1. Laboratory of Genetic Engineering, Research Faculty of Agriculture, Hokkaido University, Sapporo, Hokkaido 060-8589, Japan
2. Memuro Upland Farming Research Station, Hokkaido Agricultural Research Center, National Agriculture and Food Research Organization, Memuro, Hokkaido 082-0081, Japan
3. Central Agricultural Experiment Station, Agriculture Research Department, Hokkaido Research Organization, Takikawa 073-0013, Japan