Unlocking the potentials of nitrate transporters at improving plant nitrogen use efficiency

Abstract

Nitrate ( ${\text{NO}}_{3^{-}}$ ) transporters have been identified as the primary targets involved in plant nitrogen (N) uptake, transport, assimilation, and remobilization, all of which are key determinants of nitrogen use efficiency (NUE). However, less attention has been directed toward the influence of plant nutrients and environmental cues on the expression and activities of ${\text{NO}}_{3^{-}}$ transporters. To better understand how these transporters function in improving plant NUE, this review critically examined the roles of ${\text{NO}}_{3^{-}}$ transporters in N uptake, transport, and distribution processes. It also described their influence on crop productivity and NUE, especially when co-expressed with other transcription factors, and discussed these transporters' functional roles in helping plants cope with adverse environmental conditions. We equally established the possible impacts of ${\text{NO}}_{3^{-}}$ transporters on the uptake and utilization efficiency of other plant nutrients while suggesting possible strategic approaches to improving NUE in plants. Understanding the specificity of these determinants is crucial to achieving better N utilization efficiency in crops within a given environment.

Keywords: environmental stress; nitrate remobilization; nitrate transport and signaling; nitrate transporters; nitrate uptake; nitrogen use efficiency.

Figure 1

Key nitrate transporters involved in nitrate uptake, transport, and remobilization in plants. Nitrate transporters involved in ${\text{NO}}_{3^{-}}$ acquisition from the root include NRT2.1, NRT2.2, NPF4.6 (NRT1.2), NRT2.4, NRT2.5, and NPF6.3 (NRT1.1). NPF2.7 performs the ${\text{NO}}_{3^{-}}$ efflux function. In addition to the uptake function, NRT2.4 and NRT2.5 facilitates root-to-shoot ${\text{NO}}_{3^{-}}$ transport. NRT1.5 is responsible for xylem loading, while NRT1.8 and NRT1.9 functions to unload ${\text{NO}}_{3^{-}}$ from the xylem. NRT1.4 regulates ${\text{NO}}_{3^{-}}$ homeostasis, and the expression of NRT1.7 in the phloem of the minor vein promotes nitrate remobilization from mature to younger leaves. At shoot, NRT1.6 and NPF5.5 act as a ${\text{NO}}_{3^{-}}$ remobilizer, remobilizing ${\text{NO}}_{3^{-}}$ in the embryo. NRT2.7 enhances ${\text{NO}}_{3^{-}}$ storage in the seed vacuole.

Figure 2

Roles of nitrate transporters in plant response to adverse environmental conditions. Environmental cues including heavy metals (Cd²⁺ and Zn), salinity, drought, and pathogenic stress engender reduction in plant growth and NUE. The resulting stressed plants accumulate more ${\text{NO}}_{3^{-}}$ at the root (A) while retaining less in the shoot (B). Under Cd²⁺ or Zn stress, nitrate transporters, NRT1.1, NRT1.5 and NRT1.8 concurrently regulates Cd²⁺ or Zn uptake and ${\text{NO}}_{3^{-}}$ allocation to the root (C). The transporters involved in root-to-shoot allocation of ${\text{NO}}_{3^{-}}$ under salinity include NPF2.3, NRT1.1, NRT1.5, and NRT1.8 (D). NRT2.1 promotes plants’ tolerance to drought stress (E). In addition to NRT2.1, NRT2.2, NRT2.5 and NRT2.6 are involved in biotic stress regulation (F).