Sollod C C, Jenns A E, Daub M E
Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616.
Appl Environ Microbiol. 1992 Feb;58(2):444-9. doi: 10.1128/aem.58.2.444-449.1992.
The phytotoxin cercosporin, a singlet oxygen-generating photosensitizer, is toxic to plants, mice, and many fungi, yet the fungi that produce it, Cercospora spp., are resistant. We hypothesize that resistance to cercosporin may result from a reducing environment at the cell surface. Twenty tetrazolium dyes differing in redox potential were used as indicators of cell surface redox potential of seven fungal species differing in resistance to cercosporin. Resistant fungi were able to reduce significantly more dyes than were sensitive fungi. A correlation between dye reduction and cercosporin resistance was also observed when resistance levels of Cercospora species were manipulated by growth on different media. The addition of the reducing agents ascorbate, cysteine, and reduced glutathione (GSH) to growth media decreased cercosporin toxicity for sensitive fungi. None of these agents directly reduced cercosporin at the concentrations at which they protected fungi. Spectral and thin-layer chromatographic analyses of cercosporin solutions containing the different reducing agents indicated that GSH, but not cysteine or ascorbate, reacted with cercosporin. Resistant and sensitive fungi did not differ in endogenous levels of cysteine, GSH, or total thiols. On the basis of data from this and other studies, this report presents a model which proposes that cercosporin resistance results from the production of reducing power at the surfaces of resistant cells, leading to transient reduction and detoxification of the cercosporin molecule.
植物毒素尾孢菌素是一种能产生单线态氧的光敏剂，对植物、小鼠和许多真菌都有毒性，但产生它的真菌尾孢菌属却具有抗性。我们推测，对尾孢菌素的抗性可能源于细胞表面的还原环境。使用20种氧化还原电位不同的四氮唑染料作为7种对尾孢菌素抗性不同的真菌细胞表面氧化还原电位的指示剂。抗性真菌比敏感真菌能显著还原更多的染料。当通过在不同培养基上生长来操纵尾孢菌属的抗性水平时，也观察到染料还原与尾孢菌素抗性之间的相关性。向生长培养基中添加还原剂抗坏血酸、半胱氨酸和还原型谷胱甘肽（GSH）可降低尾孢菌素对敏感真菌的毒性。在保护真菌的浓度下，这些试剂均未直接还原尾孢菌素。对含有不同还原剂的尾孢菌素溶液进行光谱和薄层色谱分析表明，与尾孢菌素发生反应的是GSH，而非半胱氨酸或抗坏血酸。抗性真菌和敏感真菌在内源性半胱氨酸、GSH或总硫醇水平上没有差异。基于本研究及其他研究的数据，本报告提出了一个模型，该模型认为尾孢菌素抗性源于抗性细胞表面产生的还原能力，导致尾孢菌素分子的瞬时还原和解毒。
