Roles of the Hd5 gene controlling heading date for adaptation to the northern limits of rice cultivation

During the diversification of cultivated rice after domestication, rice was grown in diverse geographic regions using genetic variations attributed to the combination of alleles in loci for adaptability to various environmental conditions. To elucidate the key gene for adaptation in rice cultivars to the northern limit of rice cultivation, we conducted genetic analyses of heading date using extremely early-heading cultivars. The Hd5 gene controlling heading date (flowering time) generated variations in heading date among cultivars adapted to Hokkaido, where is the northernmost region of Japan and one of the northern limits of rice cultivation in the world. The association of the Hd5 genotype with heading date and genetical analysis clearly showed that the loss-of-function Hd5 has an important role in exhibiting earlier heading among a local population in Hokkaido. Distinct distribution of the loss-of-function Hd5 revealed that this mutation event of the 19-bp deletion occurred in a local landrace Bouzu and that this mutation may have been selected as an early-heading variety in rice breeding programs in Hokkaido in the early 1900s. The loss-of-function Hd5 was then introduced into the rice variety Fanny from France and contributed to its extremely early heading under the presence of functional Ghd7. These results demonstrated that Hd5 plays roles not only in generating early heading in variations of heading date among a local population in Hokkaido, but also in extremely early heading for adaptation to northern limits of rice cultivation.