Molecular Soil Biology 2025, Vol.16, No.2, 91-102 http://bioscipublisher.com/index.php/msb 92 et al., 2021; Xin et al., 2021; Li et al., 2022a). Promoting such low-nitrogen tolerant varieties can not only reduce the ecological pressure caused by nitrogen fertilizers, but also help farmers save costs, increase income, and promote the development of agriculture in a green and economical direction (Lee, 2021; Wang et al., 2021; Jyoti et al., 2024). This study will focus on the latest progress in low-nitrogen rice breeding, focusing on the physiological and genetic response mechanisms of rice under low-nitrogen conditions, analyzing the genes and root phenotypic characteristics related to low-nitrogen traits, and introducing methods for germplasm screening and evaluation. At the same time, we will also pay attention to the performance of these results in actual production, and combine field performance, molecular marker selection, and genome analysis technologies to provide practical ideas and theoretical basis for low-nitrogen adaptive rice breeding. 2 Scientific BASIS for Rice's Low Nitrogen Tolerance 2.1 Physiological traits related to nitrogen use efficiency (NUE) The length and thickness of rice roots directly affect its ability to absorb nitrogen from the soil. Studies have shown that in low nitrogen environments, the roots of certain rice genotypes can grow better and show stronger nitrogen absorption capacity (Li et al., 2022a). In particular, the larger the surface area of the root system and the more root hairs, the more conducive it is to absorb nutrients when nitrogen fertilizer is insufficient (Li et al., 2022a; Zhou et al., 2025). Chlorophyll content reflects the photosynthesis level and nitrogen status of rice. When nitrogen is insufficient, leaves tend to turn yellow, chlorophyll decreases, and photosynthetic efficiency decreases. However, some low-nitrogen tolerant rice varieties can maintain a high green color and photosynthetic capacity even when nitrogen supply is low (Zhang et al., 2024; Qi et al., 2025; Xu et al., 2025). For example, rice expressing the maize PCK2 gene can accumulate more photosynthetic pigments and biomass when nitrogen is insufficient, improving photosynthesis efficiency and nitrogen assimilation rate (Xu et al., 2025). In addition, these varieties also have advantages in terms of photosystem II efficiency and electron transfer rate (Zhang et al., 2024; Qi et al., 2025). In terms of yield, in a low-nitrogen environment, tolerant varieties can still maintain more tillers, longer panicles, and higher grain numbers, which are positively correlated with yield (Huang et al., 2022; Tao et al., 2022; Zhou et al., 2025). Some varieties can achieve good yield and harvest index even without nitrogen fertilizer (Huang et al., 2022; Tao et al., 2022). Whether rice can reasonably distribute nutrients, such as the ratio of roots to aboveground parts and the ratio of grains to straw, also determines its yield performance under nitrogen deficiency (Tao et al., 2022; Zhou et al., 2025). 2.2 Genetic regulation and QTL of nitrogen use efficiency In rice, there are some proteins that specifically transport nitrogen, which are encoded by genes such as NRT1.1B and OsNRT2. These genes determine the ability of rice to absorb nitrate nitrogen and are important factors in improving nitrogen use efficiency (Wang et al., 2023; Li et al., 2022a; Zhang et al., 2015). TOND1 is a key QTL. Studies have found that its overexpression can significantly improve rice yield and adaptability under nitrogen deficiency conditions (Zhang et al., 2015). Some genes related to purine metabolism, glycolysis and pentose phosphate pathways have also been found to be related to rice's low nitrogen adaptability (Wang et al., 2023). Through QTL positioning, researchers have found many important regions related to low nitrogen tolerance, which control traits such as root length, biomass, and yield (Lian et al., 2005; Li et al., 2022a; 2022b). Some of these QTLs only work at specific nitrogen levels, indicating that rice's adaptation to low nitrogen is the result of the combined action of multiple genes (Lian et al., 2005). Molecular marker-assisted breeding technology has been used to screen low nitrogen-tolerant varieties and accelerate the gathering of excellent genes (Li et al., 2022a; 2022b). The gene networks that regulate these traits are also very complex. For example, transcription factors such as NAC and TCP affect root development and nitrogen absorption (Nischal et al., 2012; Wang et al., 2023). The expression differences of some miRNAs also affect the response of rice to low nitrogen. The differences in
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