Wheat blast has left a trail of devastation on three continents since it was first reported in 1985 and is now considered a major threat to global wheat security. Therefore, finding resistance genes against this pathogen is critical to mitigating this threat. Wheat blast is caused by the fungal pathogen Magnaporthe oryzae, first identified in Brazil in 1985. The pathogen has since caused epidemics in neighboring countries of Brazil, including Bolivia and Paraguay, and further outbreaks have been reported in Zambia, India and Bangladesh. Recent studies have shown that host jumping occurs due to hybrid recombination of two species of this fungal pathogen. The search for resistance genes is urgent because breeders of modern wheat varieties have not selected to include genes for resistance to wheat blast in their programs.
On February 16, 2023, Nature Plants published a research paper entitled "A wheat kinase and immune receptor form host-specificity barriers against the blast fungus". The study used an innovative genomic discovery approach to show how we can stop an emerging and highly destructive disease: wheat blast.
In the study, the researchers identified two genes that protect experimental wheat plants from Magnaporthe oryzae, the pathogen that causes wheat blast. The researchers used a technique called AgRenSeq that allowed them to search for useful genes in traditional wheat varieties. They also searched among wheat's weed relatives. This collection of locally grown crops, along with wheat's wild grass relatives, is an important resource for researchers seeking genes that protect modern crops from emerging diseases.
To identify resistance genes, the researchers tested seedlings and spikes from the Watkins Collection with specially modified isolates of the wheat blast pathogen to identify plants that were either resistant or susceptible to the fungus. They used AgRenSeq to identify the parts of the genome that show gene activity in resistant plants. This led to the identification of a resistance candidate gene, Rwt3, which protects wheat by regulating an NLR gene. In plants, NLR genes operate by encoding defensive proteins that detect pathogen effector molecules and trigger a protective response, much like antibodies protect humans from infection. Another gene that was discovered, Rwt4, is another defense molecule called a tandem kinase. Greenhouse experiments with wheat plants that lost the function of these resistance genes showed that they were susceptible to wheat blast isolates, confirming that Rwt3 and Rwt4 protect plants from wheat blast.
The study also showed that a version of Rwt4, Pm24, also protects plants against another important disease of wheat: powdery mildew. The researchers suggest that this approach may be adaptable enough to find resistance genes that respond to geographically specific strains of the pathogen.
Reference:
Arora, S., Steed, A., Goddard, R. et al. A wheat kinase and immune receptor form host-specificity barriers against the blast fungus. Nat. Plants (2023).
