Molecular Soil Biology 2025, Vol.16, No.2, 91-102 http://bioscipublisher.com/index.php/msb 94 map and inputs the genotype and trait data into the model to predict the breeding value of each plant. This can consider the contributions of major effect genes and small effect genes at the same time, which is more suitable for complex traits, such as low nitrogen tolerance (Vinod and Heuer, 2012; Li et al., 2022b). As long as the model is well established, it is not necessary to conduct multi-generation field trials every time. It is possible to directly estimate which materials have greater potential in the early stage, significantly shortening the breeding cycle (Li et al., 2022b). 3.3 Introduction of traditional or wild rice traits Wild rice, such as Oryza rufipogon, is rich in genetic diversity and is a treasure trove of stress resistance traits. Introducing its good genes into cultivated rice through hybridization can effectively improve the adaptability to low nitrogen and the efficiency of nitrogen utilization (Wang et al., 2023). Through multi-omics analysis, it was found that a group of genes in wild rice are related to nitrogen metabolism, energy pathways and signal transduction, and these genes can be used as new breeding materials (Shen et al., 2021; Wang et al., 2023). Commonly used methods are hybridization or backcrossing. First, hybridize wild rice or traditional low-nitrogen tolerant varieties with cultivated rice to select plants with target traits. Then, through backcrossing, the background can be slowly returned to cultivated rice while retaining the gene for low nitrogen tolerance (Feng et al., 2018; Shen et al., 2021; Wang et al., 2023). It is also possible to use near-isogenic lines and specific introgression lines to combine multiple stress tolerance traits, such as drought tolerance and low nitrogen tolerance, into one variety (Feng et al., 2018). 4 Case Study: Development and Adoption of “Swarna-Sub1” and its Derivatives 4.1 Breeding background and process Swarna is a high-yield variety widely planted in eastern India, but its yield is reduced or even lost when it encounters floods. FR13A is a traditional rice variety that can survive flooding for more than ten days and is a representative of flood resistance research. The goal of breeders is to introduce the flood tolerance gene (Sub1A) of FR13A into Swarna, retaining the high-yield characteristics of Swarna while enhancing its flood resistance. They used molecular marker-assisted backcrossing (MAB) technology to ensure that only useful genes are introduced and other traits remain unchanged as much as possible (Neeraja et al., 2007; Mohapatra et al., 2021). Researchers also added genes for resistance to bacterial leaf streak and blast to Swarna-Sub1, improving its disease resistance and stability in different environments (Patroti et al., 2019; Mohapatra et al., 2021). In addition to flood resistance, breeders also pay special attention to its performance under low nitrogen conditions. They selected offspring under different nitrogen fertilizer levels and picked out plants that used less nitrogen fertilizer and had higher yields. The results showed that Swarna-Sub1 still grew well in a nitrogen-deficient environment, breaking the limitation of "low nitrogen means reduced yield" (Bhowmick et al., 2014; Singh et al., 2023). 4.2 Performance in low nitrogen environment Field trial data showed that Swarna-Sub1 had an average yield of 4.5~4.8 tons/hectare in flood years, which was significantly higher than ordinary varieties (Mishra and Kar, 2020; Singh et al., 2023) (Figure ). With less fertilizer, it can still maintain a high number of tillers and biomass, and the output per unit of nitrogen fertilizer is higher (Bhowmick et al., 2014; Singh et al., 2023). In contrast, although traditional flood-tolerant varieties can survive, their yields are low; ordinary high-yield varieties directly reduce their yields when they encounter flooding. Swarna-Sub1 does better in both aspects. In flood years, it can yield more than double that of old varieties, and in normal years, its yield is on par with mainstream varieties (Dar et al., 2013; 2017; Singh et al., 2023). Under nitrogen-deficient conditions, its nitrogen utilization efficiency is also higher, and more food is produced with less fertilizer (Dar et al., 2013; Mishra and Kar, 2020; Singh et al., 2023).
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