1. ZMBP, Center for Plant Molecular Biology, Department of General Genetics, University of Tuebingen, 72076 Tuebingen, Germany, Proteome Centre Tuebingen, Interfaculty Institute for Cell Biology, University of Tuebingen, 72076 Tuebingen, Germany, and Institute for Crop Production and Grassland Research, Bioinformatics Unit, University of Hohenheim, Fruwirthstr. 23, 70599 Stuttgart, Germany