1. Integrative Cell Biology Laboratory, School of Biological and Biomedical Sciences, Durham University, Durham DH1 3LE, United Kingdom
2. Umea Plant Science Center, Swedish University of Agricultural Science, Umea S-0901-83, Sweden
3. Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom
4. Salk Institute for Biological Studies, Carlsbad, California 92009-8711