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Low temperature leads to the increase of the activities of glutamine synthetase, isocitrate dehydrogenase, and the level of proline in rice (Oryza sativa L.) roots

Binbin Lu;Yongze Yuan;Chufu Zhang

  Many factors, including exogenous carbon motabolites influence the expression of enzymes involved in ammonium assimilation, but little is known on nitrogen metabolism under low temperature stress. To investigate the effects of low temperature stress on nitrogen metabolism, the rice (Oryza sative L.) seedlings grown at the optimal temperature (30℃) were subjected to low temperature (20℃), and expression of several key enzymes involved in ammonium assimilation, including Glutamine syntbetase (GS; EC 6.3.1.2), NADP-dependent isocitrate dehydrogenase (NADP-ICDH; EC 1.1.1.42), and NAD(H)-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2) from rice roots was assessed. It was found that with the induction of low temperature, GS and ICDH activities were increased by 80% and 75%, respectively. A positively parallel correlation was observed between the two key enzymes. Native-PAGE analysis, together with activity staining and western blot assays, showed that both GSrb activity and GSrb protein level were enhanced under low temperature. On the other hand, NADH-GDH and NAD-GDH activity were both reduced under the same conditions, though to different extent. Several physiological parameters, including biomass, NH_4~+, pH, and proline, were measured as well. At low temperature, ammonium absorption was stimulated by the elevation of GS activity. Proline content was positively linked with the increase of GS and ICDH: low temperature led to the marked accumulation of proline in rice roots (about 1.2-fold induction). The key enzymes in the rice roots with sucrose feeding showed the same behavior as those under low temperature, indicating that at least partially low temperature has the similar effect as carbon compound on the modulation of nitrogen metabolism. Our results also suggest that 2- oxoglutarate (OG), cooperating with ammonium and its derived amino acids, can regulate the co-ordination of C and N metabolism. This study represents an important contribution toward a better understanding of the effect of low temperature on ammonium assimilation in rice plants and the regulation of this metabolic pathway by the availability of 2-OG.……