1. Departments of Biological and Nutritional Sciences, Chemistry, and Biochemistry and Genetics, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K., Department of Cellular Physiology, The Babraham Institute, Babraham, Cambridge CB2 4AT, U.K., and Centre de Recherches sur les Macromolécules Végétales, Centre National Recherche Scientifique, affiliated with the Joseph Fourier University, Grenoble, BP 53, F-38041 Grenoble Cedex 9, France