with lignin. The plant cell wall may be imparting in structural integrity to support plasma membrane and sense external stimuli and mediate cellular signaling [6, 7].
The main components of plant cell wall including heterogenous mixture of polysaccharides, proteins, aromatic substances and water with trace numbers of ions. Specifically, the tensile strength of the cell wall of plant cells is due to cellulose, hemicellulose, pectic substances and their mutual interactions. Among the polysaccharide’s cellulose, hemicellulose and pectin are the most prominent composition of the cell wall with 30, 30 and 35%, abundance respectively. Cellulose and hemicellulose are responsible for rigidity in cell wall while pectin induces fluidity through gelatinous polysaccharides network (Figure 2.1) [2, 8]. The cellulose and hemicellulose embedded in the pectic are stabilized through proteins and phenolic compounds. Moreover, the hemicellulose attaches to the surface of cellulose channel to prevent the direct contact among the cellulose microfibrils while pectin is attached to hemicellulose to form gel phase [9–11]. Herein, different plant cell wall polysaccharides will be discussed.
2.2.1 Cellulose
It is considered the most frequently found polysaccharides in the nature which comprising 200–400 g/kg of the plant tissues. It is water insoluble, linear, unbranched structural polysaccharides with β-(1,4)-D-Glucose residues which are insoluble in water. Each D-glucose unit is rotated at 180° to its neighbor in a helical chain with length of 2,000–8,000 glucose units. Similarly, ~150 helical cellulose chain combined with each other through hydrogen binding for highly crystalline structure known as microfibrils. These microfibrils are approximately 3.6 nm wide and 100 nm apart without any gap, thus provide high mechanical strength and resistance to chemical and enzymatic digestion. The microfibrils have also some amorphous regions, that depend upon their source. Additionally, the algal cellulose has highly crystalline structure while the plants cellulose has least crystalline [5, 12].
Figure 2.1 The illustration depicts the cell walls of plant cell. The plant cell wall composed of cellulose microfibrils that have been cross linked with each other through glycans, pectin and other substances.
2.2.2 Hemicellulose
Hemicellulose is another heterogenous polysaccharide in plant cell wall with β (1–4) linked backbones of glucose, mannose, or xylose sugars. The term hemicellulose was coined at the archaic time when their structure and biosynthesis were not fully understood. By nature, it was not cellulose nor pectin, therefore the researchers have suggested the name of hemicellulose. Recently, the different features and structural properties of the hemicellulose has been explored but the term is still used by the researchers refereeing to the cell wall polysaccharides [13].
Hemicelluloses polysaccharides includes xylans, glucomannans, xyloglucans, mannans and β (1–3)–(1–4) glucans with the C1 and C4 equatorial configuration in their backbone. All these types of hemicelluloses are present in the cell wall terrestrial plants, expect of β (1–3)–(1–4) glucans, which only present in Poales (Order of flowering plants in the monocotyledons). The structure and abundance of the hemicellulose can vary among different special and cell wall types. The biological function of the hemicellulose is the strengthening of the cell wall of plant cells via deposition with lignin and cellulosic microfibrils [14].
2.2.2.1 Xyloglucan
Xyloglucan (XyG) is the most frequently occurring hemicellulose in the primary cell wall of the all land plants including mosses but not yet been found in the grasses and Charophytes. The hemicellulosic XyG have branched structure with alpha-D-xylose that is linked to C6 of glucose molecule. The XyG may be have β-D-galactose with less amount of L-fucose, (1–2) α-D-galactose and the galactose residues have been attached with acetyl moiety. In addition, the substituents of XyG are highly conserved and regulated during its biosynthesis. XyG is not found in the secondary cell wall although it plays a role in the interlacing of microfibrils in primary cell wall. Similarly, XyG plays an important role in the regulation of cell growth and expansion, particularly in the combination with enzyme such as xyloglucan endotransglycosylase. Most interestingly, the degraded XyG shows an anti-auxin effect, thus plays a pivotal role in cellular communication. It also helps in fruit maturing and fruit-ripening related softening [15–17].
2.2.2.2 Xylans
Xylans are also ranked as extensively found polysaccharide. It is abundantly found in the cereals, straw, sorghum, hardwood and grasses. Xylans are thought to be linked to the microfibrils of cellulose to contribute in their tensile strength properties, especially in second cell wall. Structurally, xylans have same structure like, (1–4) β connected xylose subunits with branching of (1–2) α bonded 4-O-methyl glucuronic acid and glucuronic acid subunits, respectively. The configuration and distribution of the attached substituents vary among different species. In some molecules, arabinose sugar residues have attached to the backbone of the xylans known as arabinoxylan which have been abundantly found in the endosperm of cereals. Similarly, secondary cell wall of the dicots have acetylated-xylans in which acetyl group attached to O-3 of xylose residues and very less common to O-2 [13, 18].
2.2.2.3 Mannans
Mannans have further two types. 1) Glucomannans which is an important hemicellulose that are abundantly found in the secondary cell wall of the gymnosperms and minorly in the secondary cell wall of angiosperms. Structurally, it consists of (1–4) β-linked mannose in the case of mannan while the glucose and mannose in non-repeating pattern in the case of glucomannans. In the angiosperms, a single galactose residue has been attached to the backbone of glucomannans that often known as galactoglucomannans. Functionally, glucomannans act as storage polysaccharides in the endosperm cell wall of many angiosperms. During the germination the seed-derived hydrolytic enzymes such as, beta-endomannanase, beta-endoglucanase, and alpha-galactosidase help in the deployment of glucomannan that also known as storage polysaccharides [13, 19]. 2) Glucuronomannans that primarily been found in the cell wall in very trace amount. They consist of α (1–4) linked D-mannose subunits with alternative (1–2) β-linked glucuronic acid residues. In branched side chain the arabinose and β-galactose have bonded to mannose via (1–6) and (1–3) linkages [13].
2.2.3 Callose
Callose is commonly used term for β (1–3)-linked D-glucan. It is plant derived linear polysaccharides that consist of hundreds of (1–3) β glycosidic connected glucose subunits. In comparison with cellulose that have long chain of thousands of 1–4 linked glucose residues with a crystal-like structure, Callose have relatively helical shapeless structure with 1–3 glycosidic bounds glucose residues [20].
In plants, Callose has been produced by specialized cells at different developmental stages of the cell wall, especially in cell plate formation during cell division, vascular bundles, pollen wall exine, etc. Interestingly, the Callose are deposit in the region surrounded the plasmodesmata in cell wall and then subsequently degraded, thus play important role in regulation of symplast transportation. Callose is also responsible for the formation and closing of pores in sieve plates in vascular bundles and play important role in an intracellular communication in plants. Callose is biosynthesized in plants through an enzyme known as Callose synthases and degraded by (1–3)-β-glucanase. Callose also play role in plant defense against unfavorable
