Work in our laboratories were supported by the National Basic Research Program of China (no. 2012CB126305), the National Natural Science Foundation of China (no. 31330075, 31372319 and 31110103909), the Hubei Provincial Research and Development Program (no. 2010BB023), the Hubei Hundred Talent program, the Natural Science Foundation of Hubei Province (no. 2013CFA097, 2013CFB325, 2012FFB04805, and 2011CDA131), Agriculture and Food Research Initiative Competitive Grants (no. 2011-67015-20028, 2014-67015-21770 and 2015-67015-23276) from the USDA National Institute of Food and Agriculture, and a Hatch project from Texas A&M AgriLife Research (H-8200). The authors declare no conflict of interest.