Rice endosperm, cell wall

Figure 1. Scanning electron micrograph of the cell wall preparations obtained from the different parts of rice grain (7). Caryopsis coat (upper left), aleuron layer (upper right), germ (lower left) and starchy endosperm (lower right). Bars in the picture indicate 5 /zm.

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. 

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.

A range of methods have been described in the literature, and we shall consider them in the following order (1) the methods developed at the Food Research Institute-Norwich (FRIN), to isolate cell walls from a range of vegetables and fruits (runner beans, potatoes, cabbage and apples), cereals (oats, wheat and rye) and cereal products (wheat bran and rye biscuits), and lignified tissues (parchment layers of runner bean pods) (2) the special techniques, which may include wet sieving steps, used for the isolation of cell walls from potatoes, wheat endosperm, and wheat and barley aleurone layers (3) alternative methods for the isolation of cell walls from starch and protein-rich products (rice) and (4) methods used for the isolation of cell walls from suspension-cultured tissues. 

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,... 

In the hemicellulosic fraction from rice endosperm cell walls, methylation and Smith degradation analyses as well as celiulase fragmentation studies have confirmed the presence of a mixed (1 3), (1 4)-/3-D-glucan in which blocks of... 

The oligosaccharides (11) and (12) have been isolated from the degradation products obtained from cellulase treatment of the L-arabino-D-xylo-D-glucan from tobacco leaves, Their structures were determined from the results of methylation analysis both before and after reduction with borohydride ion as well as by and n.m.r. spectroscopies. Present in the hemicellulose fraction from the endosperm cell walls of rice was a D-galacto-D-xylo-D-glucan. This polysaccharide was based on a (1-> 4)-j3-D-glucan backbone to which were attached residues of D-xylose and D-galactose at 0-6. 

The hemicellulose component of the endosperm cell walls of rice is mainly composed of L-arabino-D-xylan. This L-arabino-D-xylan is based on the usual (1 4)- 3-D-xylan backbone to which are attached L-arabinose residues at 0-3 of 78% of the D-xylose residues. ... 

In foods heteroxylans are mainly present in cereals, found in thin endosperm cell walls, the aleurone layer and lignified bran cells. The endosperm cell walls of most cereals contain 60-70% arabinoxylans, with 20% in barley and 40% in rice. Glumes (husks) of wheat grains contain about 64% heteroxylans. Wheat grains contain on average 1.4-2.1% of heteroxylans, of which 0.8-1.5% represent water soluble pentosans. Rice grains contain 7-8% heteroxylans. 

The endosperm cell walls start developing 3 to 5 days postanthesis. Cellulose is placed in cell walls starting in the periphery and gradually migrates to the central endosperm part. The mature cell walls of rice, maize, sorghum, and millets are thinner compared to wheat, barley, rye, and triticale. 

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