A novel allele of <i>Days to Heading 8</i> promotes heading, potentially widening <i>indica</i> rice adaptation in Japan

Flowering time is a critical determinant of regional adaptability. High-yielding rice (Oryza sativa L.) varieties, such as IR64, exhibit an extended vegetative growth period. As a result, IR64 is poorly adapted to high-latitude regions, such as Hirosaki-Aomori in northern Japan where prolonged day lengths and chilling stress during later growth stages induce an excessively late heading. In this study, we sequenced the whole genome of AS996, an early-heading rice cultivar derived from IR64 harboring introgressions from the wild rice species Oryza rufipogon Griff. Genetic analyses revealed that the AS996 allele of Days to Heading 8 (DTH8) accelerates heading by 10-14 days in the IR64 genetic background. Sequence analysis identified a 1-bp deletion in DTH8 that causes a frameshift and a premature stop codon, likely resulting in loss-of-function of the gene. Further gene expression analysis and protein structure modeling suggested that the truncation of DTH8 in AS996 leads to its dysfunctions by abolishing its interaction with Ghd7, another key protein regulating heading date. Notably, AS996 maintained yield stability in both Hirosaki and the southernmost location in Japan, Ishigaki Island. Sequence analysis across 378 rice varieties revealed that the AS996 haplotype represents a novel variant. These findings highlight the potential for utilizing loss-of-function alleles of DTH8 to enhance the adaptability and yield of high-performing indica rice varieties in high-latitude regions.