Molecular Soil Biology 2025, Vol.16, No.4, 162-174 http://bioscipublisher.com/index.php/msb 165 nitrogen uptake and assimilation genes (Alfatih et al., 2020) (Figure 2). Similarly, OsNLP3 modulates NUE and grain yield under nitrate-sufficient conditions by orchestrating the expression of nitrogen uptake and assimilation genes (Zhang et al., 2022). These findings underscore the complex regulatory networks involving transporters, enzymes, and transcription factors that govern NUE in rice. Figure 2 OsNLP1 improves grain yield in the field under different nitrogen levels (Adopted from Alfatih et al., 2020) Image caption: (A) Growth status of a representative WT (ZH11), osnlp1-2 mutant, and OE16 plant grown in the field in low nitrogen (LN), normal nitrogen (NN), and high nitrogen (HN) conditions. The plants were dug out from the field and potted for photography. Scale bar: 10 cm. (B) A representative panicle of WT (ZH11), osnlp1-2, and OE16 plants. Scale bar: 8 cm. (C-H) Agronomic traits. Nitrogen use efficiency (NUE), actual yield per plot, grain yield per plant, seed weight (g/1000), tiller number, and plant height were statistically analysed. Values are the means and SD (30 plants per replicate with three replicates). Different letters denote significant differences (P<0.05) from Duncan’s multiple range test. (This figure is available in color at JXB online.) (Adopted from Alfatih et al., 2020) 3.3 Molecular breeding for NUE improvement Advances in genomic tools such as quantitative trait locus (QTL) mapping, gene editing, and CRISPR-Cas9 have significantly contributed to enhancing nitrogen efficiency in rice varieties. QTL mapping has been instrumental in identifying chromosomal hotspots and candidate genes associated with NUE. For example, a meta-analysis of yield-related and N-responsive genes in rice identified 1 064 NUE-related genes, including 80 transporters and 235 transcription factors, which were further shortlisted to 62 candidate genes through hierarchical methods (Kumari et al., 2021). These genes are localized to specific chromosomes, with chromosome 1 emerging as a hotspot for NUE. Gene editing technologies like CRISPR-Cas9 have also shown promise in improving NUE. By targeting specific genes involved in nitrogen uptake and assimilation, researchers can create rice varieties with enhanced NUE. For instance, the modulation of the DNR1 gene, which is involved in auxin homeostasis, has been shown to improve NUE and grain yield in rice (Zhang et al., 2020). These molecular breeding approaches offer new avenues for developing rice varieties that require less nitrogen fertilizer while maintaining high productivity, thereby contributing to sustainable agriculture. 4 Agronomic Strategies to Improve NUE 4.1 Optimizing fertilizer application Optimizing the timing, method, and form of nitrogen (N) application is crucial for enhancing nitrogen use efficiency (NUE) in rice cultivation. Split application of N fertilizers, where the total N dose is divided into multiple smaller doses applied at different growth stages, has been shown to improve NUE by synchronizing N availability with the crop's demand (Wang et al., 2022). This method reduces N losses through leaching and
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