Covalent Cross-Linking of Primary Cell Wall Pectic Polysaccharides is Required for Normal Plant Growth

Abstract

Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide that is present in the primary walls of all higher plant cells. Recent research has revealed much about the structure and function of RG-II. RG-II exists in the wall predominantly as a dimer that is cross-linked by a 1:2 borate diol ester. The formation of the RG-II dimer in muro is proposed to generate a covalently cross-linked pectic network that contributes to the physical and biochemical properties of the wall. We have investigated the function of this pectic network using a dwarf Arabidopsis mutant (mur1) that synthesizes RG-II with an altered glycosyl residue composition and a dwarf Arabidopsis mutant (bor1) that is defective in root-to-shoot translocation of boron. The walls of these mutants contain reduced amounts of the RG-II dimer. The amounts of borate cross-linked RG-II in the walls and morphology of mur1 and bor1 plants sprayed with boric acid are comparable to wild-type plants. Our study demonstrates a major role for borate cross-linking of primary wall pectic polysaccharides in plant growth.