Qi Xie1, Zhihong Xu2. 1. State Key Laboratory of Plant Genomics, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China; China University of Chinese Academy of Sciences, Beijing 100049, China. Electronic address: qxie@genetics.ac.cn. 2. School of Advanced Agricultural Sciences, Peking University, Beijing 100871, China. Electronic address: xuzh@pku.edu.cn.