Tetcyclacis sensitivity of NO3- uptake, NO3--dependent extracellular ferricyanide reduction and intracellular NO3- reductase in roots, leaves and suspension cells of higher plants

Tetcyclacis, a growth retarding substance, known to interfere with cytochrome P-450-dependent processes, was found to selectively inhibit plant NO3- uptake. The effect was studied with roots of Zea mays, Limnobium stolonifenrm, photosynthesizing leaves and shoots of Egeria densa and with heterotrophically grown suspension cells of Catharanthus roseus. The inhibition was strongest when the NO3--uptake system, but not the NO, reductase (EC 1.6.6.2), was induced by NO2-. Tetcyclacis impaired the nitrate reductase only indirectly by inhibiting NO3- uptake. Phosphate and K+ uptake were not affected either. In addition, tetcyclacis interfered with the ferricyanide-triggered, plasma membrane-located, redox system. This inhibition was most pronounced after preincubation of the roots with NO3-. The results suggest a common intermediate of the high affinity nitrate uptake system and the redox system at the plasma-membrane level of plant cells. Since preincubation of the tissue with tetcyclacis was necessary for inhibition and since C-14-labeled tetcyclacis only slowly accumulated within the tissue with saturation after one hour, tetcyclacis must affect some reaction at the inner side of the plasma membrane and not from outside.