Nitrogen is a limiting macronutrient for plant growth and development [70,71]. Soil nitrogen availability is normally low and can be influenced by factors such as precipitation, temperature, pH and soil type. The form of nitrogen preferred by plants is intrinsically related to their adaptation and soil conditions [72]. Plants can use both nitrate (NO₃⁻) and ammonium (NH₄⁺) as sources of N [8,73,74]. NO₃⁻ is the main and most readily available source of nitrogen for most higher plants [75,76], however, NO₃⁻ concentrations in soils can be very varied, depending on environmental variations and, therefore, plants have developed several specific adaptations for the uptake of available NO₃⁻ [72,77].

Nitrate is actively transported into cells, mainly by the NO₃⁻ transcarriers of the NRT family, which depend on the energy supply and electrochemical gradient and are divided into two systems existing in the cell membranes of the roots [77,78,79,80]. One of them is the high affinity transport system (HATS), which is activated when there is a high concentration of nitrate available to the plant. The other, the low affinity transport system (LATS), is activated under conditions of low nitrate concentration [71,72,78,81].

NRT transporters belong to three main families: the first, NRT1—or NPF—contains many genes, which can be divided into 8 to 10 subfamilies in Arabidopsis, which are predominantly low-affinity transporters [71,78,80,82]. The other families—NRT2/NRT3 (NAR2)—play an important role in the high-affinity transport of NO₃⁻ [72,80]. As in Arabidopsis, NRTs have been identified in rice, sorghum and maize and show differences in gene numbers and family structure. Studies show that in the corn genome there are four copies of genes of the ZmNRT2 family, namely: ZmNRT2.1, ZmNRT2.2, ZmNRT2.3 and ZmNRT2.5, of which only two were studied: ZmNRT2.1 and ZmNRT2.2, that have about 98% homology in their amino acid sequence [81]. Both are inducible to NO₃⁻ in seedling roots, having been found transcripts of the former in the region of the root cortex while, for the latter, in the cortex, stele, and lateral primordia of the roots [83].

For the NRT3 family, two copies of the NRT3.1 gene were described in the maize genome: ZmNRT3.1A and ZmNRT3.1B, both never characterized in maize, but which are strong candidates in the NO₃⁻ transport complex [72,84,85] More recently, Wang et al. [86], after analyzing the proteome of two contrasting hybrids for the efficiency in the use of nitrogen, verified the function of the ZmTGA gene and the study found that the gene has an important role in maintaining the tolerance of plants in low conditions. N, giving greater length and area of the root system, greater shoot/root ratio, in addition to lower leaf senescence compared when comparing the mutant with the wild type. TGA transcription factors are a very important group of the bZIP (basic leucine zipper) family [87] and can bind to the -1 (as-1) activation sequence with TGACG as the core and activate or inhibit the translation of downstream target genes, having, therefore, an important role in the defense and response of plants to various biotic and abiotic stresses, such as low nitrogen availability. Furthermore, studies show that plants with overexpression of the TGA1 or TGA4 genes show a significant increase in the nitrate transporter genes NRT2.1 and NRT2.2 [88,89], whose functions we discussed earlier.

Ammonium is also a direct source of nitrogen taken up by plant roots, but, in general, the ammonium content in unfertilized soils is up to 1000 times lower than that of nitrate. [90]. However, efficient ammonium uptake is critical for optimal plant growth and development, as it confers several beneficial effects, such as root density and corn seedling length [91,92] or enhanced H+ proton extrusion, which subsequently acidifies the rhizosphere and leads to increased bioavailability of poorly soluble nutrients such as P or Fe [93]. Therefore, this NH₄⁺ absorption process is expected to be highly regulated under adverse conditions of N availability in the soil.

Whenever the ammonium concentration in the soil solution is low, the contribution of high-affinity transport systems (HATSs) becomes more relevant to the overall ammonium uptake by roots [93]. In general, high-affinity ammonium transport is mediated by AMT1-type ammonium transporters: ZmAMT1; 1a and ZmAMT1; 3, which belong to the ammonium/methylammonium permease/rhesus transporters (AMT/MEP/Rh) [93,94].

The two ZmAMTs confer high-affinity ammonium transport activities and are localized in the plasma membrane of maize root epidermal cells. Furthermore, their gene expressions are induced by ammonium, and one study revealed high correlations with high-affinity ammonium and increased root influx rates [93]. Although ZmAMT1; 1a e ZmAMT1; 3 are likely to be the main components for ammonium uptake in the root, not much is known about their physiological contribution to N uptake and use efficiency [94].

Once incorporated, nitrate is reduced to nitrite in plant cells in a reaction that takes place in the cytosol and is catalyzed by a nitrate reductase (NR) [95,96]. The nitrite is then translocated to plastids and chloroplasts, where it is reduced to ammonium by the enzyme nitrite reductase (NiR). Nitrate-derived ammonium, or that produced by photorespiration or amino acid recycling, is more assimilated in plastids by the GS/GOGAT cycle [97,98].

Ammonium is attached to glutamate to form glutamine by glutamine synthase (GS; family of Gln genes), of which there is a plastid isoform (GS2) and a cytosolic isoform (GS1). In maize, a single gene encodes GS2 (Gln2), while at least 5 genes encode GS1 (Gln1-1 to Gln1-5), which are differentially expressed during development [77,99,100,101].

Glutamate can serve as an amino acid donor to other amino acids and nitrogen-requiring compounds, or act as an amine acceptor in the GS-GOGAT cycle to regenerate glutamine. Plants have two types of GOGAT enzymes—NADH-GOGAT and Fd-GOGAT—which use NADH and ferredoxin as electron donors, respectively [102]. Different parallels of GOGAT show constitutive or tissue-specific activity in plants, including maize [99]. Ferridoxine-GOGAT is localized in leaf chloroplasts, while NADH-GOGAT is expressed in non-photosynthetic tissues, including root plastids [97]. After nitrogen assimilation, glutamine, glutamate and other amino acids, including asparagine and aspartate, are transported by vascular tissues to growing organs [97]. Nitrate and ammonium can also be stored in vacuoles before or after long-distance transport.

During senescence, leaf proteins and photosynthetic proteins are extensively degraded, becoming a huge source of nitrogen, which plants can exploit to supplement the nutrition of growing organs [97].

During the grain filling period, the absorption and assimilation of nitrogen are often insufficient for the high demand required at this stage, making re-mobilization in the different organs of the plant necessary to direct nitrogen to the seeds [98]. The contribution of this process to supplying N in cereals such as rice, wheat and corn varies according to the cultivar, at rates of 50 to 90% [97]. N remobilization also depends on the environment and is favored under conditions of nitrate limitation [103].

In this sense, glutamine (Gln) is the main amino acid translocated in cereals as a source of N. Therefore, in senescence, glutamine concentrations increase in the phloem sap, being remobilized to the reproductive organs. In this process, GS and GOGAT enzymes are important for the remobilization and reuse of N in senescent and developing organs, respectively [80,104]. Some studies have shown that GS1-1 is responsible for this process and that NADH-GOGAT1 plays a key role in the reuse of Gln transported in the developing organs of rice [104,105]. In maize, wheat, and barley, grain N content is correlated with senescence of flag leaves and appears to play a significant role in N availability for grain filling [72,100,105].