Eudicot primary cell wall glucomannan is related in synthesis, structure, and function to xyloglucan

Patterned beta-GGM resembles xyloglucan in structure, biosynthesis, and function. Hemicellulose polysaccharides influence assembly and properties of the plant primary cell wall (PCW), perhaps by interacting with cellulose to affect the deposition and bundling of cellulose fibrils. However, the functional differences between plant cell wall hemicelluloses such as glucomannan, xylan, and xyloglucan (XyG) remain unclear. As the most abundant hemicellulose, XyG is considered important in eudicot PCWs, but plants devoid of XyG show relatively mild phenotypes. We report here that a patterned beta-galactoglucomannan (beta-GGM) is widespread in eudicot PCWs and shows remarkable similarities to XyG. The sugar linkages forming the backbone and side chains of beta-GGM are analogous to those that make up XyG, and moreover, these linkages are formed by glycosyltransferases from the same CAZy families. Solid-state nuclear magnetic resonance indicated that beta-GGM shows low mobility in the cell wall, consistent with interaction with cellulose. Although Arabidopsis beta-GGM synthesis mutants show no obvious growth defects, genetic crosses between beta-GGM and XyG mutants produce exacerbated phenotypes compared with XyG mutants. These findings demonstrate a related role of these two similar but distinct classes of hemicelluloses in PCWs. This work opens avenues to study the roles of beta-GGM and XyG in PCWs.