Rice Genomics and Genetics 2025, Vol.16, No.4, 219-236 http://cropscipublisher.com/index.php/rgg 229 However, the absence of SBEIIb is also accompanied by negative effects such as slight shrinkage of the endosperm of rice grains and decreased transparency. Scientists have further utilized gene editing to precisely regulate the expression of SBEIIb, hoping to increase resistant starch while reducing the adverse effects on appearance. Some progress has been made in this regard. For instance, through gene editing, some functions of SBEIIb have been partially lost rather than completely absent, resulting in improved materials that balance both appearance and nutrition. Among SS genes, SSIIa and SSIIIa have a significant impact on the structure of amylopectin and are the focus of research. The aforementioned Alk (SSIIa) allelic variation determines the lower gelatinization temperature and soft texture of japonica rice. In breeding, superior alleles of japonica rice (such as alk/alk) are often selected in combination with low straight-chain to improve the quality of indica rice. The alteration of SSIIIa function can be used to increase the content of resistant starch. Fujita et al. (2007) created SSIIIA-deficient rice and wheat mutants. The results showed that the proportion of long branched chains in the endosperm increased, resistant starch significantly improved, and the GI value decreased simultaneously. However, the absence of SSIIIa makes the rice slightly harder in texture. During breeding, it needs to be combined and adjusted with other variations such as Wx and SSIIa to achieve a balanced taste and nutrition. In recent years, Chinese scholars have also reported the role of SSIVb in starch synthesis: although SSIV is mainly responsible for regulating the number of starch granules, its mutation can reduce the number of starch granules in endosperm, but the difference in particle size and total starch content is not significant, and its impact on quality is limited. It is worth mentioning that bifunctional enzymes are also under research. For instance, the phosphoric acid accumulated on the surface of starch granules can promote the swelling and gelatinization of starch, and altering the enzyme activity can affect this property. A deletion mutation of a phosphatase called PHS2 (the homolog of Starch Excess4 in Arabidopsis thaliana) leads to an increase in the degree of starch phosphorylation and a slight decrease in the gelatinization temperature of rice. All these indicate that through functional identification, we are constantly exploring new gene targets that can be used to improve the properties of starch. In the future, through the integration of multiple gene variations, such as Wx low straight-chain + high-quality Alk+ weak functional SBEIIb and other combined breeding methods, it is expected to cultivate ideal rice varieties with soft texture, low cold rice hardness and high resistant starch content. It can be foreseen that with the in-depth identification of functional genes, we will establish a more abundant "gene toolbox" to provide support for the targeted improvement of different rice quality traits. 5.3 Utilization of superior alleles in high-quality rice breeding Utilizing the superior alleles of the aforementioned key genes to cultivate high-quality rice is one of the important strategies in modern molecular breeding. Traditionally, the superior quality genes of different rice varieties can be introduced into the main cultivated varieties through hybridization. For instance, the Japanese "Koshihikari" japonica rice is of excellent quality, among which the low-linear Wxb and soft alk alleles play a decisive role in its soft and glutinous texture. In recent years, breeders have introduced these alleles into high-yield background varieties through backcrossing and transfer, thereby fostering new varieties that are both high-yielding and have good palatability. In China, the Wxb (low straight-chain) allele has been introduced into many restorer lines of hybrid indica rice, significantly improving the quality of hybrid rice. For instance, combinations like Taiyou 390 ensure yield while reducing amylose content to around 16%, meeting the standards of high-quality rice. For instance, the fragrant rice variety has developed fragrant glutinous rice by taking advantage of the functional deletion allele of the OsBADH2 gene (which enables rice to synthesize the aromatic substance 2-AP) and combining it with the glutinous wx allele, thus meeting the market's dual demands for flavor and texture. In addition to the conventional hybridization to polymerize superior alleles, new molecular approaches are also accelerating this process. Molecular marker-assisted selection (MAS) has been widely used to track important sites such as Wx, Alk, and BADH2. Through marker detection, individuals carrying the target allele can be precisely selected in the early generation, significantly enhancing breeding efficiency. For instance, in the project
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