BORON AND CALCIUM, ESSENTIAL INORGANIC CONSTITUENTS OF PECTIC POLYSACCHARIDES IN HIGHER-PLANT CELL-WALLS

Among 16 essential elements of higher plants, Ca2+ and B have been ter med as apoplastic elements This is mainly because of their localizatio n in cell walls, however, it has turned to be highly likely that these two elements significantly contribute to maintain the integrity of ce ll walls through binding to pectic polysaccharides. Boron in cell wall s exclusively forms a complex with rhamnogalacturonan II (RG-II), and the B-RG-II complex is ubiquitous in higher plants. Analysis of the st ructure of the B-RG-II complex revealed that the complex contains two molecules boric acid, two molecules Ca2+ and two chains of monomeric R G-II. This result indicates that pectic chains are cross-linked covale ntly with boric acid at their RG-II regions. The complex was reconstit uted in vitro only by mixing monomeric RG-II and boric acid, however, the complex decomposed spontaneously, unless Ca2+ was supplemented. Fu rthermore, the native complex decomposed when it was incubated with tr ans-1,2-diaminocyclohexane-N, N, N', N'-tetraacetic acid (CDTA) which chelates Ca2+. When radish root cell walls were washed with a buffered 1.5% (w/v) sodium dodesyl sulfate (SDS) solution (pH 6.5), 96%, 13% a nd 6% of Ca2+, B and pectic polysaccharides of the cell walls, respect ively, were released and the cell wall swelled twice. Subsequent extra ction with 50 mM CDTA (pH 6.5) of the SDS-washed cell walls further re leased 4%, 80% and 61% of Ca2+, B and pectic polysaccharides, respecti vely. Pectinase hydrolysis of the SDS-treated cell walls yielded a B-R G-II complex and almost all the remaining Ca2+ was recovered in the co mplex. This result suggests that cell-wall bound Ca2+ is divided into at least two fractions, one anchors the CDTA-soluble pectic polysaccha rides into cell walls together with B, and the other may control the p roperties of the pectic gel. These studies demonstrate that B function s to retain CDTA-soluble pectic polysaccharides in cell walls through its binding to the RG-II regions in collaboration with Ca2+.