The synthesis and origin of the pectic polysaccharide rhamnogalacturonan II – insights from nucleotide sugar formation and diversity

• Main Authors: Maor eBar-Peled, Breeanna RAE Urbanowicz, Malcolm A. O'Neill
• Format: Article
• Language: English
• Published: Frontiers Media S.A. 2012-05-01
• Series: Frontiers in Plant Science
• Subjects: RG-II; UDP-apiose; CMP-kdo; aceric acid; Golgi; wall evolution
• Online Access: http://journal.frontiersin.org/Journal/10.3389/fpls.2012.00092/full

There is compelling evidence showing that the structurally complex pectic polysaccharide rhamnogalacturonan II (RG-II) exists in the primary cell wall as a borate cross-linked dimer and that this dimer is required for the assembly of a functional wall and for normal plant growth and development. The results of several studies have also established that RG-II structure and cross-linking is conserved in vascular plants and that RG-II likely appeared early in the evolution of land plants. Two features that distinguish RG-II from other plant polysaccharides are that RG-II contains 13 different glycoses linked to each other by 22 different glycosidic linkages and that RG-II is the only polysaccharide known to contain apiose and aceric acid. Thus, one key event in land plant evolution was the emergence of genes encoding nucleotide sugar-biosynthetic enzymes that generate the activated forms of apiose and aceric acid required for RG-II synthesis. Many of the genes involved in the generation of the nucleotide sugars used for RG-II synthesis have been functionally characterized. By contrast, only one putative glycosyltransferase involved in the assembly of RG-II has been identified. Here we provide an overview of the formation of the activated sugars required for RG-II synthesis and point to the possible cellular and metabolic processes that could be involved in assembling and controlling the formation of a borate cross-linked RG-II molecule. We discuss how nucleotide sugar synthesis is compartmentalized and how this may control the flux of precursors to facilitate and regulate the formation of RG-II.