Dry-milled flour rice 'Seolgaeng' harbors a mutated fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase2.

'Seolgaeng', an opaque-endosperm rice () mutant, is used to prepare high-quality dry-milled rice flour. The mutation causing its opaque-endosperm phenotype was unknown. Map-based cloning identified a missense mutation in the gene () in Seolgaeng. Transfer DNA insertion and clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-induced mutants exhibited opaque endosperm. Rice harbors another gene, . CRISPR/Cas9-induced double mutants of and () possessed more opaque endosperm compared to single mutants, whereas the endosperm of the single mutant was normal. Grain hardness and damaged starch content were significantly reduced in mutants compared to the wild type and . Amylose content was lower than normal in mutants but not . Grain hardness and amylose content were much lower in than in . Starch polymerization analysis revealed altered amylopectin structure in and mutants. F2KP activity was lower in and much lower in the double mutants when compared to the wild types, but showed no significant difference. In coleoptiles, hypoxia induced expression but downregulated . These results suggest that OsF2KP2 functions as the main F2KP isoform in endosperm experiencing hypoxia, but OsF2KP1 may partially compensate for the absence of OsF2KP2. We propose that F2KP has a crucial role in inorganic pyrophosphate-utilizing energy metabolism for starch biosynthesis in rice endosperm.