RHAMNOGALACTURONAN II: Structure and Function of a Borate Cross-Linked Cell Wall Pectic Polysaccharide

Abstract

Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide that was first identified in 1978 as a quantitatively minor component of suspension-cultured sycamore cell walls. Subsequent studies have shown that RG-II is present in the primary walls of angiosperms, gymnosperms, lycophytes, and pteridophytes and that its glycosyl sequence is conserved in all vascular plants examined to date. This is remarkable because RG-II is composed of at least 12 different glycosyl residues linked together by more than 20 different glycosidic linkages. However, only a few of the genes and proteins required for RG-II biosynthesis have been identified. The demonstration that RG-II exists in primary walls as a dimer that is covalently cross-linked by a borate diester was a major advance in our understanding of the structure and function of this pectic polysaccharide. Dimer formation results in the cross-linking of the two homogalacturonan chains upon which the RG-II molecules are constructed and is required for the formation of a three-dimensional pectic network in muro. This network contributes to the mechanical properties of the primary wall and is required for normal plant growth and development. Indeed, changes in wall properties that result from decreased borate cross-linking of pectin may lead to many of the symptoms associated with boron deficiency in plants.

The Biochemistry and Structural Biology of Plant Cell Wall Deconstruction

“…Rhamnogalacturonan I displays a backbone composed of an alternating disaccharide, [(a-1,4)-DGalA/(a-1,2)-L-Rha] n , that contains extensive decorations at the O4 of the Rha residues (Mohnen, 2008). Rhamnogalacturonan II is the most structurally complex of the three pectic polysaccharides, consisting of 13 different sugars and over 20 different linkages (for an extensive review, see O'Neill et al, 2004).…”

Boron toxicity tolerance in wheat and barley: Australian perspectives

“…Examples of molecules that form complexes with boric acid include ribose, apiose, sorbitol and other polyols, glycoproteins and glycolipids (Loomis and Durst 1992). However, the binding of B to apiose, the central component of the rhamnogalacturonan-II complex in primary plant cell walls remains the only widely accepted role for B in plant nutrition (O'Neill et al 2004).…”

The Organization Pattern of Root Border-Like Cells of Arabidopsis Is Dependent on Cell Wall Homogalacturonan

“…The rhamnose residues are commonly substituted with polymeric b-(1/4)-linked D-galactosyl residues and/or a-(1/5)-linked L-arabinosyl residues (Ridley et al, 2001). Rhamnogalacturonan II is a highly complex but conserved molecule consisting of a homogalacturonan-like backbone substituted with four different side chains containing specific sugars (O'Neill et al, 2004).…”