1. Introduction
About 7,000 species of plants are grown by farmers somewhere in the world. Of these, only 30 species provide 90 percent of our calorific intake. The three main crops are wheat, rice and maize.
They occupy 35% of all global arable land, wheat being cultivated on 230 million hectares (mio ha) followed by rice and maize with 151 and 140 mio ha respectively (FAOSTAT, 2005).
The importance of bread wheat (Triticum aestivum L.) as a food crop originates mainly from the viscoelastic properties of the gluten proteins in the seed's endosperm that allows the dough to be expanded during fermentation and enables bread production.
For thousands of years, improvement of wheat was achieved by careful selection of the best grain from well-adapted plants. During the last century, new wheat varieties were produced by selected crossing and breeding.
Desirable traits such as ease of harvesting, shorter growing seasons, better milling and baking qualities, improved disease resistance and higher yields are combined by crossing selected wheat varieties.
It takes about ten to twelve years of testing at a cost of around 0.5 mio USD for a new variety to be released (NAMA, 2005).
For breeders and the wheat industry, the time and high costs are an incentive to accelerate the development of new elite varieties. This is achieved in modern breeding programmes by marker-assisted selection (MAS).
For diagnostic purposes, DNA is isolated from plants from which selection is to be conducted, e.g. different varieties or accessions of wheat, and is screened for sequence variations.
The knowledge that a certain DNA sequence variation is linked to a desirable trait in the plant allows the breeder to select the most promising plants within large populations without waiting for their performance in the field.
Selection takes place in the laboratory. To make marker-assisted selection efficient, the whole genome of a plant has to be covered by markers.
For wheat with its large hexaploidy genome (about 40 times the size of rice or 5 times the size of the human genome) this is a major task taken on by the wheat science community worldwide.
The application of markerassisted selection and the introgression of chromosome regions from wild relatives were particularly important for the success of wheat breeding.
Besides MAS, biotechnology offers another tool for improving wheat, namely genetic engineering (Langridge et al., 2001).
In future, wheat breeding will have to keep responding to a growing world population that has chosen wheat as one of the most favoured staple foods at a time when the increase in yield is slowing down worldwide (FAOSTAT, 2005).
In addition, wheat varieties will have to serve a wider range of end uses with differing but specific quality requirements.
As outlined by wheat breeder K. Brunckhorst (see chapter Wheat History and Kernel Composition ), current farming practices tend towards wheat growing without crop rotation and with reduced tillage, mainly for economic reasons.
Both factors promote the development of soilborne diseases and require varieties equipped with corresponding resistances.
All of these demands can only be addressed if a high level of genetic diversity exists from which the breeder can choose.
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