1. Guelph Food Research Centre, Agriculture and Agri-Food Canada, 93 Stone Road West, Guelph, ON, N1G 5C9, Canada.
2. Corresponding author. Phone: 519-780-8030. Fax: 519-829-2600. E-mail address: liuq@agr.gc.ca
3. Department of Biochemistry, Memorial University of Newfoundland, St. John's, NL, A1B 3X9, Canada.
4. Crop Development Centre, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada.