Xylans are the major hemicelluloses in secondary cell walls of dicots and are critical for normal plant growth and development. Xylan-containing lignocellulosic secondary cell walls are the most abundant repository of biomass on earth and are widely used for energy, pulping, paper-making and textiles. However, the pentose composition of xylans makes them difficult to be used efficiently. Thus, understanding the detailed mechanism of xylan biosynthesis may lead to new strategies to manipulate the xylan composition in cell walls and to modify their structures. To date the characterization of var...