Functional genomics of seed development in cereals

Abstract

In the past, the success of plant breeding in increasing cereal grain yields has been largely through the adoption of new varieties and changes in agronomic practices. Although the characteristics of cereal grains have changed over the past few decades of breeding and selection, there is still considerable scope for further improvement. For wheat, in particular, we still have only a poor understanding of the molecular basis for many of the quality characteristics that are so critical for the diverse end-uses of this cereal. Molecular markers have been important in defining many of the loci controlling quality but genomics approaches may help define the key criteria and the actual genes. A detailed understanding of the genetic control of grain quality and related characteristics also opens the path to exploring the wild relatives of our major cereals more comprehensively than has been possible in the past. An example was presented above where the knowledge of the role of β-amylase in determining malting quality allowed an exploration of wild barleys as a source of novel alleles. This led to the identification of more desirable alleles and also a better understanding of the structure and function of the protein. Can we use genomics to further expand the germplasm base for cereals? The structure of the grain also presents opportunities for improvement of grain quality, if we can define the molecular events involved in grain development. Grain shape is an important aspect of quality for all major cereals. The balance and composition of endosperm, embryo, aleurone and other tissues also impacts on the processing and nutritional quality characteristics of grains. Can these attributes be changed? For example, there exist large differences among cereal species in the overall composition of the grain. High oil lines of oat and maize are now available but there has been little progress in raising the oil content of rice or wheat. These features vary between cereals and varieties grown under optimal conditions, but we know that stresses, both biotic and abiotic, can have profound effects on the characteristics of grains. Again, genomics approaches offer a means for analysing the molecular events that occur when the plants are exposed to these stresses and may provide genetic or molecular genetic solutions. A further major target is the development of cereal grains for novel end-uses. The scope here is also large through the production of novel products, such as pharmaceuticals, or through the modification of existing components or shifting the balance of compounds. Altered starch composition has been a major target but alterations in cell wall constituents also offer new opportunities. In addition to the features of the grain that affect nutritional and processing quality, grain characteristics also have a large impact on the early development of the seedling. Even though wheat and barley are very closely related, barley seedlings show greater stress tolerance than wheat. This tolerance is at least partially due to the characteristics of the barley seedling such as long coleoptile, broad leaves and rapid germination. These features are all related to embryo characteristics and embryo size. Alterations in the early architecture of the plant may provide a mechanism for addressing several current production problems such as competition with weeds and more efficient use of surface water. The progress in developing apomictic cereals The progress in developing apomictic cereals has been discussed above. The initial proposal that this could be achieved by crossing the appropriate genes into the cereals from apomictic wild relatives has proved elusive and most research has returned to an analysis of early embryo and endosperm development. In summary, the opportunities for manipulating grain yield and quality and enhancing early seedling vigour are dependent on a detailed understanding of grain development. Genomics and the related technologies of proteomics and metabolomics provide the tools for elucidating the molecular events leading to grain formation.