title: Synthesis of borate cross-linked rhamnogalacturonan II.
abstract: In the present review, we describe current knowledge about synthesis of borate crosslinked rhamnogalacturonan II (RG-II) and it physiological roles. RG-II is a portion of pectic polysaccharide with high complexity, present in primary cell wall. It is composed of homogalacturonan backbone and four distinct side chains (A-D). Borate forms ester bonds with the apiosyl residues of side chain A of two RG-II monomers to generate borate dimerized RG-II, contributing for the formation of networks of pectic polysaccharides. In plant cell walls, more than 90% of RG-II are dimerized by borate under boron (B) sufficient conditions. Borate crosslinking of RG-II in primary cell walls, to our knowledge, is the only experimentally proven molecular function of B, an essential trace-element. Although abundance of RG-II and B is quite small in cell wall polysaccharides, increasing evidence supports that RG-II and its borate crosslinking are critical for plant growth and development. Significant advancement was made recently on the location and the mechanisms of RG-II synthesis and borate cross-linking. Molecular genetic studies have successfully identified key enzymes for RG-II synthesis and regulators including B transporters required for efficient formation of RG-II crosslinking and consequent normal plant growth. The present article focuses recent advances on (i) RG-II polysaccharide synthesis, (ii) occurrence of borate crosslinking and (iii) B transport for borate supply to RG-II. Molecular mechanisms underlying formation of borate RG-II crosslinking and the physiological impacts are discussed.
authors.affiliation: Division of Biosphere Science, Graduate School of Environmental Science, Hokkaido University , Sapporo, Japan.
authors.forename: Hiroya
authors.initials: H
authors.lastname: Funakawa
authors.affiliation: Division of Biosphere Science, Graduate School of Environmental Science, Hokkaido University , Sapporo, Japan ; Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency , Kawaguchi, Japan.
authors.forename: Kyoko
authors.initials: K
authors.lastname: Miwa
keywords: Arabidopsis thaliana; borate; glycosyltransferase; homogalacturonan; pectin; primary cell wall; rhamnogalacturonan-II; transporter