Print edition | Science and technology - The Economist
Aug 18th 2018
ONE of the most surprising things about the announcement this week that the genome of wheat has been fully mapped is how long it has taken. As well as the human genome, a draft of which was completed in 2000, scientists have tackled everything from rice to the clearhead icefish and the black cottonwood tree. The world’s most widely cultivated crop has taken all this time because it was really difficult; the “Mount Everest” of plant genetics, according to some.
That difficulty arises from the fact that wheat is not one genome but three overlapping and similar ones, the result of natural hybridisation. It is more than five times the size of the human genome and comprises some 107,000 genes (humans have about 24,000). Genomes are generally figured out by breaking them into smaller pieces, sequencing those pieces and then working out how they fit together. With so many similar-looking sequences, the international team of researchers, whose findings were reported in Science and whose efforts focused on a variety of bread wheat called Chinese Spring, had a huge job on their hands.
Their achievement comes at an opportune time. Humans have been tinkering with wheat for almost 10,000 years, but new tools are becoming available for the precise manipulation of genomes. Gene-editing using a technique called CRISPR, along with a fully annotated genetic sequence, promises a new era in wheat cultivation, introducing traits to improve yields, provide greater pest resistance and to develop hardier varieties.
Of particular interest will be how decoding the genes might contribute to understanding, and perhaps even mitigating, various immune diseases and allergies associated with eating bread. This possibility is explored by Angela Juhász of Murdoch University, in Western Australia, and her colleagues in an associated paper in Science Advances.
Coeliac disease, for instance, is an immune reaction to eating gluten; the related genes are the glutenins and gliadins that are expressed in the starchy endosperm of the wheat grain. A different set of allergens, including amylase trypsin inhibitors found in a thin layer of cells that surround the endosperm, are implicated in an illness called baker’s asthma; these could be of concern to people who suffer non-coeliac wheat sensitivity.
One possibility is using diagnostic techniques to identify wheat varieties that contain gluten which is easier to digest, says Rudi Appels, another of the associated paper’s authors. Normally the gut can break down the large proteins found in gluten, but when this process fails and those proteins arrive in the lower gut they interact with the gut’s membrane and cause immune problems. In the future, says Dr Appels, wheat might also be fine-tuned to be less allergenic. This might be done by editing the wheat genome so that it contains more digestible proteins.
Those who have trouble with gluten may find, however, that the source of their problem lies more in the processing of bread rather than the genetics of wheat. Dr Appels says that many commercial bakers use processes that eliminate part of the traditional fermentation stage in making bread. And his guess is that this fermentation would have broken up the problematic proteins into smaller and more digestible pieces. This might explain why some coeliacs (and, indeed, some others who complain that bread is indigestible) can happily eat sourdough bread, which is still made using traditional methods.
It also seems that non-coeliac wheat sensitivity might not be due to gluten at all, but a poorly absorbed carbohydrate component of wheat: fructans and galacto-oligosaccharides, along with another allergen, the amylase trypsin inhibitors which are implicated in activating the innate immune system. Again, fermented bread may have fewer of these hard-to-digest bits. Now that scientists have the genome, such theories should be easier to prove.