The present disclosure relates generally to polysaccharide synthases. The present disclosure reveals that a subset of the CesA gene family encode XynS xylan synthases. As a result of the identification of XynS nucleic acids, and corresponding amino acid sequences that encode XynS xylan synthases, the present invention provides, inter alia, methods and compositions for modulating the level and/or activity of xylan synthase in a cell and/or modulating the level of xylan produced by a cell.