1. Department of Plant Sciences, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel (A.C.A., R.F.);
2. Department of Virology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel (M.L.); and
3. Center for Agricultural Biotechnology, University of Maryland, College Park, Maryland 20742–4450 (J.N.C.)