1. Department of Biochemistry, Physiology and Microbiology, Laboratory of Protein Biochemistry and Protein Engineering, Ghent University, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium, and Faculty of Applied Bioscience and Engineering, Department of Applied Plant Sciences, Laboratory of Tropical Crop Improvement, K.U. Leuven, Kasteelpark Arenberg 13, B-3001 Leuven, Belgium