This research was supported by the National Natural Science Foundation of China [31672640], the Natural Science Foundation of Hunan Province [2017JJ3134], the Scientific Research Fund of Hunan Province Education Department [17C0935], and Blue Granary [2018YFD0900200]. We thank Mr. Xiao Ma, Ms. Dan Zeng and Ms. Qin Qin for helping with the sample collection and data analysis, an anonymous expert for figure preparing and manuscript revising, and also thank the anonymous reviewers for their helpful comments.