Plant cell primary pectic polysaccharides

The results of numerous studies have established that RG-II is present in the primary wall predominantly as a dimer that is cross-linked by a 1 2 borate-diol ester [11, 12]. A single borate ester cross-links two of the four apiosyl residues present in the dimer ([52] see Figure 5B). In vitro studies have shown that in the presence of boric acid and certain cations, two RG-II monomers rapidly self-assemble to form a dimer [52]. Moreover, the structure of RG-II itself may determine the location of the borate ester, since the location of the cross-link is the same in naturally occurring and in vitro-formed dimers. Thus, RG-II is the first example of a plant cell wall pectic polysaccharide that self-assembles to form structurally identical dimers [52]. The specificity and cation-dependence of this cross-linking suggest that there are distinct structural requirements for dimer formation and this may explain why the structure of RG-II is highly conserved in vascular plants [12, 53]. It is not known whether dimer formation in planta results from spontaneous self-assembly or is an enzymically catalyzed process. 

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 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. 

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]. 

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. 

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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