Since it was first identified in 1985, the wheat blast has wreaked havoc on three continents and is now seen to pose a serious danger to the security of the world’s wheat supply. To stop this threat, it is important to find and use genes that make organisms resistant to this pathogen.
Wheat blast, which is caused by the fungus Magnaporthe oryzae, was originally found in Brazil in 1985 owing to a “host jump” from ryegrass. Since then, the disease has produced epidemics in Brazil’s neighbors, notably Bolivia and Paraguay, with other occurrences recorded in Zambia, India, and Bangladesh.
Recent research suggests that the host shift was the result of hybrid recombination between two species of the fungal pathogen. Breeders haven’t chosen modern wheat varieties that have blast-resistant genes in their programs, so it’s important to find resistant genes as soon as possible.
The John Innes Centre has spearheaded an international research partnership that utilized cutting-edge genomic discovery techniques to demonstrate potential strategies for containing wheat blast.
In tests, scientists discovered two genes that shielded experimental wheat plants against the blast-causing fungus Magnaporthe oryzae.
The research team found the gene using a method called AgRenSeq, which enabled them to explore among a panel of historic wheat types known as the Watkins Collection for valuable genes. Additionally, they looked among wheat’s wild grass cousins.
The Watkins Collection is made up of more than 300 wheat lines or landraces that were gathered from all over the globe in the 1930s and include disease-fighting variability that was present in wheat before intense breeding. In addition to wild grass relatives of wheat, such collections of locally cultivated crops have proven to be an essential resource for scientists looking for genes that might defend current crops from developing diseases.
“We have made an important discovery on an emerging disease that threatens global food security,” remarks Professor Paul Nicholson, “and, in the process, highlighted the power of the Watkins Collection and the AgRenSeq genomic toolkit.
Researchers evaluated Watkins collected seedlings and spikes against blast pathogen isolates that had been genetically engineered to determine which plants were resistant and which were vulnerable to the fungus.
Then they identified regions of the genome that had gene activity in resistant plants using AgRenSeq, a gene identification method created by Dr. Sanu Arora, a group leader at the John Innes Centre.
This led to the discovery of the candidate resistance gene Rwt3, which protects wheat by controlling an NLR gene. Similar to how antibodies defend people against infections, NLR genes in plants produce protective proteins that bind to and neutralize pathogen effector molecules.
Another defense protein known as a tandem kinase is encoded by the second gene found, Rwt4. This gene was also detected in the collection of Aegilops tauschii, a wild grass progenitor of contemporary bread wheat.
To prove that Rwt3 and Rwt4 protected plants against blast, glasshouse studies were conducted using wheat plants in which the function of these resistance genes was eliminated.
The research, which was published in Nature Plants, also showed that an Rwt4 variant known as Pm24 shielded plants against powdery mildew, a serious disease of wheat.
It has been suggested that the outbreaks in Brazil were caused by the extensive cultivation of Rwt3-deficient wheat, which made them vulnerable to pathotypes of ryegrass.
Wheat blast outbreaks in the USA have also been linked to ryegrass pathotypes. This underlines how crucial it is to keep Rwt3 and Rwt4 in wheat cultivars in order to avoid further host leaps.
The research team says that the method could be changed enough to find resistance genes that work against strains of the pathogen that are only found in certain parts of the world. By demonstrating the feasibility of discovering resistance genes in heritage varieties or wild relatives and incorporating them into top cultivars, this study provides evidence for a potential strategy for dealing with new crop diseases.
This international cooperation, which includes organizations from Japan and Saudi Arabia, intends to make this information accessible to CIMMYT in the near future by supplying genetic markers. These make it possible for breeders to find these genes in their collections quickly and make sure they are included in the development of blast resistance wheat cultivars.
“The disastrous effect of wheat blast in the wheat belts of South America, South Asia and Africa is a warning bell for Europe,” adds first author Dr. Sanu Arora.
“We are not certain if this disease is already sitting on the horizon of Europe but the disease could potentially travel through human migration or seed import, therefore, it is critically important to defend this vital crop against the looming threat.”
“This is a really good example of an international collaboration that is contributing to United Nations sustainability goals because global plant health is important to delivering food sustainability,” remarks Dr. Jonathan Clarke, Head of Knowledge Exchange and Commercialization at the John Innes Centre.
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