Endosperm cell walls

Xyloglucans are classified as gum when they are extractable with hot water from seed endosperm cell walls, such as the tamarind seed xyloglucan, and as hemicelluloses because they are alkali-extractable from the cell walls of vegetative plant tissues where they are closely associated with cellulose [2]. Also /3-glucans with mixed linkages appear under the name gum as well as hemicellulose in the literature. 

Comparison of the Overall Composition of Cell Wall Preparations. As can be seen from Table I, cell wall preparations showed a significant difference in their polymer composition. The endosperm cell walls resembled primary walls, since they were virtually free of lignin but rich in pectic substances. On the other hand, the cell wall preparations obtained from the caryopsis coat and the aleuron tissue were highly lignified, and their pectic content was very low. The germ cell wall showed a somewhat intermediate composition between these two types, probably reflecting the fact that it consists of several different tissues. 

While the hemicelluloses obtained from the germ, aleuron, and caryopsis coat cell walls all showed a similar monosaccharide composition, this was not the case for the endosperm tissue. Thus, a major difference in the structure of hemicellulosic polysaccharides exists between the preparations obtained from the endosperm cell walls and those from the cell walls of the other parts of the grain, i.e., rice bran. (Rice bran consists of the caryopsis coat, aleuron layer and germ.) Comparison of the detailed structural features of the hemicellulosic polysaccharides of endosperm and bran cell walls will be discussed in the following sections. 

The sugar composition of the pectic polysaccharides obtained from these cell wall preparations (Table II) also suggested differences in the structural features of the different cell wall preparations. Detailed structural information is only available for the pectic polysaccharides obtained from the endosperm cell wall The main fraction of endosperm pectic polysaccharide was separated into two fractions, a neutral sugar-rich fraction and a fraction with a very high content of D-galacturonic acid (8). Structural... 

Figure 3 shows the structure of the hemicellulosic polysaccharides obtained from the bran cell wall preparations. These structures were deduced from the results of the methylation analysis of purified fractions (13). In contrast to the endosperm cell wall, no / -l,3-,l,4-glucan was obtained from... 

Figure 2. Structure of hemicellulosic polysaccharides obtained from the rice endosperm cell wall, (a) From ref. 12 (b) and (c), deduced from the results described in ref. 11.

Endosperm cell-walls of barley and oats Palm kernels, lupin seed... 

The subject of the secondary protein structure as a means of defining the performance characteristics ofwheat endosperm-known as hardness-been explored over a seven year period [39, 40]. Another approach, taken by Baron et al., involves the IMS imaging of the endosperm cell walls rather than of the protein found in the endosperm itself [61]. All of these authors performed the imaging in situ, following removal of the protein and starch, in order to study the compositional and architectural heterogeneity and, in relation to this, wheat hardness. In this case, the research was focused on kernel hardness rather than on endosperm hardness, as was the case with our studies. A further study of carbohydrate polymers by the same group involved the investigation of cereal arabinoxylans in relation to their structure and physico-chemical properties. 

Barron, C., Parker, M.L, Mills, E.N.C. and Rouau, X. (2005) FTIR imaging of wheat endosperm cell walls in situ reveals compositional and architectural heterogeneity related to grain hardness. Planta, 220, 557-77. 

A method has been developed by Mares and Stone (1973a) for the isolation of wheat endosperm cell walls using 70% aqueous ethanol as the isolation medium. Aqueous ethanol is used instead of an aqueous medium to minimize loss of water-soluble polymeric components present in the native walls. In this method the cell walls are separated from starch and protein on the basis of their size difference. The endosperm components present in a coarse wheat flour are suspended in 70% aqueous... 

Upper layer contains fine white suspension of endosperm cell walls (yield (1.5-1.8 g from 100 g wheat flour)... 

Fig. 2. Procedure for isolation of wheat endosperm cell walls.

Rice endosperm cell walls have been prepared by grinding the rice flour (obtained from the milled grains) to pass through a 50-mesh sieve and then treating the flour sequentially as follows (1) with cold water and then 1 1 ethanol/ether to defat the flour, (2) with a-amylase to remove starch and isolate the water-soluble polysaccharides from the supernatant fluid,... 

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