Callose is an atypical plant cell wall polymer which accumulation is highly restricted being present around PD, transiently in cell plates, and in few specific cell types such as pollen tubes. Studies in different plant species indicate a role for callose in cell division, growth and differentiation, and in the response to biotic and abiotic stresses. In certain cell types (such as pollen cells, dividing cells and sieve elements), the amount of callose in the cell wall is naturally enriched (specialized cell walls) presumably to enhance resistance to mechanical stress and/or to provide isolation and protection from the surrounding environment. Exerting a similar protective/defensive function, callose accumulates when cells are under stress including wounding, exposure to toxic metals and pathogenic attack.
Callose structure, synthesis and mechano-physical properties: Callose is a linear (1 → 3)-β-D-glucan polymer that forms helical structures producing an amorphous material (Fig.1). The chemical and physical properties of callose have been inferred from studies in other 1,3-beta glucans of fungal or bacterial origin, such as curdlan and pachyman. X-ray crystallography indicates that these glucans have a triple-helix backbone conformation which is stabilized by hydrogen bonding. Gelation profiles, solubility and denaturation/renaturation kinetics vary depending on the carbohydrate structure, molecular weight, amongst other factors. More importantly, there is no consensus on the basic structural requirements for biological activity, thus the precise function of callose in cell walls cannot be extrapolated from these studies.
In plants, callose is synthesized from uridine diphosphate glucose by callose synthases (CALS) localized at the plasma membrane (25). Also at the plasma membrane, callose-degrading enzymes (named beta-glucanases, BGs) have been identified, providing the machinery for rapid callose turnover in the cell wall (Fig.1). Developmental and environmental cues influence the activity of these enzymes affecting callose levels in the cell wall.
The mechanical properties of callose in plant cell walls have been inferred from digestion studies in pollen. Force deformation graphs constructed from microindentation studies indicate that after callose degradation distal stiffness of pollen cell walls is reduced affecting overall viscoelasticity. Interestingly, these changes correlate with a reduction in the accumulation of methyl-esterified pectins which are reported to increase cell wall elasticity. These results question the direct role of callose in determining the mechanical properties of cell walls and highlight its potential to act as a matrix or scaffold for the deposition or synthesis of other cell wall materials. In support of this hypothesis, the deposition of callose in the cell wall surrounding plasmodesmata also correlates with changes in cellulose and pectin distribution. Pectins are a complex set of supramolecular polysaccharides, the major two components being homogalacturonan (HG) and rhamnogalacturonan-I (RG-I). In tomato fruit pericarp, low-esterified HG and the RG-I domain (1-5)-α-L-arabinan preferentially co-localize with callose whereas the RG-I domain (1-4)-β-galactan is specifically absent. This unique combination of pectins is expected to function with callose in regulating intercellular molecular transport.
In summary, more research is required to determine the specific structural and physical properties of callose alone, in composite materials and in cell walls. Studies so far indicate a role for this polymer in modifying cell wall architecture and mechanical properties but it is not clear if this effect is direct (gelling properties of callose act as a cell wall sealant) or indirect (act as a matrix for the deposition and/or for interactions with cellulose and pectins).