Among 16 essential elements of higher plants, Ca2+ and B have been termed as apoplastic elements This is mainly because of their localization in cell walls, however, it has turned to be highly likely that these two elements significantly contribute to maintain the integrity of cell walls through binding to pectic polysaccharides.
Boron in cell walls exclusively forms a complex with rhamnogalacturonan II (RG-II), and the B-RG-II complex is ubiquitous in higher plants. Analysis of the structure of the B-RG-II complex revealed that the complex contains two molecules boric acid, two molecules Ca2+ and two chains of monomeric RG-II. This result indicates that pectic chains are cross-linked covalently with boric acid at their RG-II regions. The complex was reconstituted in vitro only by mixing monomeric RG-II and boric acid, however, the complex decomposed spontaneously, unless Ca2+ was supplemented. Furthermore, the native complex decomposed when it was incubated with trans-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% and 6% of Ca2+, B and pectic polysaccharides of the cell walls, respectively, were released and the cell wall swelled twice. Subsequent extraction with 50 mM CDTA (pH 6.5) of the SDS-washed cell walls further released 4%, 80% and 61% of Ca2+, B and pectic polysaccharides, respectively. Pectinase hydrolysis of the SDS-treated cell walls yielded a B-RG-II complex and almost all the remaining Ca2+ was recovered in the complex. This result suggests that cell-wall bound Ca2+ is divided into at least two fractions, one anchors the CDTA-soluble pectic polysaccharides into cell walls together with B, and the other may control the properties of the pectic gel.
These studies demonstrate that B functions to retain CDTA-soluble pectic polysaccharides in cell walls through its binding to the RG-II regions in collaboration with Ca2+.