Journal of Integrative Plant Biology 2007, 49 (12): 1719–1725 Regulation of the High-Affinity Nitrate Transport System in Wheat Roots by Exogenous Abscisic Acid and Glutamine Chao Cai 1,2,3 , Xue-Qiang Zhao 1 , Yong-Guan Zhu 2 , Bin Li 1 , Yi-Ping Tong 1 ∗ and Zhen-Sheng Li 1 ( 1 The State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, the Chinese Academy of Sciences, Beijing 100101, China; 2 Research Center for Eco-environmental Sciences, the Chinese Academy of Sciences, Beijing 100085, China; 3 Graduate University of the Chinese Academy of Sciences, Beijing 100049, China) Abstract Nitrate is a major nitrogen (N) source for most crops. Nitrate uptake by root cells is a key step of nitrogen metabolism and has been widely studied at the physiological and molecular levels. Understanding how nitrate uptake is regulated will help us engineer crops with improved nitrate uptake efficiency. The present study investigated the regulation of the high-affinity nitrate transport system (HATS) by exogenous abscisic acid (ABA) and glutamine (Gln) in wheat (Triticum aestivum L.) roots. Wheat seedlings grown in nutrient solution containing 2 mmol/L nitrate as the only nitrogen source for 2 weeks were deprived of N for 4 d and were then transferred to nutrient solution containing 50 µmol/L ABA, and 1 mmol/L Gln in the presence or absence of 2 mmol/L nitrate for 0, 0.5, 1, 2, 4, and 8 h. Treated wheat plants were then divided into two groups. One group of plants was used to investigate the mRNA levels of the HATS components NRT2 and NAR2 genes in roots through semi-quantitative RT-PCR approach, and the other set of plants were used to measure high-affinity nitrate influx rates in a nutrient solution containing 0.2 mmol/L 15 N-labeled nitrate. The results showed that exogenous ABA induced the expression of the TaNRT2.1, TaNRT2.2, TaNRT2.3, TaNAR2.1, and TaNAR2.2 genes in roots when nitrate was not present in the nutrient solution, but did not further enhance the induction of these genes by nitrate. Glutamine, which has been shown to inhibit the expression of NRT2 genes when nitrate is present in the growth media, did not inhibit this induction. When Gln was supplied to a nitrate-free nutrient solution, the expression of these five genes in roots was induced. These results imply that the inhibition by Gln of NRT2 expression occurs only when nitrate is present in the growth media. Although exogenous ABA and Gln induced HATS genes in the roots of wheat, they did not induce nitrate influx. Key words: abscisic acid; glutamine; high-affinity nitrate transport system; nitrate influx; Triticum aestivum. Cai C, Zhao XQ, Zhu YG, Li B, Tong YP, Li ZS (2007). Regulation of the high-affinity nitrate transport system in wheat roots by exogenous abscisic acid and glutamine. J. Integr. Plant Biol. 49(12), 1719–1725. Available online at www.jipb.net For most cultivated crops, nitrate is a major source of nitrogen (N). Nitrate uptake by root cells is a key step of nitrogen metabolism and has been widely studied at the physiological Received 16 Sept. 2006 Accepted 4 Nov. 2006 Supported by the National Natural Science Foundation of China (30390083 and 30521001) and the State Key Basic Research and Development Plan of China (2005CB120904 and 2004CB117200). ∗ Author for correspondence. Tel(Fax): +86 (0)10 6487 0483; E-mail: <yptong@genetics.ac.cn>. C  2007 Institute of Botany, the Chinese Academy of Sciences doi: 10.1111/j.1744-7909.2007.00485.x and molecular levels (Crawford and Forde 2002). Many phys- iological studies on the plant nitrate transport system suggest that at least two uptake systems are responsible for the influx of nitrate into plant roots (for reviews, see Crawford and Glass 1998; Daniel-Vedele et al. 1998; Forde 2000; Crawford and Forde 2002; Glass 2003), namely a low-affinity transport sys- tem (LATS) and a high-affinity transport system (HATS). Two classes of genes, NRT1 and NRT2, have been found to be potentially involved in LATS and HATS, respectively (Crawford and Glass 1998; Forde 2000; Orsel et al. 2002). Recent reports have demonstrated that another gene, namely NAR2 (or NRT3 as it is called in Arabidopsis), is involved in the HATS and that some of the NRT2 proteins require a second protein, specifically NAR2, to enable functional nitrate uptake in algae