Identification of Arabidopsis Mutants Impaired in the Systemic Regulation of Root Nitrate Uptake by the Nitrogen Status of the Plant    

Abstract

Nitrate uptake by the roots is under systemic feedback repression by high nitrogen (N) status of the whole plant. The NRT2.1 gene, which encodes a NO 3 2 transporter involved in high-affinity root uptake, is a major target of this N signaling mechanism. Using transgenic Arabidopsis (Arabidopsis thaliana) plants expressing the pNRT2.1::LUC reporter gene (NL line), we performed a genetic screen to isolate mutants altered in the NRT2.1 response to high N provision. Three hni (for high nitrogen insensitive) mutants belonging to three genetic loci and related to single and recessive mutations were selected. Compared to NL plants, these mutants display reduced down-regulation of both NRT2.1 expression and high-affinity NO 3 2 influx under repressive conditions. Split-root experiments demonstrated that this is associated with an almost complete suppression of systemic repression of pNRT2.1 activity by high N status of the whole plant. Other mechanisms related to N and carbon nutrition regulating NRT2.1 or involved in the control of root SO 4 2 uptake by the plant sulfur status are not or are slightly affected. The hni mutations did not lead to significant changes in total N and NO 3 2 contents of the tissues, indicating that hni mutants are more likely regulatory mutants rather than assimilatory mutants. Nevertheless, hni mutations induce changes in amino acid, organic acid, and sugars pools, suggesting a possible role of these metabolites in the control of NO 3 2 uptake by the plant N status. Altogether, our data indicate that the three hni mutants define a new class of N signaling mutants specifically impaired in the systemic feedback repression of root NO 3 2 uptake.

Glucose Elevates NITRATE TRANSPORTER2.1 Protein Levels and Nitrate Transport Activity Independently of Its HEXOKINASE1-Mediated Stimulation of NITRATE TRANSPORTER2.1 Expression

“…This suggested that blockage of OPPP metabolites destined for amino acid synthesis contributes to reduced NRT2.1 expression and that the provision of some metabolite derived from OPPP destined for amino acid synthesis can recover reduced Glc-responsive NRT2.1 expression in gin2-1. High levels of Gln reduce NR activity and the expression of NR, NiR, and NRT2.1 genes (Miller et al, 2007;Girin et al, 2010), and we showed that exogenous Gln suppressed the restoration of NRT2.1 expression by shikimate in gin2-1 (Fig. 1N).…”

“…Several studies Lejay et al, 2008;Girin et al, 2010) have shown that transcriptional regulation of high-affinity nitrate transporter genes such as NRT2.1 is centrally important for optimizing plant growth in response to soil nitrate levels. In this study, we have explored relationships between carbohydrate-and nitrate-responsive gene expression, metabolism, growth, and transport using the HXK1 mutant gin2-1 that is defective in carbohydrate responses (Moore et al, 2003).…”

“…Seedlings were taken for RNA isolation at the start of the nitrate treatment and after 4 h. NRT2.1 was induced to similar levels in response to nitrate in both Ler and gin2-1 ( Fig. 2A), showing optimal induction at low nitrate levels as reported previously (Lejay et al, 1999;Girin et al, 2010). To test if Glc-and nitrateresponsive NRT2.1 expression occurs through separate pathways, a combined analysis was performed.…”

“…Posttranscriptional mechanisms also influence nitrate uptake; in low-nitrate conditions, NRT1.1 is phosphorylated on its Thr-101 residue by CYSTATHIONINE BETA-LYASE-interacting protein kinase, switching it into a high-affinity transporter (Ho et al, 2009). NRT2.1 expression is repressed under high-N conditions (Lejay et al, 1999), possibly through systemic feedback repression by reduced N metabolites such as Gln (Gansel et al, 2001;Girin et al, 2010). NRT2.1 expression is also inhibited locally by NRT1.1 under high-nitrate conditions (Muños et al, 2004).…”

Analyzing Lateral Root Development: How to Move Forward

“…Experimental studies must therefore expose roots to heterogeneous physical or chemical environments. Split root systems (Remans et al, 2006;Girin et al, 2010;Ruffel et al, 2011) or artificial granular media are examples of systems available to understand the effects of environments on root development. However, ensuring concentrations of nutrients in growth media are within the levels recorded in soil would help to guarantee that root responses observed are physiologically relevant.…”