CELLULOSE SYNTHASE9 Serves a Nonredundant Role in Secondary Cell Wall Synthesis in Arabidopsis

*Department of Horticulture, University of Kentucky, Lexington, Kentucky 40546 (J.S., D.H., B.W., V.M., S.D.);Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 (J.G.,G.H.); and Membrane Biogenesis Laboratory, Centre National de la Recherche Scientifique and University ofBordeaux, F–33076 Bordeaux, France (F.B., Y.L.-B.)