Pedras M S, Zaharia I L, Gai Y, Zhou Y, Ward D E
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK, Canada S7N 5C9.
Proc Natl Acad Sci U S A. 2001 Jan 16;98(2):747-52. doi: 10.1073/pnas.98.2.747. Epub 2001 Jan 9.
To facilitate plant colonization, some pathogenic fungi produce phytotoxic metabolites that damage tissues; plants may be resistant to a particular pathogen if they produce an enzyme(s) that catalyzes detoxification of this metabolite(s). Alternaria blackspot is one of the most damaging and significant fungal diseases of brassica crops, with no source of resistance known within the Brassica species. Destruxin B is the major phytotoxin produced by the blackspot-causing fungus, Alternaria brassicae (Berkley) Saccardo. We have established that a blackspot-resistant species (Sinapis alba) metabolized (14)C-labeled destruxin B to a less toxic product substantially faster than any of the susceptible species. The first metabolite, hydroxydestruxin B ((14)C-labeled), was further biotransformed to the beta-d-glucosyl derivative at a slower rate. The structures of hydroxydestruxin B and beta-d-glucosyl hydroxydestruxin B were deduced from their spectroscopic data [NMR, high resolution (HR)-MS, Fourier transform infrared (FTIR)] and confirmed by total chemical synthesis. Although these hydroxylation and glucosylation reactions occurred in both resistant (S. alba) and susceptible (Brassica napus, Brassica juncea, and Brassica rapa) species, hydroxylation was the rate limiting step in the susceptible species, whereas glucosylation was the rate limiting step in the resistant species. Remarkably, it was observed that the hydroxydestruxin B induced the biosynthesis of phytoalexins in blackspot-resistant species but not in susceptible species. This appears to be a unique example of phytotoxin detoxification and simultaneous phytoalexin elicitation by the detoxification product. Our studies suggest that S. alba can overcome the fungal invader through detoxification of destruxin B coupled with production of phytoalexins.
为便于植物定殖，一些致病真菌会产生损害组织的植物毒性代谢物；如果植物产生能催化这种代谢物解毒的一种或多种酶，那么它们可能对特定病原体具有抗性。链格孢黑斑病是十字花科作物中最具破坏性且最重要的真菌病害之一，在十字花科物种中尚无已知的抗性来源。毁灭菌素B是由引起黑斑病的真菌链格孢菌（伯克利）萨卡多产生的主要植物毒素。我们已经确定，一个抗黑斑病的物种（白芥）将（14）C标记的毁灭菌素B代谢为毒性较小的产物的速度，比任何易感物种都要快得多。第一种代谢物羟基毁灭菌素B（（14）C标记）进一步以较慢的速率生物转化为β - d - 葡萄糖基衍生物。羟基毁灭菌素B和β - d - 葡萄糖基羟基毁灭菌素B的结构通过其光谱数据[核磁共振（NMR）、高分辨率（HR） - 质谱、傅里叶变换红外（FTIR）]推导得出，并通过全化学合成得到证实。尽管这些羟基化和糖基化反应在抗性物种（白芥）和易感物种（甘蓝型油菜、芥菜型油菜和白菜型油菜）中都发生，但羟基化是易感物种中的限速步骤，而糖基化是抗性物种中的限速步骤。值得注意的是，观察到羟基毁灭菌素B在抗黑斑病的物种中诱导植保素的生物合成，而在易感物种中则不然。这似乎是植物毒素解毒以及解毒产物同时引发植保素产生的一个独特例子。我们的研究表明，白芥可以通过毁灭菌素B的解毒以及植保素的产生来抵御真菌入侵。
