Yoder O C, Scheffer R P
Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48823.
Plant Physiol. 1973 Dec;52(6):513-7. doi: 10.1104/pp.52.6.513.
Susceptible corn tissues exposed to the host-specific toxin of Helminthosporium carbonum race 1 reduced more nitrate to nitrite than did control tissues, as measured by an in vivo method. There were no differences in nitrate reductase activities extracted from treated and control tissues and assayed by an in vitro method. Toxin-treated susceptible roots removed nitrate from solution and accumulated it in the tissues twice as fast as did control roots. Uptake by resistant roots was stimulated also, provided approximately 100 times higher concentrations of toxin were used. Toxin-stimulated nitrate uptake occurred in the presence of tungstate, which eliminates nitrate reductase activity. Toxin did not cause leakage of nitrate from roots under these conditions. Thus, toxin-enhanced nitrate accumulation was caused by increased nitrate uptake rather than by decreased nitrate metabolism or decreased nitrate leakage. The data indicate that toxin increases the rate of nitrate reduction in vivo by increasing the availability of substrate, not by stimulation of enzyme synthesis.
用体内法测定，暴露于玉米长蠕孢菌1号小种宿主特异性毒素的敏感玉米组织比对照组织将更多的硝酸盐还原为亚硝酸盐。用体外法测定，从处理过的组织和对照组织中提取的硝酸还原酶活性没有差异。毒素处理过的敏感根从溶液中去除硝酸盐并在组织中积累的速度是对照根的两倍。抗性根的吸收也受到刺激，前提是使用大约高100倍浓度的毒素。在存在钨酸盐（可消除硝酸还原酶活性）的情况下，毒素刺激了硝酸盐的吸收。在这些条件下，毒素不会导致硝酸盐从根中渗漏。因此，毒素增强的硝酸盐积累是由硝酸盐吸收增加引起的，而不是由硝酸盐代谢减少或硝酸盐渗漏减少引起的。数据表明，毒素通过增加底物可用性而不是通过刺激酶合成来提高体内硝酸盐还原率。
