This work was funded by the National Science and Technology Support Program ( 2012BAD19B03 ), the China 973 Program ( 2006CB708209 ), the 863 Program ( 2007AA10Z414 ), the Special Fund for Agro-scientific Research in the Public Interest of China ( 201003031 ), the Zhejiang Provincial Science and Technology Project ( 2010C12027 ), and the Zhejiang Provincial Foundation for Natural Science ( Z305165 and Y3090657 ). We thank Dr Zhiyou Du, School of Life Science, Zhejiang Sci-Tech University, for the technological help and Professor M. J. Adams, Rothamsted Research, Harpenden, UK for his help in correcting the English of the manuscript.