Molecular Soil Biology 2025, Vol.16, No.2, 91-102 http://bioscipublisher.com/index.php/msb 93 miRNA levels between different genotypes will change their adaptability to nitrogen stress (Nischal et al., 2012). 2.3 Screening and phenotypic identification strategies under low nitrogen environment Field trials are currently the most commonly used method for low nitrogen tolerance screening. The general practice is to set up two treatments, low nitrogen and normal nitrogen, under the same conditions, and then compare the yield, tillering, panicle length and other indicators of different varieties to select materials with strong adaptability (Hu et al., 2015; Lestari et al., 2019a; Zhou et al., 2025). Combining cluster analysis and membership function method, the performance of each variety can be evaluated more systematically to find the most advantageous variety (Hu et al., 2015; Zhou et al., 2025). It is generally believed that theoretical yield, effective tiller number, ear length, and number of grains per ear are important indicators for measuring low nitrogen tolerance. In addition to field trials, hydroponic and greenhouse trials are also often used for early screening and genetic research. In a controlled environment, root shape, biomass growth rate, and physiological changes can be clearly seen, which is very helpful for identifying genetic factors related to tolerance (Lian et al., 2005; Li et al., 2022b). The hydroponic system is particularly suitable for high-throughput screening and comparison between different genotypes, so it is also often used for QTL analysis and gene function verification. With technological advances, phenomics has begun to be widely used in the screening process. By using automatic photography systems, drone remote sensing, multispectral imaging and other methods, multi-dimensional information such as leaf color, root length, and plant height of germplasm materials can be quickly collected, thereby improving screening efficiency and accuracy (Zhang et al., 2024; Zhou et al., 2025). 3 Breeding Methods for Low Nitrogen Tolerance 3.1 Conventional breeding strategies Conventional breeding is the basic method for breeding low nitrogen tolerant rice. The practice is usually to plant a large number of germplasm materials in low nitrogen fields for observation, and then select varieties with better performance as parental pairs for hybridization. Then find plants that grow well under low nitrogen conditions from the offspring, select them repeatedly for several generations, and keep low nitrogen tolerant individuals. Studies have found that using traits such as biological yield, tiller number, panicle length and panicle number as screening indicators can effectively find germplasm that performs well under low nitrogen (Lestari et al., 2019b; Zhou et al., 2025). Continuing to select in a low nitrogen environment in each generation helps to accumulate favorable genes and improve the performance of offspring (Lestari et al., 2019b). In addition, statistical tools such as cluster analysis and membership function method can also be used to help comprehensively judge which materials are more tolerant to low nitrogen and improve efficiency (Zhou et al., 2025). Farmer-participated variety selection (FPVS) emphasizes combining actual production needs. At the breeding site, farmers directly participate in the screening process and evaluate which varieties are more stable and adaptable under low nitrogen based on their own experience. This method can ensure that the selected materials are closer to production practice and convenient for subsequent promotion and use (Vinod and Heuer, 2012). 3.2 Marker-assisted selection (MAS) and genomic selection (GS) Marker-assisted selection (MAS) is an important means of selecting rice at the genetic level. Researchers have found many QTL loci and genes related to low nitrogen tolerance and nitrogen use efficiency (NUE) (Lian et al., 2005; Wei et al., 2012; Shen et al., 2021; Li et al., 2022a; 2022b). With these markers, plants carrying good genes can be quickly selected in early breeding, saving time. For example, loci and genes such as TOND1, OsLHT1, and OsACR9 have been confirmed to be associated with low nitrogen and can be used as key targets in actual breeding (Wang et al., 2023; Shen et al., 2021). In addition, fine positioning using near-isogenic lines or recombinant inbred line populations can help to more accurately identify the genes that control these traits (Lian et al., 2005; Wei et al., 2012; Shen et al., 2021). Genomic selection (GS) is a further approach. Instead of looking at just a few genes, it analyzes the entire genome
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