Nitrogen (N) is the second most important crop input factor after water and is an essential macroelement. It is also an important component of most biomacromolecules and many secondary and signaling compounds in plants, such as proteins, nucleic acids, cell wall components, phytohormones, and vitamins [1, 2]. Therefore nitrogen deficiency could severely limit plant growth and development. This particularly true for wheat. Wheat plants do not establish symbiotic associations with N2 fixing microbes [3]. Therefore, chemical fertilizers have historically been used to maintain or increase crop yields. However, these chemical fertilizers have been mismanaged, resulting in environmental pollution and decreased nutrient-use efficiency (NUE) [4]. For, example, only one third of the applied nitrogen is utilized by wheat, which suggests that there is scope for increasing its NUE [5]. The remaining N is released into the environment through leaching and volatilization [6]. This means that the low wheat NUE and excess N fertilizer applications are aggravating environmental pollution and causing ecological deterioration [7, 8]. Therefore, improving the NUE will improve the sustainability of wheat production. However, achieving greater NUE is challenged by the complexity of the trait, which comprises of processes associated with nitrogen uptake, transport, reduction, assimilation, translocation, and remobilization.
Nitrogen is available to plant roots in several different forms, such as NO3−, NH4+, and organic molecules, such as amino acids [9]. Nitrate is one of the most important N sources for plants. Nitrogen uptake is the first step in nitrate assimilation and can be manipulated to enhance NUE. Plants have evolved regulated, energy dependent systems for the uptake of NO3− that use both high and low affinity transporters. The Nitrate Transporter 1 (NRT1)/Peptide Transporter (PTR) family (NPF), NRT2 family, Chloride Channel (CLC) family, and Slow Anion Channel (SLAC) protein family are the four protein families that play key roles in NO3− transport [10, 11]. The NRT1 and NRT2 families have been identified as being involved in low-affinity nitrate transporter systems (LATSs) and high-affinity nitrate transporters systems (HATSs), respectively. The LATS is activated when nitrate concentrations are high (> 1 mM), whereas the HATS is activated when nitrate concentrations are low (< 1 mM) [12, 13]. The NRT2s, which are thought to be involved in the major transporter system responsible for nitrate uptake in plants, are membrane associated proteins and contribute specifically to the nitrate-inducible step.
The first NRT2 family transporters were discovered in a chlorate-resistant mutant (crnA) of Aspergillus nidulans [14, 15]. Subsequently, numerous studies have investigated the functional roles of the plant NRT2 family and important progress has been made. It has been reported that there are 7 NRT2 genes in Arabidopsis [10, 16], 4 in rice [17], 4 in maize [18], 31 in Brassica napus [19, 20], 13 in poplar [21], 4 in tomato [22], and 5 in wild soybean (Glycine soja) [23]. In Arabidopsis, four AtNRT2 transporters (AtNRT2.1, AtNRT2.2, AtNRT2.3, and AtNRT2.4) are involved in nitrate uptake. The AtNRT2.1 and AtNRT2.2 genes play key roles in the regulation of high-affinity NO3– uptake and nrt2.1nrt2.2 reduces the inducible high-affinity transport system (IHATS) by up to 80% in A. thaliana [24, 25]. AtNRT2.4 has a role in both the roots and shoots under N starvation [26] and AtNRT2.5 is the most abundant transcript in adult plants among the seven AtNRT2 family members after long-term nitrogen starvation [27]. Furthermore, AtNRT2.7 is specifically highly expressed in reproductive organs, reaches a maximum in dry seeds, and AtNRT2.7 is the only NRT2 transporter located in the tonoplast [28].
In crops, the homologs of AtNRT2s have been shown to perform numerous roles in N uptake, transport, and utilization processes across all the developmental stages. In rice, OsNRT2.1 and OsNRT2.2 share the same coding sequences (CDSs) with different 5′- and 3′-untranscription regions (UTRs) and have high similarities with maize ZmNRT2 genes, while OsNRT2.3 is more closely related to AtNRT2.5, and OsNRT2.4 is more closely related to AtNRT2.7 [17]. OsNRT2.3 mRNA has been previously spliced into OsNRT2.3a and OsNRT2.3b [29], OsNRT2.3a plays a key role in long-distance nitrate transport from the root to the shoot at low nitrate supply levels [30], OsNRT2.3b plays a critical role in sensing the cytosolic pH of phloem cells, and increased OsNRT2.3b expression improves grain yield and NUE [31]. OsNRT2.4 has been shown to be a dualaffinity nitrate transporter and is required for nitrate-regulated root and shoot growth [32]. In wheat, TaNRT2.5 is expressed in the root, leaf, embryo, and shell, and can increase seed vigor, grain nitrate accumulation, and yield [33]. In maize, only ZmNRT2.1 has been reported to play a role in nitrate uptake along the root axis [34]. In summary, NRT2 homologs play key roles in nitrate uptake and utilization in plants.
Wheat (Triticum aestivum L.) is one of the three main cereal crops across the globe. The ability to uptake N is heavily dependent on the functional efficiency of the nitrate transporter, which is genetically determined in many crops. However, TaNRT2 family members have not been systematically identified and their expressions also have not been analyzed under nitrate deficiency conditions in wheat. This is due to the complexity of its genome. In this study, a genome-wide identification of TaNRT2 members in wheat was performed. The gene structures, chromosomal locations, cis-elements, and conserved motifs of all TaNRT2s were also analyzed. Furthermore, a transcriptome analysis of all TaNRT2s was conducted under nitrate starvation conditions. This study reveals the characteristics of NRT2 genes in wheat and provides valuable information and candidate gene resources for future functional analyses that could be used to genetically improve NUE in wheat.