Cell Wall Polysaccharide Composition and Covalent Crosslinking

Abstract

Genetics now potentially lets us modify the production, crosslinking and degradation of cell wall polysaccharides. There remains, however, the need to test experimentally whether intended modifications of polysaccharide metabolism have successfully been effected in vivo . Simple methods for this are described, including in‐vivo radiolabelling, enzymic dissection (e.g. with Driselase) and chromatographic/electrophoretic fractionation of dissection products. After an overview of polysaccharide chemistry, I discuss the structures and taxonomic distribution of wall polysaccharides in charophytes and land plants. Primary and secondary walls are compared. The major wall polysaccharides are cellulose [microfibrillar β‐(1→4)‐ d ‐glucan], pectins (α‐ d ‐galacturonate‐rich) and hemicelluloses (lacking galacturonate; hydrogen‐bonding to cellulose; extractable by 6 M NaOH at 37 °C). Land‐plant pectins are anionic polymers built of about four glycosidically interconnected domains (homogalacturonan, rhamnogalacturonans I and II, xylogalacturonan). Hemicelluloses occurring in most/all land plants are α‐xylo‐β‐glucans, β‐xylans (including α‐arabino‐β‐xylans, α‐glucurono‐β‐xylans, etc.) and β‐mannans (including α‐galacto‐β‐mannans, β‐gluco‐β‐mannans, etc.). Another hemicellulose [mixed‐linkage β‐(1→3)(1→4)‐ d ‐glucan) is confined to Equisetum and some Poales. Other taxonomically restricted features of angiosperm primary walls occur in Poales (xylose‐poor xyloglucans; feruloylated arabinoxylans); Solanales and Lamiales (characteristic xyloglucans); Caryophyllales (feruloylated pectins); and Alismatales (apiogalacturonan). I also summarize characteristic wall features of charophytes, bryophytes, lycopodiophytes, fern‐allies and gymnosperms. The making or breaking of a ‘crosslink’ (defined as an individual chemical bond, not a whole ‘tethering’ chain) may cause wall tightening/loosening. Covalent crosslinks include phenolic coupling products, uronoyl esters and amides, and borate diesters.

A cell wall reference profile for Miscanthus bioenergy crops highlights compositional and structural variations associated with development and organ origin

“…Abundant MLG epitopes in 4 M KOH PC extracts suggest that all harvests contain proportions of tightly bound MLG, only released after delignification. High MLG detection in harsher extracts was also observed in glycome profiles of switchgrass (Shen et al ., ), sugarcane (de Souza et al ., ) and corn stover (Li et al ., ), and may derive from tight MLG–cellulose associations (Carpita, ; Fry, ; Kiemle et al ., ). XG epitopes, particularly nonfuc XG‐1, nonfuc XG‐2 and nonfuc XG‐3 groups of mAbs, were prominent in postchlorite extracts, suggesting that despite their reduced abundance in grasses, structural features associated with XG may inhibit sugar release from the cell wall matrix.…”

“…Xylans frequently contain acetyl, arabinosyl and/or glucuronyl substituents attached to some backbone xylose residues (Carpita, ; Scheller & Ulvskov, ), hence the designations, arabinoxylan (AX) and glucuronoarabinoxylan (GAX). Grass cell walls also contain small amounts of pectins, which are α‐galacturonate‐rich polysaccharides, and are thought to consist essentially of three interconnected domains joined together by glycosidic bonds: homogalacturonan (HG), rhamnogalacturonan‐I (RG‐I) and rhamnogalacturonan‐II (RG‐II) (O'Neill et al ., ; Fry, ; Atmodjo et al ., ). HG makes up most of cell wall pectin and comprises unbranched chains of α‐galacturonate residues, joined by 1→4‐bonds, which may be methyl‐esterified (Atmodjo et al ., ).…”

Implications of the galactosidase activity of yieldin in the regulatory mechanism of yield threshold that is fundamental to cell wall extension

“…Considering that the role of YLD is to control the yield threshold that is required to allow initiation of wall extension, the substrate of YLD is probably part of the load‐bearing network in cell walls. However, because the xyloglucan‐cellulose network (the main load‐bearing network in dicotyledonous plants) does not contain any alpha‐ d ‐galactose residues (Carpita and Gibeaut , Fry ), it is unlikely that yieldin liberates alpha‐ d ‐galactose residues from the hemicellulosic wall polysaccharides. Previous reports have shown binding of pectic polysaccharides to cellulose, suggesting that the pectin‐cellulose networks may bear the tension in extending plant cell walls (O'Neill et al , Vincken et al , Pena et al , Zykwinska et al , Marcus et al ).…”

Dual functions of Expansin in cell wall extension and compression during cotton fiber development

“…Chitin is a polymer of glucosamine and specifically present in some cell walls. Cellulose is a polymer of glucose featured with Micelle, Microfibril and microfibrils arrangements (Fry 2018). A bundle of nearly 100 cellulose chains gives rise to an elementary fibril structure called a micelle.…”

“…For this purpose, various cellular secretions with longer, stickier composition become involved in sticking up the polymers together. Pectin methyltransferases catalyze the formation of gel aggregates with pectin polymers (Fry 2018). Both esterified and non-esterified polymers clumped together through Calcium and ionic bridges and give rise to compact consistency.…”