The first epidemics of a deadly wheat fungus could have occurred due to widespread cultivation of varieties missing a specific gene.
The loss of a single gene in wheat makes it vulnerable to a fungus called Pyricularia oryzae, which causes a highly destructive disease that emerged in Brazil in 1985 and has since spread to South America, Bangladesh and Zambia. The finding was published in Nature Plants.
There is still much to learn about how to control the disease, known as wheat blast, which is a threat to global production. Intensive breeding can reduce the diversity of genes that resist the fungus, rendering crops more vulnerable to outbreaks.
An international research team searched for resistance genes in a wild grass ancestor of modern wheat and in 300 wheat heritage lines, which are part of a collection assembled at the beginning of the 20th century, before the emergence of intensive breeding. This included analyses of plants from field sites in Egypt by Ahmed Elkot from the Field Crops Research Institute.
The team found that the wheat resistance genes Rwt3 and Rwt4 regulate defence molecules that recognize the fungal proteins PWT3 and PWT4 respectively. Further experiments showed that wheat plants in which the function of one of these resistance genes was lost were susceptible to wheat blast, confirming the hypothesis that the first wheat blast epidemics in Brazil occurred due to the widespread cultivation of wheat varieties lacking Rwt3.
The scientists were able to use their analyses to learn more about the evolution of the interactions between different pathogens and wheat genes. For example, they found that Rwt4 is related to another gene, called Pm24, which confers resistance against another wheat disease called powdery mildew.
″The mechanism by which Rwt3 and Rwt4 recognise PWT3 and PWT4 is not known, but we have initiated research to establish how the fungal and host components interact with one another. The technologies are being used to identify genes that confer resistance against pathogens present in Bangladesh and Zambia,″ says Paul Nicholson from the John Innes Centre in the United Kingdom, who co-authored this study, with Brande Wulff from King Abdullah University of Science and Technology, in Saudi Arabia.
″The discovery of two wheat resistance genes is hugely significant in light of our limited understanding of the genetic basis of wheat blast specificity. The potential for the emergence of highly virulent and aggressive blast strains is unknown and requires further study," says plant pathologist, Md. Motaher Hossain from the Bangabandhu Sheikh Mujibur Rahman Agricultural University in Bangladesh, who was not involved in this study.
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