Recently, an arabinogalactan protein has been found in Arabidopsis, which covalently links xylan to pectic polysaccharides. Albeit present only in low abundance in the wall, this finding has important implications for polymer biosynthesis, network formation and apoplastic polymer metabolism (Tan et al. 2013). In addition to the sidechains, a tetrasaccharide sequence of β-Xyl-1-3-α-Rha-1-2-α-GalA-1-4-β-Xyl has been observed at the reducing end of GX in a range of dicots and gymnosperm species such as spruce, birch, and Arabidopsis and has been proposed to play a role in xylan biosynthesis (see below).