description
- Cereal grains are composed of three main components: husk, starchy endosperm and germ. The germ (also called the embryo) accounts for only a small proportion of grain weight (2-3% in barley and wheat) but compared to the larger endosperm, it is rich in nutrients. Very often, cereal grains are processed to produce white flour which involves removing the husk and germ. However, whole grains are richer than white flour in fibre, nutrients, vitamins and minerals and there is an increasing public demand for products made from whole grains. This is because we now know that people who eat whole-grain foods have a lower risk of almost all chronic diseases. Our project aims to increase germ size, thus enhancing whole-grain quality for human health and animal nutrition. It is a partnership between cereal scientists at NIAB, Cambridge and at the James Hutton Institute (JHI) in Dundee. Our project aims to identify the factors responsible for the reduction in relative germ size that has occurred during the domestication of cereals. It has been shown that endosperm size has increased during domestication in wheat, whilst germ size has remained constant. By comparing collections of wild and domesticated grains we hope to identify when and how relative germ size became sub-optimal during the course of domestication. Identification of the genetic factors responsible for this will provide the tools needed to allow rapid breeding for improved grain quality. Studies of variants of cultivated barley and other cereals (rice and maize) have shown that it is possible to drastically increase germ size (although this is accompanied by a decrease in endosperm size). At present three genes have been discovered that control relative germ size. These are: GIANT EMBRYO (GE) in rice and maize, BIG EMBRYO 1 (BIGE1) in maize and PROLAMIN-BINDING FACTOR (PBF) in barley. We will investigate these genes and the genetic and biochemical pathways in which they work. Our aim is to find a way to increase relative germ size without causing deleterious decreases in endosperm size, grain size or yield. Of the three genes known thus far to control germ size, PBF has the most commercial potential as it impacts on barley grain development only, without any detrimental effects on plant growth. However, the negative effect on endosperm size that accompanies reductions in PBF function is greater than that seen with GE and BIGE variants. To find a way to overcome this unfavourable impact on yield, first we will test whether it is possible to separate the favourable effects of PBF on the germ from the unfavourable effects on the endosperm using genetic manipulation. We will produce a barley plant with a functional PBF protein in the endosperm but lacking functional PBF in the germ. Secondly, we will investigate the mode of action of PBF to identify germ-specific and endosperm-specific target genes. Selecting for variation specifically in these target genes will allow plant breeders to optimize relative germ size. Finally, we will build on our knowledge of the control of germ size in barley and other cereals to produce wheat with improved nutritional value. We will transfer the trait for optimal germ size to wheat by selecting for variation in the three genes known to control this in rice, barley and maize: GE, BIGE1 and PBF. We will use publicly-available germplasm resources in durum/pasta wheat and bread wheat to identify variant genes and then combine these together in different combinations. The resulting large-germ wheat will be assessed for improved grain quality. This approach provides a non-GM route to deliver wheat plants with enhanced nutritional quality.