Investigating the role of flavonoid biosynthesis in coat-imposed dormancy to facilitate the breeding of white-grained varieties of wheat

The vast majority of bread consumed in the UK, whether made from white, brown or wholemeal flour, is produced from red-grained wheat. The 'red' colour of the grain (actually more of a reddish-brown) is due to the presence of coloured compounds, called flavonoids, in the seed coat. As well as colour, these flavonoids (related to the tannins found in tea and red wine) give wholemeal bread its somewhat bitter taste that is disliked by many people. For this and other cultural reasons, most bread produced in the UK and Europe is made from white flour, which has had all of the outer layers of the seed (the bran) removed during milling. However, removal of the bran results in a reduction in the nutritional quality of the grain, as much of the fibre, minerals, vitamins and other nutrients of the wheat grain are found in these outer layers. There is overwhelming evidence that consumption of wholemeal bread and wholegrain products containing these components contributes to a healthier lifestyle by offering protection against a number of diseases including diabetes, heart disease and bowel cancer. However, surveys suggest that one third of British adults and a quarter of children do not eat any wholegrain foods at all, and promotion of the health benefits of wholemeal grains has had relatively little effect on this sector of the population. White-grained wheats lack the red flavonoids of the seed coat and the associated bitter taste; wholemeal bread made from white-grained varieties has therefore been found to be more acceptable to some consumers; there also an appreciable market for wholegrain breakfast cereals, cakes and biscuits made from white wheat. Although white wholegrain products lack the health benefits imparted by the tannin components, they still contain far more fibre, micronutrients and phytochemicals than those made from refined flour. However, nearly all wheat varieties grown in the UK are of the red-grained type, as white grains are prone to germinating before harvest, a particular problem in countries such as the UK where cool, wet weather before harvest is common. This 'pre-harvest sprouting' results in a loss of grain quality and even a small proportion of sprouted grains can result a serious loss of value for the crop. For this reason, white-grained wheats are mainly grown in warmer, drier parts of the world such as Australia, necessitating the costly importation of grain by UK millers and bakers. In an existing programme that involves other academic partners as well as most of the UK's commercial wheat breeders, our colleagues are studying the underlying biology of pre-harvest sprouting. This current work is leading to the identification of UK wheat varieties that are more sprouting resistant and should provide information that will help breeders to improve their performance still further. Our new project has several aims: first, we want to develop genetic markers for the genes that confer the white grain colour, so that wheat breeders can more easily produce new white varieties with good sprouting resistance. Second, we want to establish whether breeding white grain colour into the sprout-resistant varieties identified above will generate sprouting-resistant white wheat lines that can withstand the UK's cool, wet climate. Finally, we want to explore the possibility that compounds earlier in the biochemical pathway that produces the red pigment are protective against sprouting, in which case we should be able to develop white wheats that are as sprout-resistant as red-grained varieties and also retain the antioxidant properties of the flavonoids. This project thus aims to provide the knowledge and tools to facilitate the development of white-grained wheats that can be grown reliably under UK conditions.

The consumption of wholegrains is known to have significant health benefits: there is evidence that the fibre and phytochemicals that are present in wholewheat but less abundant in white flour protect against bowel cancer, diabetes and other diseases. However, most consumers reject wholegrain products, at least partly because of the bitter taste imparted by the proanthocyanidin (PA) pigments in the bran. White-grained wheat, which lacks PAs, can be used to make wholegrain products that are similar in taste and appearance to those made from white, refined flour but contain higher levels of fibre and other nutrients. White grain colour also confers a higher yield and protein content of refined flour. However, white wheat is a difficult target for breeders as grain colour is a maternal character controlled by three homoeologous loci. Also, white grained wheat is prone to pre-harvest sprouting (PHS) in which grain germinates in the ear, resulting in amylase accumulation and loss of bread-making quality. The aim of this project is to underpin the genetic improvement of white wheats through the development of molecular markers for grain colour and by investigating the interaction between grain colour, dormancy and sprouting resistance. The R genes for grain colour encode transcription factors that control the PA biosynthetic pathway. We aim to survey the range of red and white alleles of the R genes present in UK germplasm and to develop allele-specific markers that can be directly utilised by breeders. We will assess the potential for improving the sprouting resistance of white wheats by introgressing white r alleles into PHS-resistant germplasm identified within the current HFN LINK programme. We will also identify genetic determinants that protect against sprouting in a mapping population of white wheats. In addition we will investigate alternative approaches to the development of PHS-resistant white wheat involving manipulation of the PA biosynthetic pathway.

We anticipate direct impacts on several sectors: Wheat breeders Breeding white wheats is currently difficult because grain colour is controlled by three independently segregating R (Red) loci; a single R gene is sufficient to confer red grain colour, but the genotype of most UK varieties is unknown. Furthermore, grain colour is a maternal character, and environmental effects on grain colour can make discrimination between red and white types difficult. These difficulties have severely limited the genetic improvement of white types. Wheat breeders will therefore clearly benefit from a survey of the R alleles present in UK germplasm and from the development of genetic markers to identify these alleles, which can be directly used in their breeding programmes for selection of parents and subsequent marker assisted selection. In addition, the white grain character is associated with reduced resistance to pre-harvest sprouting (PHS), so most wheat varieties grown in the UK are of the red type. However, within a current LINK project our LINK partners are assessing the sprouting tolerance of a wide range of UK germplasm. This has identified within mapping populations a range of sprouting resistance that exceeds that of the parents. Thus, a second objective will be to investigate the performance of white alleles in PHS-resistant backgrounds and to identify new genetic loci that determine sprouting sensitivity in white wheat under UK conditions. Together these will enable us to assess the potential for improving the sprouting resistance of white wheats. Commercial wheat breeders in the UK are fully supportive of this proposal as indicated by their willingness assist with marker validation, field phenotyping of a mapping population for sprouting and provision of germplasm (see example letter of support from KWS UK Ltd). Outputs from this programme will directly benefit the breeders and our continuing close interactions with this community through various existing programmes will ensure rapid take-up of these materials for wheat improvement. Growers and millers Pressure from consumers and health professionals for healthier versions of traditional white loaves has seen the development of high fibre white bread, achieved either through the addition of fibre or the use of flour milled from white wheat. As very little white wheat is produced in the UK most is imported from abroad at significant extra cost. The development of white wheats suitable for growing in the UK could see a rapid expansion in the market for such varieties, widening the choice for growers and millers. In addition, flour extraction rate is up to 7% higher for white-grained wheat than for red types. The protein within the wheat grain is also distributed non-randomly, with a higher concentration in the sub-aleurone close to the bran layers. Increased extraction of this material could significantly increase the protein content of white, refined flour, increasing its value. The development of white wheats suitable for the UK could therefore also increase extraction rates and protein levels for white flour, reducing wastage and increasing profits for growers and millers alike. UK consumers Although the consumption of wholegrains has been shown to protect against a range of diseases, white bread outsells wholemeal in the UK by a ratio of almost 10:1 and a large proportion of the population eat no wholegrain foods at all. Encouraging consumers to eat wholegrain products seems to have had limited impact on this sector of the population. Food made from white-grained wheat varieties would be more acceptable to consumers, while retaining many of the health-promoting properties of wholegrain, irrespective of the loss of the antioxidant properties of the proanthocyanidins. The development of white wheats for the UK could thus have enormous impact on the range of wholegrain products available, with the potential to confer significant health benefits on the population.