogens similar to camalexin, a phytoalexin that disrupts the cell membrane of the bacterium Pseudomonas syringae [7]
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