Dicotyledonous plants primary cell-wall polysaccharides

Hemicelluloses have traditionally been defined by extraction procedures, i. e., hemicelluloses are those polysaccharides extracted by alkaline solutions from plant tissues after removal of low-molecular-weight substances with hot aqueous alcohol, removal of waxes and other lipid-soluble substances, delignification, and removal of pectin with an aqueous solution of a calcium ion chelator. The most abundant hemicelluloses in the primary cell walls of dicotyledons are xyloglucans. Xyloglucans have a backbone chain whose chemical stmcture is identical to that of cellulose. About 75% of the 8-D-glucopyranosyl units in that chain are substituted... 

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. 

A hitherto unobserved component of the primary cell walls of dicotyledonous plants, rhamnogalacturonan II, has been isolated and partially characterized. It is a very complex polysaccharide containing residues of ten different monosaccharides including o-apiose, 2-O-methyl-D-xylose, and 2-O-methyl-L-fucose. The polysaccharide, which accounts for 3—4% of the primary cell walls of suspension-cultured sycamore cells, is also characterized by the presence of 2-linked D-glucosyluronic acid, 3,4-linked L-fucosyl, and 3-linked L-rhamnosyl residues. These linkages have not previously been detected in polysaccharides of sycamore primary cell walls. Evidence was also presented that similar polysaccharides are present in the primary cell walls of pea, pinto bean, and tomato. 

Arabans have been isolated from the cell walls of many dicotyledonous plants. Until recently, no homo-araban has been isolated specifically from primary cell walls. However, methylation analysis of the walls of suspension-cultured sycamore 123) and pea (55) cells strongly suggested that these primary cell walls possess arabans which are structurally similar to arabans obtained from other tissues or organelles. Recently, an araban, essentially free of other polysaccharides, has been isolated from a methylated sycamore cell wall polysaccharide fraction 5). 

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