1. Department of Crop and Soil Sciences, 277 Johnson Hall, P.O. Box 646420, Washington State University, Pullman, WA 99164-6420, USA.
2. Department of Plant Science, McGill University, Macdonald Campus, 21111 Lakeshore Road, Sainte-Anne-de-Bellevue, QC H9X 3V9, Canada.
3. Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA.