1. Laboratory of Molecular Plant Physiology, Institute of Botany and Microbiology, K. U. Leuven, Kasteelpark Arenberg 31, B-3001 Heverlee, Belgium, and Laboratory of Medicinal Chemistry, Rega Institute of Medical Research, BioMacs, K. U. Leuven, Minderbroedersstraat 10, B-3000, Leuven, Belgium