Krouk Gabriel, Tillard Pascal, Gojon Alain
Biochimie et Physiologie Moléculaire des Plantes, Unité Mixte de Recherche 5004, Agro-M, Centre National de la Recherche Scientifique, Institut National de la Recherche Agronomique, Montpellier, France.
Plant Physiol. 2006 Nov;142(3):1075-86. doi: 10.1104/pp.106.087510. Epub 2006 Sep 22.
The NRT2.1 gene of Arabidopsis thaliana encodes a major component of the root high-affinity NO(3)(-) transport system (HATS) that plays a crucial role in NO(3)(-) uptake by the plant. Although NRT2.1 was known to be induced by NO(3)(-) and feedback repressed by reduced nitrogen (N) metabolites, NRT2.1 is surprisingly up-regulated when NO(3)(-) concentration decreases to a low level (<0.5 mm) in media containing a high concentration of NH(4)(+) or Gln (>or=1 mm). The NRT3.1 gene, encoding another key component of the HATS, displays the same response pattern. This revealed that both NRT2.1 and NRT3.1 are coordinately down-regulated by high external NO(3)(-) availability through a mechanism independent from that involving N metabolites. We show here that repression of both genes by high NO(3)(-) is specifically mediated by the NRT1.1 NO(3)(-) transporter. This mechanism warrants that either NRT1.1 or NRT2.1 is active in taking up NO(3)(-) in the presence of a reduced N source. Under low NO(3)(-)/high NH(4)(+) provision, NRT1.1-mediated repression of NRT2.1/NRT3.1 is relieved, which allows reactivation of the HATS. Analysis of atnrt2.1 mutants showed that this constitutes a crucial adaptive response against NH(4)(+) toxicity because NO(3)(-) taken up by the HATS in this situation prevents the detrimental effects of pure NH(4)(+) nutrition. It is thus hypothesized that NRT1.1-mediated regulation of NRT2.1/NRT3.1 is a mechanism aiming to satisfy a specific NO(3)(-) demand of the plant in relation to the various specific roles that NO(3)(-) plays, in addition to being a N source. A new model is proposed for regulation of the HATS, involving both feedback repression by N metabolites and NRT1.1-mediated repression by high NO(3)(-).
拟南芥的NRT2.1基因编码根部高亲和力硝酸盐转运系统（HATS）的一个主要成分，该系统在植物吸收硝酸盐过程中起关键作用。尽管已知NRT2.1受硝酸盐诱导并被还原态氮（N）代谢产物反馈抑制，但在含有高浓度铵（NH4+）或谷氨酰胺（Gln）（≥1 mM）的培养基中，当硝酸盐浓度降至低水平（<0.5 mM）时，NRT2.1出人意料地上调。编码HATS另一个关键成分的NRT3.1基因表现出相同的响应模式。这表明NRT2.1和NRT3.1都通过一种独立于涉及N代谢产物的机制，被高浓度外部硝酸盐供应协同下调。我们在此表明，高硝酸盐对这两个基因的抑制是由NRT1.1硝酸盐转运体特异性介导的。这种机制保证了在存在还原态氮源的情况下，NRT1.1或NRT2.1在吸收硝酸盐方面是活跃的。在低硝酸盐/高铵供应条件下，NRT1.1介导的对NRT2.1/NRT3.1的抑制被解除，这使得HATS重新激活。对atnrt2.1突变体的分析表明，这构成了对铵毒性的关键适应性反应，因为在这种情况下HATS吸收的硝酸盐可防止纯铵营养的有害影响。因此推测，NRT1.1介导的对NRT2.1/NRT3.1的调控是一种机制，旨在满足植物对硝酸盐的特定需求，这与硝酸盐除作为氮源外所起的各种特定作用有关。提出了一种新的HATS调控模型，涉及N代谢产物的反馈抑制和高硝酸盐介导的NRT1.1抑制。
