Blast from the past

A major breakthrough in efforts to halt the progress of wheat blast, a growing threat to international food security, has come from a surprising source.

New research unexpectedly reveals that wheat varieties with resistance to another pathogen, powdery mildew, also offer protection against wheat blast.

When looking for disease resistance, it is common to look among old landraces or varieties from regions where the disease originated. Since wheat blast is a disease of humid subtropical regions, efforts to control the disease have focused on finding resistance genes among wheat varieties adapted to warmer climates.

Using gene discovery methods developed at the John Innes Centre, they have identified the first gene that protects wheat plants against strains of the blast fungus containing the protein effector. AVR-Rmg8.

Surprisingly, find, located on chromosome 2A of the wheat genome, IS 4 in the afternoona gene that gives wheat resistance to powdery mildew, a disease of the colder, wetter climates of the northern hemisphere.

European plant breeders have selected wheat with 4 in the afternoon for many years for resistance to powdery mildew; now those in the southern hemisphere will be prompted to do the same as wheat blast protection.

“These findings were completely unexpected and they suggest that if you want to find resistance to wheat blast, you should also look at varieties that come from non-tropical regions where they already have resistance to the mildew,” said Professor Paul Nicholson, a leader of the the group. at the John Innes Center and coordinator of the study, which also includes contributions from Mexico-based CIMMYT and Saudi Arabia-based KAUST.

“We have to be open to the idea of ​​looking in unusual places because blast is a disease of high-temperature, high-humidity environments while mildew is a disease of high-humidity, low-temperature environments, so nobody he wouldn’t have thought to look at European varieties before because one looks for commonalities”.

The research team made the discovery by examining over 300 wheat varieties in the Watkins Collection, a panel of diversity collected from around the world in the 1930s. Of this population, only three percent showed resistance to pathogenic wheat blast strains that produce AVR-Rmg8.

Disturbingly, all varieties that were highly resistant to the blast carried it 4 in the afternoon gene, indicating that only a single resistance was present among this highly diverse population. This highlights the need to identify additional resistances to ensure strong and durable resistance to this new threat.

The team will now use the same gene discovery methods to search among wheat varieties bred in Europe for further blast resistance genes, increasing the genetic armory that can be used against this devastating disease.

Dr Tom O’Hara, lead author of the study, said: “This is the first cloned blast resistance gene – unlike previous blast resistances, we have been able to pinpoint the exact gene – even identifying small variations in the gene that make it dysfunctional. This means that our findings could be of great immediate benefit to breeders.

“Our goal from the beginning was to find resistance that could be established in Bangladesh and potentially in other countries where the outbreak has spread. The added pleasure is that our study has taken an unexpected turn.”

Wheat powdery mildew resistance gene 4 in the afternoon also gives resistance to wheat blast appears in Plants of nature.

Growing threat of wheat blast

Almost all cereal diseases have existed for thousands of years, and the pathogens that cause them have evolved along with their hosts. In contrast, wheat blast is an extremely new disease, having first appeared in 1985 in Brazil, and thus the host has not had time to adapt to this new threat.

After its appearance, wheat blast spread throughout South America in humid tropical climates.

In 2016, wheat blast was reported in Bangladesh and in 2018 it was identified in Zambia. In both cases it appears that the disease was imported into the wheat from South America.

Fortunately, the strains causing outbreaks of wheat blast outside Brazil all produce a small protein effector called AVR-Rmg8. This molecule is part of the machinery used by the fungus to suppress wheat defenses. The presence of this protein, however, is a potential ‘Achilles heel’ if the wheat variety can detect this protein and initiate its defense responses.