Rice Genomics and Genetics 2024, Vol.15, No.3, 142-152 http://cropscipublisher.com/index.php/rgg 145 The genetic factors that affect rice yield are multifaceted, involving the number of grains per panicle, grain size and weight, plant height, biomass and other traits. High-yielding varieties often have specific genetic markers, and QTL play a crucial role in increasing yield. Molecular pathways, including those related to photosynthetic efficiency and stress tolerance, further influence yield outcomes. These insights provide valuable opportunities for breeding programs aimed at improving rice yield and nutritional content (Sakamoto and Matsuoka, 2008). 4 Breeding Strategies to Improve Nutritional Content and Yield 4.1 Traditional breeding methods Traditional breeding methods, such as cross breeding and selection, have been fundamental in improving both the nutritional content and yield of rice. These methods involve the deliberate crossing of different rice varieties to combine desirable traits from each parent, followed by selection of the best-performing progeny over successive generations. One of the key strategies in traditional breeding is the identification and utilization of quantitative trait loci (QTLs) that influence both yield and nutritional content. For instance, a study involving 209 recombinant inbred lines derived from a cross between indica rice Xieqingzao B and Milyang 46 identified 22 QTLs affecting traits such as grain yield, protein content, and fat content. Notably, two QTL clusters were found on chromosomes 6 and 10, which were responsible for multiple traits, indicating the potential for simultaneous improvement of yield and nutritional content through targeted breeding (Huang et al., 2009). Efforts to enhance the micronutrient density in staple crops like rice have also been pursued through conventional breeding approaches. Studies have shown that it is possible to exploit genetic variation in seed concentration of essential minerals such as iron and zinc without negatively impacting yield. This approach involves selecting varieties with high mineral content and combining these traits with high-yield characteristics through cross breeding and selection (Li et al., 2019). Marker-assisted selection (MAS) has emerged as a powerful tool in traditional breeding, allowing for the precise selection of desirable traits based on genetic markers. This method has been particularly effective in improving grain yield and nutritional content. For example, MAS has been used to identify and introgress major QTLs for yield and yield component traits, leading to significant improvements in rice productivity. The use of well-characterized QTLs through introgression and gene pyramiding has proven effective, especially under abiotic stress conditions (Xing et al., 2010). Biofortification, a process of increasing the nutritional content of crops through breeding, has been a focus of rice breeding programs. QTL mapping for grain yield and micronutritional traits has identified several QTLs that influence both yield and nutrient content. For instance, QTLs for zinc, manganese, and copper content have been identified, with some QTLs showing synergistic effects on both yield and micronutrient content, suggesting the potential for simultaneous improvement through QTL pyramiding (Yu et al., 2009). Breeding for improved nitrogen utilization efficiency (NUE) has shown promise in enhancing both yield and grain protein concentration. Studies have demonstrated significant variation in NUE among different rice genotypes, with positive correlations between NUE, grain yield, and grain protein concentration. These findings suggest that selecting for high NUE can lead to rice varieties that are both high-yielding and nutritionally superior (Guo and Ye, 2014). Traditional breeding methods, including cross breeding, selection, and the use of marker-assisted selection, have been instrumental in improving the nutritional content and yield of rice. By leveraging genetic variation and QTL mapping, breeders can develop rice varieties that meet the dual goals of high yield and enhanced nutritional quality. These strategies, combined with biofortification and improved nutrient use efficiency, offer promising avenues for the development of nutritionally superior and high-yielding rice varieties. 4.2 Breeding strategies to improve nutritional content and yield Marker Assisted Selection (MAS) has emerged as a powerful tool in modern rice breeding, enabling the precise introduction of desirable traits into rice cultivars. MAS leverages DNA markers that are closely linked to target genes or quantitative trait loci (QTLs) to facilitate the selection process. This method has been particularly effective in improving traits such as disease resistance, abiotic stress tolerance, and yield components.
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