Written by Sarah Kellett
Wheat is Australia’s main winter crop. Sown in autumn and harvested in spring or summer, it provides us with flour to make our daily bread. We eat wheat as toast with vegemite, or sandwiches for lunch. But wheat is not for everyone, as eating it can cause trouble for some people, such as those with coeliac disease.
Songs and hymns have been sung for bread, yet even today it has its mysteries. Like all living things, it contains a genome – the collection of its genetic material, such as DNA. Scientists have found the DNA sequence for the genomes of humans, lions, tigers and bears. Oh my, but bread wheat is a whole other story.
Tiny seed, giant genome
“With bread wheat, it is a big challenge,” says Ute Baumann at the Australian Centre for Plant Functional Genomics. “Bread wheat has a genome more than five times larger than the human genome. It’s massive. Rice is tiny by comparison.”
Why so big? Today’s bread wheat came from three ancestor species, which combined thousands of years ago and shared their DNA. Each grain of wheat contains three sub-genomes from the three ancestor species.
In July, an international team of scientists announced that they had made a rough draft of the whole genome of bread wheat, and a detailed version of one single chromosome, 3B. A team in Australia is working now to create a detailed version of chromosome 7A. “There are a number of important traits on chromosome 7A,” says Ute. “It has genes involved in salt tolerance, yield and flour quality.”
The rough draft was made using a method called shotgun sequencing, which breaks the genome into small parts and then sequences them. It’s like a big jigsaw, and the trick is putting all the little pieces together. It’s very difficult, because the wheat genome is very repetitive and a lot of the pieces are the same.
A more detailed version, like the one Australian scientists are working on for chromosome 7A, uses a method called BAC by BAC sequencing. BAC stands for bacterial artificial chromosome, and this method cuts the DNA into much larger sections and grows them in bacteria. This method is slower, but makes it possible to piece the whole chromosome together.
With a map of wheat’s genome, it’s much faster for plant breeders to grow varieties of wheat that are more productive, or better suited to Australian conditions. As the world’s population grows bigger and bigger, better bread wheat needs to be on the menu.
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Wow, wheat has a really big genome. Do you find it surprising that it’s five times larger than the human genome?
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