1. Carnegie Institution of Washington, Department of Plant Biology, 260 Panama Street, Stanford, CA94305, USA,
2. Department of Botany, Duke University, Durham, NC27708, USA,
3. Biological Sciences, University of Wollongong, Wollongong, NSW 2522, Australia, and
4. Department of Biochemistry, School of Medical Sciences, University of Bristol BS8 1TD, UK