Primary pectic polysaccharides

Rhamnogalacturonan 11 (RG-11) is a structurally complex, pectic polysaccharide that is present in the primary cell-walls of higher plants. It is composed of 60 glycosyl residues, and is a very complex molecule indeed. For example, on acid hydrolysis, at least ten different monosaccharides are formed, including the novel aceric acid (30), which is the only branched-... 

The pectin network.-The second polysaccharide network present in primary cell walls is composed of pectic polysaccharides. The pectin network appears to coexist with the cellulose/hemicellulose network, that is, both networks appear to be able to share the same space [16-19]. However, the proportions of the two networks appear to vary from location to location within a single cell wall as well as from the primary wall of one type of cell to the primary wall of a another type of cell [9,20-22]. 

There are three pectic polysaccharides in all primary cell walls that have been studied these are rhamnogalacturonan n, rhamnogalacturonan I, and homogalacturonan. 

The pectin network revisited.--The importance of the interconnections of the pectic polysaccharides to the integrity of the pectin network has been highlighted by the recent discovery that RG-II is present in primary walls as a mixture of monomers and dimers [54]. The dimers are covalently cross-linked by borate diesters [55,56]. If single molecules of homogalacturonan are covalently attached to both RG-I and RG-II, the covalently cross-linked RG-n dimers would explain how the network of the three types of pectic polysaccharides is covalently connected and covalently cross-linked. 

The primary walls of growing plant cells are composed of 90% carbohydrate and 10% protein (51). Carbohydrate in the primary wall is present predominantly as cellulose, hemicellulose, and pectin. The pectic polysaccharides, are defined as a group of cell wall polymers containing a-l,4-linked D-galactosyluronic acid residues (62,76). Pectic polysaccharides are a major component of the primary cell waU of dicots (22-35%), arc abundant in gymnosperms and non-graminaceous monocots, and are present in reduced amounts (-10%) in the primary walls of the graminaceae (27,62). 

O Neill, M., Albersheim, P., and DarviU, A. (1990) The pectic polysaccharides of primary cell walls. In Methods in Plant Biochemistry, Volume 2, edited by P.M. Dey, pp. 415-441. Academic Press, London. 

The plant cell wall contains different types of polysaccharides, proteins (structural glycoproteins and enzymes), lignin and water, as well as some inorganic components (1, 14-16). The plant cell suspensions, however, grow as a population of cells with a primary cell wall(17). The main components of these walls are cellulose-free polysaccharides and pectic polysaccharides in particular, which constitute 1/3 of their dry weight. (18). Some fragments, e g. methanol, acetic, ferulic and p-cumaric acids, are connected with the pectic polysaccharides by ester bonds with the carboxylic and hydroxylic groups. 

Figure 3.5 A simplified model of the molecular architecture of a primary cell wall rich in pectic polysaccharides, such as a potato cell wall. Two co-extensive, but independent polysaccharide networks are shown a cellulose-xyloglucan network and a pectic-polysaccharide network. The middle lamella is located between the primary cell walls of adjacent cells and is responsible for cell-cell adhesion. Reprinted with permission from McCann and Roberts (1991).

The results of a recent study have called this into question. The primary walls of a double mutant of Arabidopsis thaliana in which two xyloglucan xylosyltransferase genes were not expressed were found to contain no detectable xyloglucan (Cavalier et al., 2008). Compared to the wild type, the double mutant was slightly smaller and had abnormal root hairs, yet there was no catastrophic effect on cell-wall integrity as would be predicted from current cell-wall models. A possible explanation is that the pectic polysaccharide network between the... 

Pectic polysaccharides in Rosa glauca bark157 and lucerne leaves and stems139 have been shown to contain 3- and 6-linked galactosyl residues. The sycamore, primary cell-walls contain appreciable proportions of terminal and 3-, 6-, 3,6-, and 2,6-linked galactosyl residues,55 125 and it is possible that many of these residues are components of an arabinogalac-tan (see later). 

The possibility of other types of noncovalent interactions between the pectic polysaccharides and other cell-wall polymers must be borne in mind,30 251 but there is at present little evidence to indicate that such interactions do exist in the primary cell-wall (see also, Section IX,3). 

There is evidence684,685,685 for the attachment of phenolic components (ferulic and coumaric acids) both to / -(l— 4)-linked D-galactose and a-(l— 3)-linked L-arabinose in the primary cell-wall, suggesting feruloylation-coumaroylation of pectic neutral side-chains. Earlier papers686-688 had also suggested the attachment of these phenolic compounds to primary-wall polysaccharides which remained uncharacterized. 

Fractionation and Chemistry of Citrus Pectic Polysaccharides. Pectic polysaccharides, commonly known as pectin, appear early in plant cell-wall formation. A series of complex biochemical steps results in the formation of cell plates followed first by its growth in area (primary cell wall) then in thickness (secondary cell wa.ll). Exclusive of randomly oriented cellulose fibrils, primary cell wall is composed mainly of pectic polysaccharides (34). These pectic polysaccharides are rich in D-galacturonic acid, D-galactose and L-arabinose residues. With growth in thickness of cell wall (secondary cell wall),there appears to be a replacement of pectic polysaccharide deposition with polysaccharides rich in D-glucuronic acid or 4-0-methyl-D-glucuronic acid,... 

Plants contain signiFcant concentrations of polysaccharides of which the potentially negatively charged oxygen functions can bind cations electrostatically or chelate them via polyhydroxy groups [89]. Particular attention was attracted by a structurally complex pectic polysaccharide rhamnogalacturonan-II (RG-II) [90]. This ubiquitous component of primary plant cell walls forms dimers cross-linked by 1 2 borate diol esters (dRG-II) that were found to complex in vitro sped be divalent cations and the majority of Ba, Pb, Sr, and rare earth elements (REEs) in fruit and vegetables [45, 91]. 

Chapman, H.D., Morris, V.J., Selvendran, R.R. and O Neill, M.A. 1987. Static and dynamic light scattering studies of pectic polysaccharides from the middle lamellae and primary cell walls of cider apples, Carbohydr. Res., 165 53-68. 

The primary cell wall of dicotyledonous plants consists of cellulose microfibrils dispersed within a matrix of predominantly non-cellulosic polysaccharides, including xyloglucans and pectic polysaccharides. The xyloglucans are neutral polysaccharides which bind to the cellulose microfibrils through secondary interactions, and have the ability to crosslink the fibrillar cellulose network. This fibrillar network is then dispersed in a network of the pectic polysaccharides.1 The pectic polysaccharide network also forms the middle lamella in dicotyledons and is responsible for cell-cell adhesion. 

Main characteristics of the three major domains of pectic polysaccharides found in the primary cell walls of... 

After 70 years of effort, a biochemical function for boron in plants was identified. In primary cell walls, boric acid cross-links two chains of pectic polysaccharide at the rhamnogalacturonan-II region through borate-diester bonding between two side-chain apiosyl residues, thus forming a network of pectic polysaccharides (O Neill et al. 1996, Matoh 1997). Boron is suspected to have additional essential functions... 

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