J.S. Swanston1, R.P. Ellis1 and S.A. Tiller2
Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK
1Dept. of Cell and Molecular Genetics.
2Dept. of Cellular and Environmental Physiology.
Swanston et al. (1995) compared inbred lines from a cross between a high amylose and a waxy parent. Four populations were obtained, characterised by expression of one, both or neither of the mutant genes. There were apparent additive effects of the two genes on a range of malting quality characters, including water uptake, cell wall modification and hot water extract. However, the high amylose lines were much higher in protein content than the waxy lines and this resulted in a tightly compacted matrix surrounding the starch granules, when observed by scanning electron microscopy.
The similarity of the effect on cell wall modification, for both waxy and high amylose, appeared to conflict with previous data on the major cell wall constituent, beta-glucan. Ullrich et al. (1986) showed the waxy gene to be associated with an increase both in beta-glucan content and in the viscosity of acid extracts of barley flour, associated with beta-glucan. By contrast Ellis et al. (1979) showed no increase in acid extract viscosity in several high amylose lines, although they did not make a direct measurement of beta-glucan content.
In the development of the extract viscosity test, Greenberg & Whitmore (1974) assumed that very little starch would come into solution, as the temperature used (40oC) was below that required for gelatinisation. However Stark & Yin (1986) showed that there was preferential extraction of amylopectin, in cold water, if starch granules were damaged, e.g. by milling grain to flour. Extract viscosity may not, therefore, be suitable for comparing lines differing widely in the proportion of amylopectin, so beta-glucan contents were determined, by the enzymic method of McCleary & Glennie-Holmes (1985), in the four populations of inbred lines, grown as previously described (Swanston et al., 1995). Results were expressed as a percentage of the dry weight of the grain. In addition, as the protein surrounding the starch was considered to be a barrier to enzymes modifying the endosperm during malting, it was decided to test if this also hindered starch extraction from the grain. Starch extractions were carried out on samples of 10g of grain, by the method of Sulaiman & Morrison (1990), with the weight of dried starch recorded.
Although the ranges of values showed slight overlap, mean beta-glucan contents of the waxy and high amylose populations were identical and higher than that of wild type lines (Table 1). Lines carrying both genes had a very high mean level of beta-glucan, exceeding a simple additive effect. A greatly reduced mean level of starch was extracted from the lines with both genes, but comparative ease of laboratory scale starch extraction, in waxy and high amylose types, was not affected by storage protein. Although high amylose starch granules appeared to be more tightly packed into the protein matrix than those of the waxy type (Swanston et al., 1995), similar levels of starch were obtained from the waxy and high amylose lines. This probably reflected reductions in grain size of similar magnitude, in comparison to the wild type (Swanston et al., 1995). Starch extraction may, however, be problematic in the combined waxy plus high amylose lines. Previous data (Swanston et al., 1995) showed them not to differ from the waxy types in grain size, or from the high amylose lines in grain nitrogen content, but the reduction, here, in extracted starch was much greater than that associated with either single mutant type.
Smith et al. (1987) suggested that high levels of beta-glucan may be associated with reduced starch content, due to genetic effects on partitioning of photosynthate. By restricting starch synthesis, the waxy and high amylose genes may have a similar effect. Increases in beta-glucan, associated with both genes, suggest that reduced cell wall modification (Swanston et al., 1995) may occur for the same reason in waxy and high amylose lines. However, although waxy types are unsuitable for malting, they are now considered to be a valuable source of soluble fibre for human diets (Newman et al., 1989). These preliminary data suggest that the high amylose and combined (waxy plus high amylose) types may also be worth investigating for their nutritional properties, as Bjork et al. (1990) suggested that dietary fibre was increased in high amylose Glacier and the resistant starch, which forms after processing of amylose, may also function in a similar manner to dietary fibre.
Table 1: Beta-glucan and extracted starch quantities from four populations of inbred lines from the cross Waxy Hector x BE285(High Amylose).
| Population Starch Type | No. of Lines> | Beta-glucan (% dry wt.) | Extracted starch (% dry wt.) | ||
|---|---|---|---|---|---|
| Mean | S.Dev. | Mean | S.Dev. | ||
| Wild type | 10 | 3.48 | 0.541 | 60.0 | 5.97 |
| Waxy | 20 | 4.36 | 0.562 | 54.1 | 7.23 |
| High amylose | 10 | 4.36 | 0.381 | 54.8 | 8.07 |
| Waxy + high amylose | 14 | 6.46 | 1.065 | 42.4 | 4.99 |
References:
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