Nitrate Signaling, Functions, and Regulation of Root System Architecture: Insights fromArabidopsis thaliana

Root system architecture (RSA) is required for the acquisition of water and mineral nutrients from the soil. One of the essential nutrients, nitrate (NO3-), is sensed and transported by nitrate transportersNRT1.1andNRT2.1in the plants. Nitrate transporter 1.1 (NRT1.1) is a dual-affinity nitrate transporter phosphorylated at the T101 residue by calcineurin B-like interacting protein kinase (CIPKs); it also regulates the expression of other key nitrate assimilatory genes. The differential phosphorylation (phosphorylation and dephosphorylation) strategies and underlying Ca(2+)signaling mechanism ofNRT1.1stimulate lateral root growth by activating the auxin transport activity and Ca2+-ANR1 signaling at the plasma membrane and the endosomes, respectively. NO(3)(-)additionally functions as a signal molecule that forms a signaling system, which consists of a vast array of transcription factors that control root system architecture that either stimulate or inhibit lateral and primary root development in response to localized and high nitrate (NO3-), respectively. This review elucidates the so-far identified nitrate transporters, nitrate sensing, signal transduction, and the key roles of nitrate transporters and its downstream transcriptional regulatory network in the primary and lateral root development inArabidopsis thalianaunder stress conditions.