**9. Acknowledgment**

This work was mainly supported by the National Natural Science Foundation of China (Grant No. 30170656, 30971986); Transformation Fund for Agricultural Science and Technology Achievements Project, The Ministry of Science and Technology, China (Grant No. 2010GB2E020600); Special Fund for Agro-scientific Research in the Public Interest, Ministry of Agriculture, China (Grant No. 201003073); Key Scientific and Technological Research Project, Chongqing Science and Technology Commission, China (Grant No. CSTC, 2009AB1183); The Natural Science Foundation, Chongqing Science and Technology Commission, China (Grant No. CSTC, 2010BB1129); The Fundamental Research Funds for the Central Universities (Grant No. XDJK2009C123, XDJK2009C127); The Ph.D. Programs Foundation of Southwest University (Grant No. SWU111016, SWU111017) and the 211 Project of Southwest University.
