Pectins are a mixture of heterogenous polysaccharides rich in D-galacturonic acid; one major function
is to provide charged structures for ion exchange in cell walls (67)
. Under acidic conditions, aluminum
binds strongly to negatively charged sites in the root apoplast, sites consisting mostly of free carboxyl
groups on pectins. Klimashevskii and Dedov (103)
isolated cell walls from pea roots, exposed them
to aluminum, and found that aluminum decreased plasticity and elasticity of cell walls. Blamey et al.
demonstrated in vitro a rapid sorption of aluminum by calcium pectate and proposed that aluminum
phytotoxicity is due to strong binding between aluminum and calcium pectate in cell walls.
Reid et al. (105)
proposed that aluminum could disrupt normal cell wall growth either by reducing
concentration below that required for cross-linking of pectic residues or through formation of
aluminum cross-linkages that alter normal cell wall structure. Using x-ray microanalysis, Godbold and
showed that aluminum displaced calcium and magnesium from root cortical cell walls
of Norway spruce. Using a vibrating calcium-selective microelectrode, Ryan and Kochian (107)
observed that addition of aluminum commonly resulted in an initial efflux of calcium from wheat
roots, probably due to displacement of calcium from cell walls.
Pectin is secreted in a highly esterified form from the symplasm to the apoplast, where
demethylation takes place by pectin methylesterase (PME), resulting in free carboxylic groups
available to bind aluminum (108)
. Transgenic potato (Solanum tuberosum L.) overexpressing PME
is more sensitive to aluminum based on inhibition of root elongation relative to unmodified control
plants, indicating that increased binding sites for aluminum in the apoplast are associated with
increased aluminum sensitivity (108)
Modification of Synthesis or Deposition of Polysaccharides
In addition to external binding to cell wall components, aluminum also could interfere with the
internal synthesis or deposition of cell wall polysaccharides. Exposure of wheat seedlings to 10 µM
Al for 6 h decreased mechanical extensibility of subsequently isolated cell walls (109)
. Tabuchi and
showed that aluminum treatment modified cell wall components, increasing the
molecular mass of hemicellulosic polysaccharides, thus decreasing the viscosity of cell walls, and
perhaps restricting cell wall extensibility.
Uridine diphosphate glucose (UDGP) is the substrate for cellulose synthesis. Using 31P-NMR,
Pfeffer et al. (87)
demonstrated that a 20-h exposure of excised corn roots to 0.1mM Al decreased
UDGP by 65%, and they speculated that such suppression could limit production of cell wall polysaccharides.
In barley, one of the most aluminum-sensitive cereals, callose was excreted from the
junction between the root cap and the root epidermis after 38 min of exposure to 37 µM Al, and
Kaneko et al. (110)
proposed that aluminum-induced inhibition of root elongation could be due to
reduced cell wall synthesis caused by a shortage of substrate to form polysaccharides.