T. Matoh et M. Kobayashi, BORON AND CALCIUM, ESSENTIAL INORGANIC CONSTITUENTS OF PECTIC POLYSACCHARIDES IN HIGHER-PLANT CELL-WALLS, Journal of plant research, 111(1101), 1998, pp. 179-190
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+.