Nitrate Reductase Activity is Endogenously Induced by Zeatin Riboside in Habituated Suspension Cultured Chenopodium rubrum Cells

Abstract

Summary The natural cytokinins IP, IPA, Z, ZR, DHZ and DHZR of suspension cultured photoautotrophic and phytohormone-independent cells of Chenopodium rubrum were examined for their ability to induce nitrate reductase (EC 1.6.6.1) activity in resting cells of the batch culture. Each of the applied cytokinins was readily taken up from the medium and nitrate reductase activity was rapidly enhanced by all of them, ZR being a little more effective than the others. Nitrate reductase activity was also enhanced by adding ammonium to the cell culture in the G o phase. Subsequently, the endogenous level of ZR increased transitorily up to 2.5-fold, peaking about 4 h after addition of ammonium. The levels of the other endogenous cytokinins remained more or les unchanged. Supply of cytokinins to the stationary cell culture did not result in resumption of cell division. Induction of nitrate reductase activity by both ZR and ammonium could be inhibited with cycloheximide, indicating that protein de-novo -synthesis was involved in the enhancement of the enzyme activity. The time-span between the rise of the cytokinin level and the increase of nitrate reductase activity was about 2 h, irrespective of whether induction was triggered with ZR or ammonium nitrate. From these results, ZR was considered to be the endogenous cytokinin species that is involved in the induction of nitrate reductase in Chenopodium rubrum .