1. Advanced Microscopy Facility, Vienna Biocenter Core Facilities, A-1030 Vienna, Austria.
2. Gregor Mendel Institute, Austrian Academy of Sciences, Vienna Biocenter, A-1030 Vienna, Austria.
3. Laboratoire Reproduction et Développement des Plantes, Univ Lyon, ENS de Lyon, UCB Lyon 1, CNRS, INRA, F-69342 Lyon, France.