Gene stacking may help fight off wheat stem rust

Australian researchers say bundling together disease resistance genes makes it more difficult for rust fungus to overcome the resistance

Researchers at Australia’s Commonwealth Scientific and Industrial Organization are trying to improve wheat rust resistance by stacking five resistance genes together.

Lead researcher Mick Ayliffe said bundling disease resistance genes together in wheat makes it harder for the rust fungus to overcome the resistance.

“Cereal rust diseases have always been big a big problem in wheat because they constantly evolve to overcome resistant wheat cultivars. That is, rust disease resistance lacks durability. We know that deploying a single resistance gene generally leads to it being overcome by the fungal rust disease in a short period of time.”

He said combining multiple resistance genes is possible through conventional breeding but it is difficult and expensive. As well, keeping those genes together in subsequent breedings is more difficult because they are usually located in different parts of the genome and therefore not always inherited together.

“We have overcome this problem by physically combining five different resistance genes and inserting them together in one region of the genome,” he said.

According to a CSIRO news release, a disease outbreak of one of the world’s most virulent strains of rust, Ug99, could cost the industry about C$1.8 billion over a decade.

Stripe, leaf and stem rust diseases already cause more than $1.3 billion in global crop damage annually with different strains of each fungal disease occurring around the world.

So far, the concept has proven consistently stable.

“The material that we tested in Minnesota field trials that performed so well had been through five generations and the genes were all stably inherited,” said Ayliffe. “They showed very high levels of field resistance in the fifth generation.”

Now wheat growers and the industry must decide if genetically modified wheat is a path they want to take. GM wheat is not commercially grown because many fear loss of market acceptance.

“We believe this is likely to change in the future and, in the interim, we are making more gene stacks using different genes and starting to target stripe rust as well as stem rust,” Ayliffe said. “This material also serves as an important backup should a catastrophic rust epidemic emerge. All the genes in our five-gene stack are derived from wheat or wheat relatives and all are being used in breeding programs.”

Dealing with pathogens also means dealing with their ability to mutate. Ayliffe said during the gene stacking research, a new isolate of stem rust was detected in Europe. It has already overcome three of the five resistance genes in the stack.

“This suggests that even gene stacks may be vulnerable to pathogen evolution,” he said. “However, now that we have developed this technology, we are in a position to rapidly make new stacks and possibly even larger gene combinations to keep ahead of the pathogen.”

He said with more resistance genes, the stack will become more stable and resistance will potentially last longer. That means making larger stacks is advantageous.

“The other way we are planning to increase the number of genes is by combining two separate gene stacks by conventional breeding, which requires only a modest breeding effort.”

Moving forward, scientists are making larger gene stacks and targeting not only stem rust disease but also wheat stripe rust using new genes and new gene combinations. They are also starting to combine gene stacks by breeding.

The research was published in Nature Biotechnology.

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