1. School of Life Sciences, Tsinghua University, Beijing 100084, China, and Tsinghua-Peking Center for Life Sciences, Beijing 100084, China.
2. State Key Laboratory of Plant Genomics and National Center for Plant Gene Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China.
3. The Sainsbury Laboratory, Norwich Research Park, Norwich NR4 7UH, UK.