Abstract text: Hemicellulose polysaccharides influence assembly and properties of plant primary cell walls (PCW), perhaps by interacting with different cellulose microfibril surfaces to affect the deposition and bundling of cellulose microfibrils. However, the functional differences among plant cell wall hemicelluloses such as β-galactoglucomannan (β-GGM), xyloglucan and xylan remain clear. Xyloglucan and β-GGM, accounting for most of the Arabidopsis PCW hemicellulose, have been shown to have structure and biosynthesis similarities and play redundant functions for Arabidopsis plant growth, but plants devoid of xyloglucan and β-GGM show relatively mild phenotypes.

Here, we characterized a xylan that is found only in some PCW cell types in Arabidopsis. Unlike previously described acetylated glucuronidated xylan, this xylan is not acetylated and every other xylose along the xylan backbone is decorated solely with β-1,2-Xylp- or decorated with β-1,2-Xylp-, α-1,2-Araf and the disaccharide α-1,2-Arap-α-1,2-GlcA-. Although Arabidopsis PCW xylan synthesis mutants show no obvious growth defects, genetic crosses between xyloglucan and xylan synthesis mutants cause sterility, and swollen short roots, similar to some PCW CESA mutant phenotypes. These findings suggest a related role for these two distinct classes of hemicelluloses in PCWs and raise the possibility these two hemicelluloses may together affect the deposition of PCW cellulose fibrils.