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 environmental conditions and phytopathogens [21, 22].
2.2.4 Pectic Polysaccharides
Pectic polysaccharides often know as pectin were first time discovered within tamarind fruits in the 18th century. Due to the peculiar nature no specific name was assigned to it. Until, the 19th century it was revealed that this substance is an active fruit component and responsible for gel formation, therefore they named it ‘pectin’ which mean congeal, curdle or solidify. Pectin is heterogeneous and there are more complex polysaccharides in nature that consist of α (1–4) D-galacturonic acid and rhamnose containing backbone, branched with side chains that have enrich with arabinose, galactose and trace amount of other sugars. Being complex in nature, pectin consists of seventeen different types of monosaccharides with more than twenty different types of linkages. Furthermore, pectin performs many biological functions like, cell to cell adhesion, cell wall expansion, cell wall porosity, bonding of ions, seed hydration, fruit development, pollen tube growth, lead abscission and modulation of growth regulators [7, 23, 24].
2.2.4.1 Homogalacturonan (HG)
HG is frequently existing pectin, nearly 60% present in plant cell wall. Structurally, it comprises of α (1–4) linked D-Glucuronic acid with linear chain. In the HG, the –COOH at C-6 of the α-D-Glucuronic acid are methyl-esterified, which conventionally known as high methyl-esterified HG [24, 25]. Homogalacturonan have further two types such as, Rhamnogalacturonan-I (RG-I) which represent 20–35% of pectin. The backbone of RG-I is linear, homologues and composed of the repeating units of galacturonic acid and rhamnose. At position O-3 and O-4 the rhamnose sugars are partially substituted with neutral glucosyl units with side-chain that frequently contain of α (1–5) L-arabinans, arabinogalactans and β (1–4) D-galactans [24, 26]. The second type of homogalacturonan is Rhamnogalacturonan II (RG-II). It is a branched pectic polysaccharides whose backbone is made of repeating units of α (1–4) D-Galacturonic acid α (1–2) L-Rhamnose. The branched side chains of the RG-II are consisting of 12 different glycosyl residues which are bonded together through 22 different glycosidic bonds. So, the covalent cross-linkage between RG-I, RG-II an HG in the cell wall and responsible for flexibility, toughness and dynamicity of the plant cell wall [24, 27].
2.2.4.2 Arabinan
Arabinan is also branched pectic sugar that has a backbone of α (1–5) L-Arabinose with helical structure. The branch consists of single chain of arabinose residues which are linked via (1–2) or (1–3) bonds. The branches of arabinan often degraded during cell expansion and fruit ripening [28, 29]. Arabinan have further two classes such as, Arabinogalactan I and Arabinogalactan II. AG-I is the most common arabinan which abundantly found in several fruits as well as in the cell wall of dicot plants. The backbone of AG-I is made of (1–4) linked β-D-galacturonic acid with small side chain having α (1–5) linked arabinose attached at the O3 position [30]. AG-II is highly complexed pectic polysaccharides that oftenly conjugated with proteins known as arabinogalactan proteins (AGPs). These proteins have 90% polysaccharides and less than 10% of amino acids. Structurally, AG-II has branched backbone which consisting of β (1–3), (1–6) linked D-Galacturonic acid subunits. AG-II has a small branch of one to two residues that also β (1–6) linked D-Galacturonic acid linkages [30, 31].
2.3 Algal Cell Wall Polysaccharides
Algae possess carbohydrates-rich cell walls which composed highly coordinated network of different polysaccharides, water and metals. All polysaccharides that present in algae, either they will be in the form of reserve food or structural component of their cell wall. Polysaccharides that present in algae, act as main representative of their corresponding taxa, and it could be speculated that algal cell wall polysaccharides also act as taxonomical and structural marker. Structurally, the algal cell wall has crystalline and fibrous polysaccharides like, cellulose, hemicellulose and xylans which have embedded in to jelly matrix of carboxylic and sulfated polysaccharides. Alongside the proteins, proteoglycan and phenolic also help in cell wall formation. Similar, different algae produce different polysaccharides such as, the cell wall of green algae have structurally different sulfated or carboxylic polysaccharides. The cell wall of red algae has composite structures of different sugars such as, xylans fibrils, mannan, cellulose, carrageenan and ager. The cell wall of brown algae is made up of cellulosic fibrils, alginates and fucoidans as matrix polysaccharides [32, 33].
Algal cell wall polysaccharides have significant diversity in structure, molecular mass, composition, glycosidic linkages, the arrangement of the sugar residues and the existence or configuration of the functional moieties such as, sulfate semi-esters, methyl-ether, carboxylates, acetyl-ester, etc. Also, the cell wall polysaccharides have linear or branched structure and it may be varied due to the size of side chains or the numbers of branching to internal or peripheral side [33, 34].
2.3.1 Alginates
It is matrix polysaccharides of the algal cell wall, especially in the cell wall of brown algae. Alginate have a linear structure and made of (1–4)-linked β-D-mannuronic acid subunits with epimerically attached α-L-guluronic acid at C-5 position. During biosynthesis of algal cell