1. Department of Microbiology, Swedish University of Agricultural Sciences, SLU, Uppsala S-75007, Sweden
2. Laboratoire de Biologie Moléculaire des Plantes Supérieures (LBMPS), University of Geneva, Sciences III, CH-1211 Geneva 4, Switzerland
3. Plant Genetics Research Unit, USDA Agricultural Research Service, and Department of Agronomy, University of Missouri, Columbia, Missouri 65211