Rice Genomics and Genetics 2025, Vol.16, No.4, 199-210 http://cropscipublisher.com/index.php/rgg 205 indicating that there may be a feedback mechanism regulating this network (Perez et al., 2019). Similarly, if the factors regulating OsGBSSI are knocked out, the content of amylose can also be reduced, and the edible quality of rice will be improved. These results provide a clear direction for the improvement of rice quality. Figure 2 Proposed model of the role of O3 in maintaining ER homeostasis and regulating endosperm storage protein and starch biosynthesis in rice (Adopted from Cao et al., 2022) Under normal conditions in the WT, O3 is located in the ER and the nucleus. It simultaneously regulates ER protein processes and secretion as well as storage protein and starch biosynthesis in endosperm cells by binding to specific motifs, such as pUPRE-II, O2, and the GCN4 box, to activate transcription of UPR genes and storage protein and starch biosynthesis genes, ultimately ensuring normal development of rice grains. However, mutation of O3 leads to downregulated expression of ER stress-related genes, such as OsBIP1 and PDIL1-1, as well as genes related to storage protein and starch biosynthesis, resulting in ER stress and an impaired protein folding process in the ER. This leads to excessive accumulation of the 57-kDa glutelin precursor and reduces the contents of starch and storage proteins. High temperature can aggravate ER stress and lead to more abnormal grain development in the o3mutant. As physiological feedback, more O3 (mutated) is transferred to the nucleus from the ER, together with OsbZIP50 and other unknown TFs, to activate expression of key UPR genes to maintain ER homeostasis, especially under high-temperature conditions (Adopted from Cao et al., 2022)
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