Effects of Helminthosporium carbonum Toxin on Nitrate Uptake and Reduction by Corn Tissues

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.