Thermo-responsive allele of sucrose synthase 3 (Sus3) provides high-temperature tolerance during the ripening stage in rice (Oryza sativa L.).

High-temperature stress during the ripening stage leads to quality deterioration due to an increase in chalky grains in brown rice (Oryza sativa L.). In a previous study, we identified a QTL for Appearance quality of brown rice 1 (Apq1) using chromosome segment substitution lines of the indica cultivar 'Habataki' in the japonica cultivar 'Koshihikari' background and narrowed down the locus to a 48-kb region on chromosome 7. To verify the function and mechanisms of this QTL in grain appearance, in this study, we fine-mapped the gene and conducted high-temperature tolerance tests. As a result of the genetic mapping, we narrowed down the candidate region of Apq1 to a 19.4-kb region including three predicted genes. Among these, the temporal expression pattern of sucrose synthase 3 (Sus3) corresponded well with the high temperature-sensitive period during ripening, and expression of the 'Habataki' allele of Sus3 was increased under high-temperature condition. In addition, we transformed the 'Habataki' Sus3 gene into 'Nipponbare', and the transformants obtained high-temperature tolerance. Therefore, we conclude that the causal gene underlying the QTL Apq1 is the thermo-responsive Sus3 allele, and the increase in Sus3 expression under high-temperature condition during ripening leads to high-temperature tolerance in rice.

Overexpression of SRS5 improves grain size of brassinosteroid-related dwarf mutants in rice (Oryza sativa L.).

Grain size is a trait that is important for rice (Oryza sativa L.) yield potential. Many genes regulating grain size have been identified, deepening our understanding of molecular mechanisms of grain size determination in rice. Previously, we cloned SMALL AND ROUND SEED 5 (SRS5) gene (encoding alpha-tubulin) from a small and round seed mutant and revealed that this gene regulates grain length independently of the brassinosteroid (BR) signaling pathway, although BR-related mutants set small grain. In this study, we showed that overexpression of SRS5 can promote grain length and demonstrated that the overexpression of SRS5 in BR-related mutants rescued the shortened grain length, which is an unfavorable phenotype in the yield potential of BR-related mutants, while preserving the useful semi-dwarf and erect leaf phenotypes.