Acknowledgements We are grateful to Chatchawan Jantasuriyarat and Bo Zhou for experimental assistance, to David Ow (Plant Gene Expression Center, USDA, Albany, CA 94710, USA) for providing the Cre gene, and to Yoshihito Suzuki (Department of Applied Biological Chemistry, University of Tokyo, Tokyo 113-8657, Japan) for the DNA construct containing the 4x CaMV 35S enhancers.