Nitrate is both an important nutrient and a signalling molecule for plants. Although several components of the nitrate signalling pathway have been identified, their hierarchical organization remains unclear. Here we show that the localization of NLP7, a member of the RWP-RK transcription factor family, is regulated by nitrate via a nuclear retention mechanism. Genome-wide analyses revealed that NLP7 binds and modulates a majority of known nitrate signalling and assimilation genes. Our findings indicate that plants, like fungi and mammals, rely on similar nuclear retention mechanisms to instantaneously respond to the availability of key nutrients.

Plants have evolved a variety of mechanisms to adapt to N starvation. NITRATE TRANSPORTER2.4 (NRT2.4) is one of seven NRT2 family genes in Arabidopsis thaliana, and NRT2.4 expression is induced under N starvation. Green fluorescent protein and b-glucuronidase reporter analyses revealed that NRT2.4 is a plasma membrane transporter expressed in the epidermis of lateral roots and in or close to the shoot phloem. The spatiotemporal expression pattern of NRT2.4 in roots is complementary with that of the major high-affinity nitrate transporter NTR2.1. Functional analysis in Xenopus laevis oocytes and in planta showed that NRT2.4 is a nitrate transporter functioning in the high-affinity range. In N-starved nrt2.4 mutants, nitrate uptake under low external supply and nitrate content in shoot phloem exudates was decreased. In the absence of NRT2.1 and NRT2.2, loss of function of NRT2.4 (triple mutants) has an impact on biomass production under low nitrate supply. Together, our results demonstrate that NRT2.4 is a nitrate transporter that has a role in both roots and shoots under N starvation. INTRODUCTIONNitrate (NO 3 -) uptake from the soil and distribution through the plant can profoundly affect plant growth and productivity. Nitrogen (N) limitation decreases crop yield worldwide. To meet expanding food demands, the global use of N fertilizer in agricultural production is projected to increase threefold to reach 249 million tons annually by the year 2050 (Tilman et al., 2001). However, the recovery of N fertilizer by crops is low, with in some cases only 30 to 50% of the applied N being taken up by the crop (Peoples et al., 1995;Sylvester-Bradley and Kindred, 2009). The remainder is partly used by subsequent crops but can also be lost from the agro-ecosystem, and fertilizer runoff into aquatic systems leads to environmentally harmful eutrophication (Tilman, 1998). Therefore, improving N uptake efficiency is important to reduce the costs of crop production and pollution damage. Beside N uptake, N remobilization is another key step to improve N use efficiency in crops (Mickelson et al., 2003;Masclaux-Daubresse et al., 2008).Plants have evolved versatile mechanisms to cope with N limitation and N starvation, and besides major adaptive changes of the root system architecture (Drew and Saker, 1975), root NO 3 -uptake characteristics are regulated in response to N availability (Clarkson et al., 1986;Lejay et al., 1999;Glass, 2003). Physiological studies have led to the conclusion that at least three NO 3 -uptake systems are responsible for the influx of NO 3 -into roots (reviewed in Crawford and Glass, 1998;Daniel-Vedele et al., 1998;Forde, 2000). Two high-affinity transport systems (HATS) operate to take up NO 3 -at low concentrations in the external medium, and both display saturable kinetics as a function of the external NO 3 -concentration, with saturation in the range of 0.2 to 0.5 mM. The first one, constitutive HATS, is active in plants that have not been supplied with NO 3 -, whereas the second HATS is induced by NO ...

Nitrate signaling improves plant growth under limited nitrate availability and, hence, optimal resource use for crop production. Whereas several transcriptional regulators of nitrate signaling have been identified, including the Arabidopsis thaliana transcription factor NIN-LIKE PROTEIN7 (NLP7), additional regulators are expected to fine-tune this pivotal physiological response. Here, we characterized Arabidopsis NLP2 as a top-tier transcriptional regulator of the early nitrate response gene regulatory network. NLP2 interacts with NLP7 in vivo and shares key molecular features such as nitrate-dependent nuclear localization, DNA binding motif, and some target genes with NLP7. Genetic, genomic, and metabolic approaches revealed a specific role for NLP2 in the nitrate-dependent regulation of carbon and energy-related processes that likely influence plant growth under distinct nitrogen environments. Our findings highlight the complementarity and specificity of NLP2 and NLP7 in orchestrating a multi-tiered nitrate regulatory network that links nitrate assimilation with carbon and energy metabolism for efficient nitrogen use and biomass production.