Structural diversity of xylans in the cell walls of monocots

Xylans in the cell walls of monocots are structurally diverse. Arabinofuranose-containing glucuronoxylans are characteristic of commelinids. However, other structural features are not correlated with the major transitions in monocot evolution. Most studies of xylan structure in monocot cell walls have emphasized members of the Poaceae (grasses). Thus, there is a paucity of information regarding xylan structure in other commelinid and in non-commelinid monocot walls. Here, we describe the major structural features of the xylans produced by plants selected from ten of the twelve monocot orders. Glucuronoxylans comparable to eudicot secondary wall glucuronoxylans are abundant in non-commelinid walls. However, the α-D-glucuronic acid/4-O-methyl-α-D-glucuronic acid is often substituted at O-2 by an α-L-arabinopyranose residue in Alismatales and Asparagales glucuronoxylans. Glucuronoarabinoxylans were the only xylans detected in the cell walls of five different members of the Poaceae family (grasses). By contrast, both glucuronoxylan and glucuronoarabinoxylan are formed by the Zingiberales and Commelinales (commelinids). At least one species of each monocot order, including the Poales, forms xylan with the reducing end sequence -4)-β-D-Xylp-(1,3)-α-L-Rhap-(1,2)-α-D-GalpA-(1,4)-D-Xyl first identified in eudicot and gymnosperm glucuronoxylans. This sequence was not discernible in the arabinopyranose-containing glucuronoxylans of the Alismatales and Asparagales or the glucuronoarabinoxylans of the Poaceae. Rather, our data provide additional evidence that in Poaceae glucuronoarabinoxylan, the reducing end xylose residue is often substituted at O-2 with 4-O-methyl glucuronic acid or at O-3 with arabinofuranose. The variations in xylan structure and their implications for the evolution and biosynthesis of monocot cell walls are discussed.