1. Department of Biology, ETH Zurich, Universitaetstrasse 2, 8092 Zurich, Switzerland. Fax: +41 44 6321044; Tel: +41 44 6323491
2. Faculty of Life Sciences, University of Manchester, Brunswick Street, Manchester M13 9PL, UK
3. Institute of Botany, Chinese Academy of Sciences, Haidian Xiangshan Nanxincun 20, Beijing 100093, China