1. Institute of Molecular Genetics, National Research Council, 27100 Pavia, Italy
2. Department of Genetics and Microbiology, University of Pavia, 27100 Pavia, Italy
3. Institut de Biotechnologie des Plantes, Unité Mixte de Recherche, Centre National de la Recherche Scientifique 8618, Plateau du Moulon, Université Paris-Sud, 91405 Orsay, France