Author contributions
Lijun Li and Hui Ni designed research, Wenjing Li and Zheyu Wu performed experiments; Lijun Li and Wenjing Li analyzed data, Jianye Gong performed computational experiments and analyzed, Lijun Li, Wenjing Li and Jianye Gong wrote the paper, Yanyan Xu, Zedong Jiang, Hui Ni and Yi-Sheng Chen modified the paper. All authors read and approved the final manuscript.