Cell Wall Composition of Wheat

The wheat grain comprises three main parts which differ in their contents and composition of cell wall polysaccharides. The major storage tissue is the endosperm, which accounts for about 90 % of the dry weight of the mature grain (Barron et al. 2007). The outer layer of endosperm cells, called the aleurone layer, have thick cell walls which account for about 40 % of the dry weight. These surround the starchy endosperm cells, which have thin walls (about 2–3 % dry wt.) and contain mainly starch and storage proteins. The second major tissue is the embryo, which accounts for about 3 % of the mature grain (Barron et al. 2007). These tissues are sounded by several outer layers, the nucellar epidermis, testa and inner and outer pericarps.

Table 46.1 Compositions of cell wall types in wheat grain tissues (% dry weight)

Cellulose

Lignin

Xylan

β-glucan

Glucomannan

Refs.

Starchy endosperm

2

0

70

20

7

A

Total bran

29

8

64

6

B

Aleurone

2–4

0

62–65

29–34

C

Outer pericarp (beeswing)

30

12

60

D

References: A, Mares and Stone 1973; B, Selvendran et al. 1980; C, Bacic and Stone 1981; Rhodes and Stone 2002; Antoine et al. 2003; D, Du Pont and Selvendran 1987

On milling the outer layers, embryo and aleurone form the bran fraction and the starchy endosperm cells the white flour.

The major cell wall polysaccharides in the cell walls of wheat grain are arabinoxylan (AX) and (1,3; 1,4)-β-D-glucan (β-glucan). AX comprises a backbone of β-Dxylopyranosyl residues linked through (1,4) glycosidic linkages, with some residues being substituted with α-L-arabinofuranosyl residues, while β-glucan comprises only glucose residues which are linked by (1,4) and (1,3) bonds. AX accounts for

about 70 % of the total cell wall polysaccharides in white flour and 62–65 % in aleurone, while β-glucan accounts for about 20 % and 29–35 % in these tissues, respectively (Table 46.1). Cellulose is a minor component in both tissues (2–4 %) while the starchy endosperm cell walls also contain about 7 % glucomannan. By contrast, the composition of the outer bran layers has high contents of cellulose, lignin and complex glucuronarabinoxylans but no β-glucan (Table 46.1).

Genetic Variation and Heritability of AX Content

Analyses of wheat genotypes have shown extensive variation in the content and composition of AX in whole grain and white flour. A study carried out under the EU FP6 HEALTHGRAIN programme (Poutanen et al. 2008), showed that total DF varied from 11.5 % to 15.5 % dry wt (mean 13.4 %) in wholegrain of 129 wheat lines grown on the same site, while total AX (TOT-AX) ranged from 5.53 % to 7.42

% (mean 6.49 %) (Andersson et al. 2013). A more detailed study of 151 lines grown on the same site (including those studied by Andersson et al. 2013) compared the contents of TOT-AX and water-extractable AX (WE-AX) in flour and bran fractions (Gebruers et al. 2008; Ward et al. 2008). Although the bran fractions contained 13–22 % TOT-AX, WE-AX only ranged from 0.30 % to 0.85 %, corresponding to between 2 % and 5 % of the whole fraction. By contrast, white flour contained only 1.35–2.75 % TOT-AX but between 20 % and 50 % of this was water soluble (0.30–1.4 % dry wt. WE-AX). A small number of lines from this study were subsequently grown in an additional five environments (sites and/or years), allowing the broad sense heritability of the AX fractions to be determined. The ratios of genetic variance to total variance were 0.39 and 0.71 for TOT-AX in bran and flour, respectively, and 0.47 and 0.59 for WE-AX in bran and flour (Gebruers et al. 2010; Shewry et al. 2010). These high heritabilities, particularly for the flour fractions, indicate that the contents of both TOT-AX and WE-AX should be amenable to manipulation by plant breeding.

 
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