1. College of Computer Science and Technology, Jilin University, Changchun 130012, P. R. China
2. Key Laboratory of Symbolic Computation and Knowledge Engineering of Ministry of Education, Jilin University, Changchun 130012, P. R. China
3. School of Natural and Computing Sciences, University of Aberdeen, Aberdeen, AB24 3UE, UK
4. Plant Science Department, South Dakota State University, Brookings, SD 57007, USA