1. Department of Biophysical Chemistry, GBB, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Plant Biology Laboratory, Department of Plant Biology, The Royal Veterinary and Agricultural University, 40 Thorvaldsenvej, Frederiksberg C, DK-1871, Denmark, and Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands