1. B. Baker and S. P. Dinesh-Kumarare are in the Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, and the Plant Gene Expression Center, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan Street, Albany, CA 94710, USA. P. Zambryski and B. Staskawicz are in the Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA.