Nitrogen-dependent posttranscriptional regulation of the ammonium transporter AtAMT1;1

Molecular Plant Nutrition, Institute of Plant Nutrition, University of Hohenheim, D-70593 Stuttgart, Germany.
Plant physiology (Impact Factor: 6.84). 03/2007; 143(2):732-44. DOI: 10.1104/pp.106.093237
Source: PubMed


Ammonium transporter (AMT) proteins of the AMT family mediate the transport of ammonium across plasma membranes. To investigate whether AMTs are regulated at the posttranscriptional level, a gene construct consisting of the cauliflower mosaic virus 35S promoter driving the Arabidopsis (Arabidopsis thaliana) AMT1;1 gene was introduced into tobacco (Nicotiana tabacum). Ectopic expression of AtAMT1;1 in transgenic tobacco lines led to high transcript levels and protein levels at the plasma membrane and translated into an approximately 30% increase in root uptake capacity for 15N-labeled ammonium in hydroponically grown transgenic plants. When ammonium was supplied as the major nitrogen (N) form but at limiting amounts to soil-grown plants, transgenic lines overexpressing AtAMT1;1 did not show enhanced growth or N acquisition relative to wild-type plants. Surprisingly, steady-state transcript levels of AtAMT1;1 accumulated to higher levels in N-deficient roots and shoots of transgenic tobacco plants in spite of expression being controlled by the constitutive 35S promoter. Moreover, steady-state transcript levels were decreased after addition of ammonium or nitrate in N-deficient roots, suggesting a role for N availability in regulating AtAMT1;1 transcript abundance. Nitrogen deficiency-dependent accumulation of AtAMT1;1 mRNA was also observed in 35S:AtAMT1;1-transformed Arabidopsis shoots but not in roots. Evidence for a regulatory role of the 3'-untranslated region of AtAMT1;1 alone in N-dependent transcript accumulation was not found. However, transcript levels of AtAMT1;3 did not accumulate in a N-dependent manner, even though the same T-DNA insertion line atamt1;1-1 was used for 35S:AtAMT1;3 expression. These results show that the accumulation of AtAMT1;1 transcripts is regulated in a N- and organ-dependent manner and suggest mRNA turnover as an additional mechanism for the regulation of AtAMT1;1 in response to the N nutritional status of plants.

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    • "Rice (Oryza sativa L.) is a model monocot and the staple food of approximately half of the world's population . The N uptake and assimilation pathways in rice are well documented (Wang et al. 1993; Kronzucker et al. 2000; Von Wiren et al. 2000; Suenaga et al. 2003; Tabuchi et al. 2007; Yuan et al. 2007a). "
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    ABSTRACT: Phomopsis liquidambari can establish a mutualistic symbiotic relationship with rice. It promotes the growth and yield of the host plant and reduces the amount of nitrogen (N) fertilizer required for plant growth. However, the mechanisms responsible for the effects of the fungal endophyte on N use in rice are largely unknown. We conducted a hydroponic experiment to investigate the effects of P. liquidambari on N uptake and N metabolism in rice plants. Rice plants were cultivated in the presence or absence of P. liquidambari under three N levels. Under the low-N treatment, fungal infection significantly increased the biomass, and the total N, soluble protein, free amino acid, free NH4+, and chlorophyll contents of rice roots and shoots. The activities of nitrate reductase and glutamine synthetase were increased in infected rice plants. Some genes related to N uptake (OsAMT1;1, OsAMT1;3, OsAMT2;2, OsAMT3;2, OsAMT3;3, OsNRT2;1) and N metabolism (OsNR1, OsGS1, OsGS2, OsNADH-GOGAT) were also up-regulated in infected plants under the low-N treatment. However, these effects gradually weakened as the N level increased. The colonization rate of the endophyte substantially decreased with increasing N levels. Taken together, these results suggest that low-N fertilization induces a physiological state in rice that is favorable for the P. liquidambari symbiosis. The greater extent of P. liquidambari colonization under low-N conditions stimulated the expression of several genes involved in N uptake and N metabolism in rice, thereby enhancing N utilization. These results have implications for enhancing plant growth in low-input systems at nutrient-poor sites.
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    • "Hence, the greater NH 4 + uptake in transgenic lines is most probably due to higher NH 4 + transport through the OsAMT1;1 protein in the plasma membrane. This is in agreement with the study of Yuan et al. (2006) in which AtAMT1;1-transformed tobacco plants had 30% higher NH 4 + influx into roots compared with the WT. However, these authors determined the NH 4 + uptake per unit root dry weight rather than per unit surface area. "
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    • "Lowaffinity transport of NH 4 + , which dominates at NH 4 + concentrations above 0.5–1 mol m −3 , has been linked to K + transporters and non-selective cation channels (ten Hoopen et al., 2010). NH 4 + uptake is regulated by mechanisms that act at the transcriptional and posttranscriptional levels (Rawat et al., 1999; Yuan et al., 2007; Lanquar et al., 2009; Rogato et al., 2010a,b). NH 4 + taken up by roots is assimilated into amino acids via the glutamine synthetase (GS)/glutamate synthase (GOGAT) pathway (Fig. 1; Lea & Miflin, 2011; Hirel et al., 2011). "
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