Regulation of activity and transcript levels of NR in rice (Oryza sativa): Roles of protein kinase and G-proteins

The aim of the present study was to determine the effects of activators and inhibitors of signaling pathways on nitrate-induced NR activity and transcript levels in rice leaves. Rice plants (Oryza sativa indica var. Panvel I) were grown hydroponically (without nutrients) on germination paper under continuous white light for 10–12 days. Leaves, adapted for darkness for 2 days, were treated with signaling agents both in light and dark. The results presented in this work show that okadaic acid (protein phosphatases 1 and 2A inhibitor) and lithium (inhibitor of IP3 pathway) specifically inhibit activity and mRNA levels of NR under light conditions with no effect on NiR activity. Both NR and NiR activities are inhibited and stimulated in the dark by PMA and Bisindolylmaleimide (BIM) (PKC activator and inhibitor), respectively. Cholera toxin specifically enhances NR activity and steady state levels of mRNA in the dark showing the involvement of G-proteins. Calcium has a stimulatory effect on both the enzymes, with an increase in NR mRNA levels albeit to a lesser extent. These results suggest that the activities of nitrate-assimilating enzymes in rice are regulated independently by G-protein and IP3 mediated pathways and co-regulated by PKC and calcium.