1. Crop, Soil and Water Sciences Division, International Rice Research Institute, MCPO Box 3127, 1271 Makati City, Philippines;
2. Present address: Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA
3. Centre for High Resolution Imaging and Processing, MSI/WTB Complex, School of Life Sciences, University of Dundee, Dundee DD1 5EH, UK;