Koshihikari: a premium short-grain rice cultivar - its expansion and breeding in Japan.

Koshihikari, a Japanese short-grain rice cultivar, was developed in 1956, more than 60 years ago. Despite its age, it has been the most widely grown cultivar in Japan for more than 35 years, making it the most important rice for the Japanese people. In its early days, there was no reason to predict that Koshihikari would become so widely disseminated. However, since the end of the post-World War II food shortage in the 1960s, Japanese preferences changed from high productivity to good eating quality. This triggered wide expansion of Koshihikari cultivation due to the cultivar's excellent taste and texture. With increasing cultivation of Koshihikari in Japan, several good agronomic characteristics beyond its high eating quality became apparent, such as its good adaptation to different environments, tolerance to pre-harvest sprouting, and cold tolerance during the booting stage. These characteristics outweigh drawbacks such as its low blast resistance and low lodging resistance. The popularity of Koshihikari influenced subsequent rice breeding trends at regional agricultural experimental stations, and the characteristics of newly developed rice cultivars in Japan are usually rated relative to Koshihikari, which is used as the benchmark. Koshihikari was the first japonica rice cultivar whose whole genome has been sequenced by means of next-generation sequencing. Furthermore, comparison of the genomes of Koshihikari and Nipponbare has provided detailed insights into the genetic diversity of Japanese rice cultivars relative to that in rice populations elsewhere in the world. Further progress in rice genomics is gradually unlocking the mechanisms that underlie the agronomic characteristics of Koshihikari. To support both research and the development of novel rice cultivars, a series of isogenic and near-isogenic lines in the Koshihikari genetic background have been continuously developed. These new findings and materials will facilitate genomics-assisted rice breeding, eventually leading to superior cultivars.