1. National Key Laboratory of Crop Genetic Improvement; Huazhong Agricultural University; Wuhan 430070 China
2. Department of Agronomy and Plant Genetics; University of Minnesota; St Paul MN 55108 USA
3. Department of Plant and Microbial Biology; University of Minnesota; St Paul MN 55108 USA
4. Department of Agronomy; Iowa State University; Ames IA 50011 USA