1. Institute of Plant Sciences, Swiss Federal Institute of Technology, Universitätstrasse 2, 8092 Zürich, Switzerland,
2. Department of Biological Sciences, John Tabor Laboratories, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK and Environmental Biology and Instrumentation Division, Building 318, Brookhaven National Laboratory, Upton, New York 11973, USA