Molecular Targets for Improvement of Crop Nitrogen Use Efficiency: Current and Emerging Options

Unused fertilizers are the single largest source of nitrous oxide greenhouse gas emissions globally, apart from causing N-pollution in ground and surface waters and eutrophication. While short-term improvements in N-use efficiency (NUE) can be made by agronomic practices, long-term crop improvement is only possible through biological interventions. The lack of clearly defined phenotype and genotype has delayed this process till recently, but the advent of omics and reverse genetics is opening up new avenues to improve crop NUE. It is becoming increasingly evident that several genes and pathways contribute to NUE. Many N-responsive regulators of root development, sensing and signaling, transportation, utilization and remobilization have been targeted to improve NUE, such as C-terminally encoded peptides (CEPs), CLAVATA3/endosperm surrounding region-related peptides (CLE), MADS-box transcription factors, and NAC transcription factors. The nitrate transporter of NRT1.1 is proposed to be a sensor or transceptor, which regulates the crop yield by acting as a component in the Ca2+-mediated signaling cascade. Several other signaling pathways involving target of rapamycin (TOR) complex, general amino acid control non-derepressible 2 (GCN2), ionotropic glutamate-like receptor (iGLR), and PII proteins have been found to play a important roles in maintaining proper N balance in plants. In cereals, cytosolic glutamine synthase, glutamate synthetase, and alanine/aspartate aminotransferases are important targets, as they are involved in remobilization of N from senescing leaves during grain filling and in maintaining proper C/N balance. Several post-transcriptional regulators such as non-coding small RNAs and post-translational regulators such as kinases and phosphatases regulate the expression level of genes involved in N-response/NUE and are emerging as novel targets.