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Published on December 15, 2006; 10.1104/pp.106.093237

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Received November 16, 2006
Accepted December 6, 2006

Nitrogen-dependent Posttranscriptional Regulation of the Ammonium Transporter AtAMT1;1

Lixing Yuan , Dominique Loqué , Fanghua Ye , Wolf B. Frommer , and Nicolaus von Wirén *

Molecular Plant Nutrition, Institute of Plant Nutrition, University of Hohenheim, D-70593 Stuttgart, Germany; Carnegie Institution, Department of Plant Biology, 260 Panama Street, Stanford, CA 94305, USA

* Corresponding author; email: vonwiren{at}uni-hohenheim.de.

Ammonium transporter 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 AMT1;1 gene was introduced into tobacco. 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 approx. 30% increase in root uptake capacity for 15N-labelled ammonium in hydroponically-grown transgenic plants. When ammonium was supplied as the major nitrogen form but at limiting amounts to soil-grown plants, transgenic lines overexpressing AtAMT1;1 did not show enhanced growth or nitrogen acquisition relative to wild type plants. Surprisingly, steady-state transcript levels of AtAMT1;1 accumulated to higher levels in nitrogen-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 nitrogen-deficient roots suggesting a role for nitrogen 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'-UTR of AtAMT1;1 alone in nitrogen-dependent transcript accumulation was not found. However, transcript levels of AtAMT1;3 did not accumulate in a nitrogen-dependent manner, even though the same T-DNA insertion line amt1;1-1 was used for 35S:AtAMT1;3 expression. These results show that the accumulation of AtAMT1;1 transcripts is regulated in a nitrogen- and organ-dependent manner and suggest mRNA turnover as an additional mechanism for the regulation of AtAMT1;1 in response to the nitrogen nutritional status of plants.




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