Cell-wall polysaccharides xyloglucans

This essay was written in an attempt to explain our overview of primary cell walls and to reach consensus on the nomenclature of primary cell wall polysaccharides. We present evidence supporting the hypothesis that cellulose, xyloglucan, arabinoxylan, homogalacturonan, RG-I, and RG-II are the six polysaccharides common to all primary cell walls of higher plants. In many cells, these six polysaccharides account for all or nearly all of the primary wall polysaccharides. Like the physically interacting proteins that constitute the electron transport machinery of mitochondria, the structures of the six patently ubiquitous polysaccharides of primary cell walls have been conserved during evolution. Indeed, we hypothesize that the common set of six structural polysaccharides of primary cell walls have been structurally... 

Xyloglucan is perhaps the most thoroughly understood of the non-cellulosic polysaccharides of primary cell walls 31). Xyloglucans were first characterized as an amyloid component of seeds 21, 60, 61, 79,... 

Considerably later, xyloglucans were isolated from the medium of suspension-cultured sycamore (24) cells, and, finally, from the primary cell walls of suspension-cultured sycamore cells (31). The basic structure of xyloglucans was elucidated by Kooiman (79) who studied the amyloid of Tamarindus indica seeds. The xyloglucans of primary cell walls were isolated and structurally characterized before it was recognized that the xyloglucans are very similar to the amyloids 31). The widespread occurrence of the amyloids 21, 67, 79, 115, 119) and xyloglucans 24, 29, 31) shows that polysaccharides isolated from tissues other than primary cell walls can, at times, serve as excellent models for the cell wall polysaccharides. 

The plant cell wall biology studies require more sensitive and specific probes to target individual wall components. Traditionally, antibodies have been the primary workhorses for the spatial localization of cell wall polysaccharides. Currently, nearly 30 monoclonal antibodies directed toward specific arabinan, galactan, xylan, galacturonan, fucosylated xyloglucan, and cell wall glycoprotein epitopes are available, from academic and commercial sources [172], Nevertheless, CBMs may be used for this purpose, since they present intrinsic specificity for individual carbohydrates. 

In the absence of suitable cell wall mutants, DCB-adapted tomato cells provide an opportunity to characterise the pectin network of the plant cell wall. It should be noted that synthesis and secretion of hemicellulose is not inhibited but, in the absence of a cellulose framework for it to stick to, most of the xyloglucan secreted remains in soluble form in the cells culture medium (9, 10) while other non-cellulosic polysaccharides and other uronic-acid-rich polymers predominate in the wall. 

Figure 3.5 A simplified model of the molecular architecture of a primary cell wall rich in pectic polysaccharides, such as a potato cell wall. Two co-extensive, but independent polysaccharide networks are shown a cellulose-xyloglucan network and a pectic-polysaccharide network. The middle lamella is located between the primary cell walls of adjacent cells and is responsible for cell-cell adhesion. Reprinted with permission from McCann and Roberts (1991).

The results of a recent study have called this into question. The primary walls of a double mutant of Arabidopsis thaliana in which two xyloglucan xylosyltransferase genes were not expressed were found to contain no detectable xyloglucan (Cavalier et al., 2008). Compared to the wild type, the double mutant was slightly smaller and had abnormal root hairs, yet there was no catastrophic effect on cell-wall integrity as would be predicted from current cell-wall models. A possible explanation is that the pectic polysaccharide network between the... 

Such products would be expected to arise from similar treatment of xyloglucans from dicots. However, if xyloglucan is present in primary cell-walls of monocots, it is clearly there in much smaller proportions than in dicot primary-walls, and it may not assume the structural significance that this polysaccharide has in the latter. 

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