Dietary Fibre: Wheat Genes for Enhanced Human Health
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Abstract Dietary fibre (DF) has been shown to be a vital component of diet for human health, decreasing the risk of cardiovascular disease, type II diabetes and possibly bowel cancer. DF in wheat flour is derived from the cell walls of the starchy endosperm, which is principally composed (~70 %) of the polysaccharide arabinoxylan (AX). Diversity screens of elite wheat germplasm have established that variation in total and water-extractable AX within flour exists and has high heritability. Identification of genes which determine AX content will assist in introduction of high DF alleles into appropriate backgrounds. We identified candidate genes for the synthesis and feruloylation of AX from bioinformatics approaches. Using RNAi suppression of genes in wheat endosperm, we have shown that a glycosyl transferase (GT) family 61 gene is responsible for nearly all mono-substitution of xylose by arabinose on AX, and that genes in GT43 and GT47 families are responsible for the synthesis of the xylan backbone in AX. Using mapping populations derived from crosses of high AX x normal AX varieties, we are also seeking to identify QTLs for high AX. This combination of forward and reverse genetics will accelerate the introduction of the high fibre alleles into modern commercial wheat varieties.
Wheat as a Source of Dietary Fibre
In addition to providing calories and protein to much of the world's population, wheat is also an important source of dietary fibre (DF), with bread products alone contributing 20 % of the DF in the adult UK diet. There is also a significant shortfall in the intake of DF in many countries. For example, in the UK the daily intake of DF is only about 13 g per day (Buttress and Stokes 2008) compared with a recommended daily intake of 25–40 g a day in most other countries. Most of the DF consumed is non-starch polysaccharides, with resistant starch contributing about 20–25
% of the total in European diets (Cummings 1983). There is strong evidence that DF in wholegrain provides protection against the risk of a number of chronic diseases, including type 2 diabetes, obesity and cardiovascular disease, with both soluble and insoluble forms contributing to the beneficial effects (Slavin 2004; Topping 2007; Wood 2007; Lunn and Buttriss 2007; Buttress and Stokes 2008; Anderson et al. 2009; Fardet 2010). DF is not digested in the upper gastrointestinal (GI) tract and many of the beneficial effects result from fermentation in the colon. However, it also has effects throughout the GI tract, with insoluble fibre providing faecal bulk to speed up transit, while soluble fibre may increase the viscosity of the digesta to reduce the rates digestion and the uptake of nutrients in the small intestine, and hence lower the glycaemic load. The wide consumption and low cost of staple foods made from wheat mean that it is an excellent vehicle to deliver the health benefits conferred by DF to large populations at low cost. However, in order to achieve this it is necessary to increase the fibre content of wheat and, in particular, in the part of the grain that is most widely consumed: the starchy endosperm which forms white flour on milling.