1. Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
2. Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA.
3. Pioneer Hi-Bred International, Inc.—A DuPont Company, Johnston, IA 50131, USA.
4. Department of Biology, University of North Dakota, Grand Forks, ND 58202, USA.