An 'important breakthrough' in efforts to halt the advance of wheat blast, an emerging threat to food security, has come from a surprising source, researchers say.
A new study has unexpectedly revealed that wheat varieties with resistance to another pathogen, powdery mildew, also confer protection against wheat blast.
When seeking resistance to diseases it is common to search among varieties or old landraces from regions where the disease originated.
As wheat blast is a disease of humid sub-tropical regions, efforts to control the disease have focused on finding resistance genes among wheat varieties adapted to warmer climates.
A research collaboration led by the UK's John Innes Centre challenged this approach, suggesting that researchers should not ignore resistance in wheat varieties that have been bred to withstand other diseases including those of colder climes, like powdery mildew.
Using gene discovery methods developed at the centre, they have identified the first gene that protects wheat plants against the strains of the blast fungus that contain the protein effector AVR-Rmg8.
Surprisingly, the gene, located on chromosome 2A of the wheat genome, is Pm4, a gene that gives wheat resistance to powdery mildew, a disease of the cooler, wetter climates of the northern hemisphere.
European plant breeders have been selecting wheat with Pm4 for many years for resistance to powdery mildew; now those in the southern hemisphere will be urged to do the same as protection against wheat blast.
Professor Paul Nicholson a group leader at the John Innes Centre and coordinator of the study, said the findings were completely unexpected.
"They suggest that if you want to find resistance to wheat blast you should also look in varieties that come from non-tropical regions, where they already have resistance to mildew.
“We need 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 low temperature high humidity environments.
"No one would have thought of looking in European varieties previously because one is looking for commonalities.”
The research team made the discovery by screening over 300 varieties of wheat in the Watkins Collection, a diversity panel gathered from around the globe in the 1930s.
Out of this population, just three percent showed resistance to wheat blast pathogen strains that produce AVR-Rmg8.
Worryingly, all the varieties that were highly resistant to blast carried the Pm4 gene, indicating that only a single resistance was present among this highly diverse population.
The researchers said this emphasised the need to identify additional resistances to ensure robust, durable resistance against this new threat.
The team will now use the same gene discovery methods to search among European-bred wheat varieties for further resistance genes to blast, increasing the genetic armory which can be deployed against this destructive disease.
What is wheat blast?
Almost all cereal diseases have existed for thousands of years and the pathogens causing them have co-evolved with their hosts.
Wheat blast, by contrast, is a new disease, first emerging in 1985 in Brazil, and so there has been no time for the host to adapt to this new threat.
Following its appearance, it has spread throughout South America in humid tropical climates.
In 2016 wheat blast was reported in Bangladesh and in 2018 was identified in Zambia. In both instances it appears that the disease has been imported on grain from South America.
Fortunately, the strain(s) 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 defences.
The presence of this protein, however, is a potential ‘Achille’s heel’ if the wheat variety can detect this protein and initiate its defence responses.
