Wild wheat, plant genomics, and food security
Posted 24th February 2020 by Liv Sewell
Wheat is the most widely grown crop in the world, providing 20% of the calories and proteins consumed by humankind. More than one fifth of the projected yield is lost every year to disease. Dr Brande Wulff, Group Leader at the John Innes Centre, and speaker at the 8th Plant Genomics and Gene Editing Congress, Europe, is leading research aiming to reduce the proportion of wheat lost to pathogens and increase global food security…
When wheat was domesticated 10,000 years ago, it went through a genetic bottleneck whereby important diversity for disease resistance and other traits was lost. The diversity is retained in the wild ancestors of wheat in the Fertile Crescent, where wheat was domesticated. Our aim is to discover and harness the wild genes to improve cultivated wheat. We have developed new methods that allow us to very rapidly clone disease resistance genes. Once the genes have been molecularly identified, this opens up several avenues for mobilizing the genes into cultivated wheat.
Identifying disease resistance genes
The wheat genome is very large and complex and until recently it was fiendishly difficult to clone a gene in wheat. We have developed enabling technologies that combine genetic structuring with sequencing and bioinformatics to accelerate gene discovery and cloning in complex genomes. In one method we mutagenize seed from a resistant plant, grow up the mutant population, and then screen for plants that are no longer resistant. If we sequence the DNA from multiple, independently derived mutants, and look for a gene which has been mutated in all of them, then that is, most likely, the disease resistance gene. In this way, we can arrive at a single gene out of the 120,000 genes in the wheat genome. Another approach takes advantage of the naturally occurring genetic structure accumulated over eons in a wild wheat population. By sequencing the individuals in such a population and then correlating their disease resistance profiles with their DNA sequences we can sometimes arrive at the genes which underpins the resistance.
It’s quite expensive to sequence a whole wheat genome. However, we can reduce the complexity using hybridisation-based enrichment for the part of the genome encoding immune-receptors. This makes it cheaper and computationally easier to deal with.
Introducing resistance genes into domesticated crop varieties
Once we have identified a resistance gene, we can use transformation to introduce the gene into the genome of a cultivated wheat variety. However, a wheat line with just one resistance gene would most likely be overcome very quickly through evolution of resistance-breaking strains of the pathogen. Therefore, we aim to create a stack of multiple resistance genes – from first principle, this should make it very difficult for the pathogen to overcome, providing much more durable resistance in the field.
Another way to introduce disease resistance into cultivated wheat is by crossing the wild wheat with its elite domesticated brethren. This is akin to crossing a racehorse with a donkey and it takes many years of backcrossing and cleaning up to create a crop which combines the best of both worlds. This problem is exacerbated when trying to introduce several genes at once, such as when generating a stack. An important limitation is the long generation time of wheat – about five months. This is why we developed methods to accelerate the growth of wheat.
Speed breeding to introduce resistance
To reduce the limitation of generation time, we worked with Dr Lee Hickey and others to develop a method – dubbed ‘speed breeding’ – for growing wheat and other crops much faster in a controlled environment. In speed breeding, the plants are provided intense red and blue light, the part of the spectrum optimal for photosynthesis. We also increase the day length to 22 hours and pamper them with a rich nutrient regime. In these conditions we can reduce the generation time from about five months down to eight weeks, allowing a theoretical maximum of six generations of wheat in a year.
Thanks to its simplicity, the technology has been adopted by colleagues and researchers around the world. Dr Lee Hickey has written about how he is utilising speed breeding at the University of Queensland in a recent post. The formula that we’ve come up with works across the board for a lot of different species including barley, oats, pea, grasspea, brassicas, and others.
The future of GM wheat
To take full advantage of rapid gene cloning technologies, we need to overcome the social and political stigma associated with GM. There is so much scope for creating crops that are more nutritious, better for the environment, that can be grown more sustainably, and produce higher yields. If we could create crops with greater disease resistance, we would not need as much pesticide to maintain the health of these plants. While GM acceptance appears to be rising globally, the EU has dug its heels in and ruled that even genetically engineered crops be regulated in the same way as GM crops.
It is possible that the UK government will change the regulations surrounding the use of GM and gene-edited crops in the UK after Britain’s exit from the European Union. However, a lot of the agricultural products produced in the UK are exported to the European Union and if the European Union does not accept gene edited products, the UK would lose that export market. If we are to see more genetically engineered crops grown in the UK it will have to depend on the crop: if the crops are used or consumed locally or exported to other parts of the world where gene-editing is not regulated the same way, this could work. But it will be on a crop-by-crop basis.
Collaboration is the future
As I look ahead to the future, I would love to see an international consortium come together, working on wheat, to clone a very large number of disease resistance genes against the major diseases of wheat such as Septoria tritici blotch, Fusarium head blight, the three rusts, wheat blast, etc; and then create an atlas of cloned disease resistance genes. This resource could be used by breeders, and those institutions generating transgenic stacks, to allow exploitation of this very precious genetic diversity in a much more judicious manner. If that could happen, I think we could very significantly reduce the reliance on pesticides for growing wheat.
Dr Brande Wulff is Group Leader at the John Innes Centre in Norwich, UK.
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