New Zealand Journal of Forestry Science 39 (2009) 153-160 http://nzjfs.scionresearch.com Mannans in primary and secondary plant cell walls† Laurence D. Melton1,*, Bronwen G. Smith1, Roshita Ibrahim1 and Roswitha Schröder2 1Food Science, Department of Chemistry, University of Auckland, Auckland, New Zealand 2New Zealand Institute for Plant and Food Research, Mt Albert Research Centre, Auckland, New Zealand (Submitted for publication 10 September 2008; accepted in revised form 18 May 2009) *Corresponding author: [email protected] Abstract A brief overview of the structure of mannans in plant cell walls and other organisms is presented. In particular, mannans, galactomannans and glucomannans in seed endosperm and vegetative tissues such as bulbs and tubers, galactoglucomannans (GGMs) in primary cell walls, and glucomannans and GGMs in secondary walls of hardwoods and softwoods are covered. Possible roles of mannans in primary plant cell walls other than as storage polysaccharides are discussed. Keywords: galactomannans; galactoglucomannans; mannans; plant cell walls; polysaccharides † Based on a paper presented at the 3 rd Joint New Zealand-German Symposium on Plant Cell Walls, 13-15 February 2008, Auckland, New Zealand. Introduction the walls of the Chinese caterpillar fungus (Cordyceps Mannans are widely distributed among living militaris) consist of a (1→4)-α- D-mannose backbone organisms. They are found as pure mannans branched at O-3 with side chains of (1→4)-α- D-glucose (homomannans), galactomannans, glucomannans and (1→6)-β-D-galactose residues with β-D-galactose and galactoglucomannans. Also, cell walls from at the terminal position (Yu et al., 2009). From the non-plant sources such as fungi or bacteria contain edible mushrooms, Auriculana auricula-judae, (Sone mannose-based polysaccharides. However, here the et al., 1978) and Tremella fuciformis (Kakuta et al., mannose residues are alpha-linked, whereas in the 1979) heteromannans have been isolated in which the plant cell wall, mannose residues in polysaccharides backbone is (1→3)-α-D-mannan. Lichen (Thamnolia are beta-linked. In yeast cell walls, including bakers’ vermicularis var. subuliformis) had a galactofuranosyl yeast (Saccharomyces cerevisiae) (Korn & Northcote, oligosaccharide linked to an (1→6)-linked α-manno- 1960), Kluyveromyces lactis (Raschke & Ballou, 1972) oligosaccharide (Omarsdottir et al., 2006). As the and Candida utilis (Ruszova et al., 2008), mixed structures of the mannans from such organisms linkage mannans containing (1→6) and (1→2)-linked are generally unrelated to those of higher α-mannosyl residues have been reported. plant cell wall mannans, they will not be Galactomannan has been isolated from cell walls of discussed further. It is noteworthy that the the fungus Lineolate rhizophorae (Giménez-Albian galactoglucomannan found in moss (Fontinalis et al., 2007) and from the bacteria Rahnella aquatilis (Zdorovenko et al., 2006) with both containing (1→6) antipyretica) has a backbone of (1→4)-linked and (1→3)-linked α-mannosyl residues. Mannans from β-mannose and glucose residues (Geddes & Wilkie, © 2009 New Zealand Forest Research Institute Limited, trading as Scion ISSN 0048 - 0134 154 Melton et al.: New Zealand Journal of Forestry Science 39 (2009) 153-160 1972). The remainder of this review will focus on. galactose side chains arranged to give hairy and mannans in primary and secondary plant cell walls. smooth regions (Dea & Morrison, 1975). The mannose to galactose ratio is 3.5 : 1 in locust bean gum (Daas et Storage Mannans, Galactomannans and al., 2000), while the mannose to galactose ratio in guar Glucomannans gum is 1.5 : 1, with the galactose side chain randomly attached throughout the polymer. Tara ( Caesalpinia Mannans, glucomannans and galactomannans occur spinosa) gum has a mannose to galactose ratio of 3 : 1 as storage polysaccharides in the endosperm cell (BeMiller, 2007) (Figure 1b). Konjac gum from corms of walls of seeds (mostly mannans and galactomannans) Amorphophallus konjac is a well known glucomannan, or in vegetative tissues such as bulbs or tubers and is utilised in the food industry (BeMiller, 2007) (mostly glucomannans) (Matheson, 1990). Their (Figure 1c). It has a mannose to glucose ratio of 1.5- principal chemical structures are shown in Figure 1. 2.0 : 1 with the β-D-mannopyranosyl units linked (1 →4) in a straight chain. The glucose sugars appear to be Mannan (a homopolymer of (1→4)-linked β-D-mannosyl randomly distributed along the chain (BeMiller, 2007). residues) (Figure 1a) has been found as the major For further information on storage mannans, consult component in endosperm cell walls of palm seeds Matheson (1990), Buckeridge et al. (2000), and such as ivory nut (Phytelephas macrocarpa), where it Srivastava and Kapoor (2005). can make up as much as 60% of the seed (Stephen, 1982; Matheson, 1990). Ivory nut mannan is highly Mannans in Primary Cell Walls water-insoluble, whereas the other storage mannans, galactomannan and glucomannan, are water-soluble. Apart from being storage polysaccharides, mannans are also structural polysaccharides in the primary plant Galactomannans found in seed endosperm such as cell wall. Although they are ubiquitous, they are usually locust bean (carob) (Ceratonia siliqua) gum and guar only present in small amounts. Generally, mannose is (Cyamopsis tetragonolobus) gum are well-known 2 mol % or less of the dry cell wall, and in some cases for their use in food products. They are found in the it was not detected (Thimm et al., 2002). For example, endosperm walls and vacuoles of the seeds (Brennan ripe kiwifruit (Actinidia chinensis) cell walls contain 1.2 et al., 1996). They consist of a (1→4)-linked β-mannan to 1.9 mol% depending on the tissue zone (Redgwell backbone with variable amounts of α-galactose et al., 1990) and cabbage (Brassica oleracea) cell residues linked (1→6) to mannose residues with the walls 2 mol% (Smith et al., 1998). A notable exception (a) Ivory nut mannan →4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→ (b) Tara gum (galactomannan) →4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→ (1→6)- -(1→6)- - p p -Gal -Gal D -D - α α (c) Konjac gum (glucomannan) →4)-β-D-Manp- (1→4)-β-D-Glcp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Glcp- (1→ FIGURE 1: Structures of: (a) ivory nut mannan, an example for a structure of a pure mannan; (b) tara gum, an example for a structure of a galactomannan; and (c) konjac gum, an example for a structure of a glucomannan. © 2009 New Zealand Forest Research Institute Limited, trading as Scion ISSN 0048 - 0134 Melton et al.: New Zealand Journal of Forestry Science 39 (2009) 153-160 155 is cell wall material isolated from ripe tomato which had isolated and characterised (Figure 2a) (Schröder et al., 11.7 mol % mannose (Seymour et al., 1990). The walls 2001). It has a mannose : glucose : galactose ratio of of the vegetative tissue of the palms Rhopalostylis 3 : 3 : 2. More recently, a GGM isolated from tomato sapida and Phoenix canariensis (Carnachan & Harris, (Lycopersicon esculentum ) peel and outer pericarp 2000) have been reported to contain small amounts has been shown to have a signifi cant amount of xylose, (2 mol % or less) of mannose. Fern walls have also with a mannose to glucose to galactose to xylose ratio been reported to contain mannans (Bremner & Wilkie, of 3 : 3 : 1 : 1 (Schröder et al., 2007). 1971). Popper and Fry (2003) found cell walls of some liverworts and mosses to be rich in mannose while the green algae Chara corallina and Ulva lactuca Mannans in Secondary Cell Walls contained much lower amounts. Interestingly, the β-D-mannan in green seaweed (Codium fragile) is It has been frequently noted that thickened cell walls found as crystalline microfi brils similar to cellulose contain higher levels of mannose. Arabidopsis thaliana microfi brils, therefore replacing cellulose as the stems which contain secondary cell walls were found principal skeletal component (Mackie & Preston, 1968). by one group (Brown et al., 2005) to contain ~ 8% of the noncellulosic neutral sugars in the cell wall material as Little work has been done on elucidating the structures mannose, while another group found ~7 mol% of the cell of mannans in primary cell walls. This is likely to be wall material (CWM) excluding cellulose was mannose due to a number of factors, the most obvious being (Harholt et al., 2006). However, a third group (Zhong the small amount of mannose present in primary cell et al., 2005) reported 4.8% mannose. Sclerenchyma walls (Thimm et al., 2002) and the diffi culty isolating cell walls in A. thaliana contain 5% mannose along them. A GGM from kiwifruit primary cell walls has been with 31% cellulose and 48% xylose (Ha et al., 2002). β -D (a) GGM from kiwifruit -Gal p -(1→4)- α - D α -Gal - D-Gal p -(1→6)- p -(1→6)- →4)-β-D-Manp- (1→4)-β-D-Glcp- (1→4)-β-D-Manp- (1→4)-β-D-Glcp- (1→4)-β-D-Manp- (1→4)-β-D-Glcp- (1→4)-β-D-Manp- (1→ (b) Typical gymnosperm GGM, acetylated and water-soluble α -D -Gal p -(1→6)- →4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Glcp- (1→4)-β-D-Manp- (1→ -(1→6)- -Acetyl- p O -Gal -D α (c) Typical gymnosperm GGM, water-insoluble →4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Manp- (1→4)-β-D-Glcp- (1→4)-β-D-Manp- (1→ -(1→6)- p -Gal -D α FIGURE 2: Structures of galactoglucomannan from: (a) kiwifruit primary cell walls (Schröder et al., 2001); (b) a typical GGM isolated from gymnosperm softwood, water-soluble through acetylation (Timell, 1967); and (c) a typical water-insoluble GGM isolated from gymnosperm softwood (Timell, 1967).