Callose and Related Glucans


Callose is a water‐insoluble, linear (1→3)‐β‐d‐glucan found among embryophytes. It occurs in specialized cell walls of reproductive tissues, as a transient component of the cell plate in dividing cells and as deposits on plasma membranes following wounding or in physiological or pathological stress. Related glucans are found as bacterial capsules, as storage polysaccharides in euglenoid protozoa and chromistans (brown algae, yellow‐green algae, diatoms, water moulds and haptophytes), and as wall components and surface mucilages of yeasts and fungi.

Keywords: chemistry; conformation; microfibrils; polymorphic forms; taxonomic distribution; cell walls; carbohydrate reserves; biosynthesis; depolymerization; plant defence; immunomodulation; applications; animal defence

Figure 1.

(a) A (1→3)‐β‐d‐Glucan. In the unbranched chain, the β‐d‐glucopyranose units are joined by glycosidic linkages between the hemiacetal hydroxyl at C1 on one glucose unit and the hydroxyl at C3′ on the next unit. The glucopyranose units are in the chair conformation, designated 4C1 (carbon 4 high and carbon 1 low). Each carbon on the glucose ring carries an axial hydrogen atom. For clarity these are omitted. (b) Portion of a side‐chain‐branched (1→3, 1→6)‐β‐d‐glucans with a single β‐d‐glucopyranose unit attached to the (1→3)‐β‐d‐glucan backbone through a (1→6)‐glucosidic linkage. The degree of side branching varies with the source and may be as low as one in every five units and as high as two in every three glucose units. In some examples, the side branches are two or three unit (1→3)‐β‐d‐oligoglucosides. (c) Cyclic (1→3,1→6)‐β‐d‐glucan from B. japonicum. The macrocycle is composed of two blocks of three (1→3)‐linked glucose units separated by a block of three (1→6)‐linked glucose units with a single branch glucose unit at C(O)6. Some molecules are substituted at C(O)6 by phosphocholine. The diameter of the macrocycle is 1.5 nm. (d) Cyclic (1→3)‐β‐d‐glucan from a B. japonicum ndvC mutant A1 and a recombinant strain of R. meliloti  TY7 composed of ten glucose residues and an extracyclic (1→6)‐β‐linked laminaribiose residue. (e) The capsular polysaccharide of S. pneumoniae type 37 is composed of a (1→3)‐β‐d‐glucan backbone with every residue substituted by a (1→2)‐linked‐β glucopyranose residue.

Figure 2.

(a) Curdlan triple helix. For clarity, only one of the three helical chains is shown in detail. Only the backbones of the other helices are shown. Each helix has a repeat of six residues. (b) End on view of a curdlan triple helix showing two triads of C(O)2 hydrogen bonds holding the individual helical chains together. Reproduced from Deslandes Y, Marchessault RH and Sarko A (1980) Macromolecules 13: 1466–1471.

Figure 3.

(a) Capsules of Agrobacterium sp. strain ATCC 31749 stained with the (1→3)‐β‐d‐glucan specific Aniline Blue fluorochrome (Fluorescence micrograph, McIntosh M). (b) Microfibrils of curdlan in the capsule of Agrobacterium sp. strain ATCC 31749. Bar, 200 nm (Electron micrograph, McIntosh M). (c) Section through an E. gracilis paramylon granule showing the very regular concentric and parallel orientation of the (1→3)‐β‐d‐glucan microfibrils. Reproduced from Kiss JZ, Roberts EM, Brown RM Jr and Triemer RE (1988) Protoplasma 146: 150–156. (d) Chrysolaminarin (leucosin) in the vacuoles of the diatom, Thalassiosira pseudomonana, detected with a gold‐labelled (1→3)‐β‐d‐glucan antibody. Reproduced from Chiovitti A, Molino P, Crawford SA, Teng R, Spurck T and Wetherbee R (2004) European Journal of Phycology 39: 117–128.

Figure 4.

A model for the ultrastructure of an oriented curdlan gel at room temperature. The micelles forming the gel average about 80 Å in diameter and the intermicellar distance is about 120 Å. The space between the micelles is occupied by water molecules (W). The micelles consist of single helical molecules hydrogen‐bonded to one another and to water molecules. Some parts of the micelles are occupied by triple‐stranded helices. Reproduced from Kasai N and Harada T Ultrastructure of curdlan. In: French AD and Garner KH (eds) Fibre Diffraction Methods, pp. 363–383. American Chemical Society Washington DC (American Chemical Society Symposium Series, No141).

Figure 5.

Model of the cell wall of the yeast S. cerevisiae. The branch‐on‐branch (1→3,1→6)‐β‐d‐glucan forms the central component of the wall. The proportions of (a), (b) and (c) chains are about equal but their exact lengths are not known. The nonreducing ends of (a) and (b) chains are attachment sites for chitin chains at the plasma membrane surface of the wall and for the reducing ends of (1→6)‐β‐d‐glucan chains at the outer surface of the wall. The (1→6)‐β‐d‐glucan chains are in turn substituted by mannoproteins through the C‐terminal amino acid of an amino acid‐ethanolamine‐phosphodiester‐(Man)5 remnant of a GPI anchor, Pir cell wall proteins (not shown) are linked directly to (1→3,1→6)‐β‐d‐glucan. Chitin chains may also be attached to the (1→6)‐β‐d‐glucan chains (not shown). After Manners DJ, Masson AJ and Patterson JC (1973) Biochemical Journal 135: 19–30; de Nobel H et al. ; Kollár R, Reinhold BB, Petráková E et al. (2001) Journal of Biological Chemistry 272: 17762–17775.

Figure 6.

(a) Special callose walls around the tetrad of microspores in Helloborus foetidus stained with the Aniline Blue fluorochrome. Reproduced from Waterkeyn L (1961) Comptes Rendus des Séances de l’Académie des Sciences, Paris, Series D 252: 4025–4027. (b) Longitudinal section through a pollen tube growing in the style of Nicotiana alata showing the callose‐rich, inner wall layer detected with gold‐labelled (1→3)‐β‐d‐glucan antibody. tt, transmitting tract; em, extracellular matrix; pt, pollen tube. Bar, 1 μm. Reproduced from Meikle PJ, Bõnig I, Hoogenraad NJ, Clarke AE and Stone BA (1991) Planta 185: 1–8. (c) Callose deposition at the cell plate (arrow) in tobacco BY2 cells during cytokinesis detected with the Aniline Blue fluorochrome. N shows positions of daughter nuclei. Reproduced from Verma DPS and Hong ZL Plant Molecular Biology (2001) 47: 693–701. (d) Plasmodesmatal callose in Echium wildpreti petioles. Right: plugs of callose, penetrated by plasmodesmata, fill the pores. Left: pores open, lined with thin layer of callose, and most are occluded with P‐protein, some with starch grains. Reproduced from Botanical Society of America, Katherine Esau. (e) Callose in developing cotton (Gossypium hirsutum) seed hairs at 40–50 days postanthesis. Left: bright field view. Right: stained with the Aniline Blue fluorochrome. An innermost layer of callose lines the lumen as secondary wall thickening is proceeding. 500× on the original. Reproduced from Waterkeyn L (1981) Protoplasma 106: 49–67. (f) Wound callose at the edges of a cut site on leaves detected with the Aniline Blue fluorochrome. Bar, 200 μm. Reproduced from Jacobs et al. . (g) Papillary callose in leaf cells at sites of penetration by the powdery mildew fungus, Blumeria graminis. Left: light microscope image, Coomasie Blue stain sp, conidiospores; sh, secondary hyphae. Right: callose deposits (arrows) detected with Aniline Blue fluorochrome. Bar, 100 μm. Reproduced from Jacobs et al. .

Figure 7.

(a) Experimentally derived membrane topology of the Agrobacterium curdlan synthase protein, CrdS, showing the cytoplasmic region with catalytic and substrate‐binding residues. Reproduced from Karnezis et al. . (b) Deduced membrane topologies of Arabidopsis thaliana callose synthase‐associated (AtGsl5) protein and Saccharomyces cerevisiae FSK1 protein. Arrows indicate proposed UDPGlc‐binding sites. Reproduced from Ostergaard L, Petersen M, Mattson O and Mundy J (2002) Plant Molecular Biology 49: 559–566.

Figure 8.

Schematic representation of interactions of a fungal pathogen with a plant host cell showing potential roles of fungal (1→3,1→6)‐β‐d‐glucans and derived oligoglucosides, host (1→3)‐β‐d‐glucan hydrolases and their fungal inhibitors. Adapted from Stone BA and Clarke AE (1992) The Chemistry and Biochemistry of (1→3)‐β‐Glucans. Melbourne, Australia: La Trobe University Press and Rose et al. .



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Further Reading

Dijkraaf GJP, Li H and Bussey H (2001) Cell wall glucans of Saccharomyces cerevisiae. In: De Baets S, Vandamme E and Steinbüchel A (eds) Biopolymers, Polysaccharides II: Polysaccharides from Eukaryotes Chap. 7, pp. 179–213, Germany: Wiley‐Vch.

Giavasis I, Harvey LM and McNeil B (2001) Scleroglucan. In: De Baets S, Vandamme E and Steinbüchel A (eds) Biopolymers, Polysaccharides II: Polysaccharides from Eukaryotes Chap. 2, pp. 37–60, Germany: Wiley‐Vch.

Lee I‐Y (2002) Curdlan. In: Vandamme EJ, De Baets S and Steinbüchel A (eds) Biopolymers, Polysaccharides I: Polysaccharide from Prokaryotes Chap. 5, pp. 135–157. Germany: Wiley‐Vch.

McIntosh M, Stone BA and Stanisich VA (2005) Curdlan and other bacterial (1→3)‐β‐d‐glucans. Applied Microbiology and Biotechnology 68: 163–173.

Rau U (2001) Schizophyllan. In: De Baets S, Vandamme E and Steinbüchel A (eds) Biopolymers, Polysaccharides II: Polysaccharides from Eukaryotes Chap. 3, pp. 61–91. Germany: Wiley‐Vch.

Stone BA and Clarke AE (1992) The Chemistry and Biochemistry of (1→3)‐β‐Glucans, p. 803. Melbourne, Australia: La Trobe University Press.

Zhang H, Nishinari K, Williams MAK, Foster TJ and Norton IT (2002) A molecular description of the gelation mechanism of curdlan. International Journal of Biological Macromolecules 30: 7–16.

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Stone, Bruce(Jan 2006) Callose and Related Glucans. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0004111]