Fig. 1

Xylan structures from spruce, poplar, and switchgrass secondary walls. Graphical representation of the main structural features of (a) arabinoglucuronoxylan (AGX) from spruce (b) acetylated glucuronoxylan (AcGX) from poplar, and (c) acetylated glucuronoarabinoxylan (AcGAX) from switchgrass. Spruce GX and poplar AcGX contain a distinct glycosidic sequence at their reducing ends, which is absent in switchgrass AcGAX, which often has substituted reducing xylosyl residues at the reducing end [25, 28, 43]. The GlcA and Ara substituents are in even positions and regularly distributed in the main domain of spruce AGX [27, 46]. The substituents in the main domain of Arabidopsis AcGX and poplar are also likely to be evenly distributed [22, 45]. The pattern of distribution of AcGAX substituents in switchgrass secondary walls is still unknown, but they are less branched than the AcGAX in primary walls and other tissue-specific grass xylans (see text for more details)