Mechanisms of Enhanced Rice Growth and Nitrogen Uptake by Nitrate

Rice is being increasingly cultivated in intermittently irrigated regions and also in aerobic soil in which Nitrate （NO3^- ） plays important role in nutrition of plant. However, there is no information regarding the influence of nitrate on the overall growth and uptake of nitrogen （N） in rice plant. Solution culture experiments were carried out to study the effects of NO^3- on the plant growth, uptake of N, and uptake kinetics of NH4^＋ in four typical rice （Oryza sativa L.） cultivars （conventional indica, conventional japonica, hybrid indica, and hybrid japonica）, and on plasma membrane potential in roots of two conventional rice cultivars （indica and japonica） at the seedling stage. The results obtained indicated that a ratio of 50/50 NH4^＋-N/NO3^--N increased the average biomass of rice shoots and roots by 20% when compared with that of 100/0 NH4^＋-N/NO3^--N. In case of the 50/50 ratio, as compared with the 100/0 ratio, total N accumulated in shoots and roots of rice increased on an average by 42% and 57%, respectively. Conventional indica responds to NO3^- more than any other cultivars that were tested. The NO^3- supply increased the maximum uptake rate （Vmax） of NH4^＋ by rice but did not show any effect on the apparent Michaelis-Menten constant （Km） value, with the average value of Vmax for NH4^＋ among the four cultivars being increased by 31.5% in comparison with those in the absence of NO3^-. This suggested that NO3^- significantly increased the numbers of the ammonium transporters. However, the lack of effect on the Km value also suggested that the presence of NO3^- had no effect on the affinity of the transporters for NH4^＋. The plasma membrane potential in the roots of conventional indica and japonica were greatly increased by the addition of NO3^- , suggesting that NO3^- could improve the uptake of N by roots of the rice plant. In conclusion, the mechanisms by which NO3^- enhances the growth and N uptake of rice plant was found by the increased value of Vmax of NH4^＋ and increased plasma membrane potential. Thus promotion of nitrification in paddy soil is of great significance for improving the production of rice.

Phosphorylation regulation of nitrogen,phosphorus,and potassium uptake systems in plants

The uptake of ammonium,nitrate,phosphorus,and potassium ions by roots is mediated by specific ion transporter or channel proteins,and protein phosphorylation regulation events occurring on these proteins and their regulators determine their ultimate activity.Elucidating the mechanism by which protein phosphorylation modification regulates nutrient uptake will advance plant breeding for high nutrientuse efficiency.In this review,it is concluded that the root nutrient absorption system is composed of several,but not all,members of a specific ion transporter or channel family.Under nutrient-starvation conditions,protein phosphorylation-based regulation of these proteins and associated transcription factors increases ion transporter-or channel-mediated nutrient uptake capacity via direct function activity enhancement,allowing more protein trafficking to the plasma membrane,by strengthening the interaction of transporters and channels with partner proteins,by increasing their protein stability,and by transcriptional activation.Under excessive nutrient conditions,protein phosphorylation-based regulation suppresses nutrient uptake by reversing these processes.Strengthening phosphorylation regulation items that increase nutrient absorption and weakening phosphorylation modification items that are not conducive to nutrient absorption show potential as strategies for increasing nutrient use efficiency.