Wheat blast fungus is a pathogen that has been devastating wheat crops in South America since the mid-1980s and recently caused heavy yield losses in Asia. An international research team has clarified the evolutionary steps that led to the spread of this pathogen.
This study was carried out by a Kobe University research group led by Professor TOSA Yukio (Graduate School of Agricultural Science, Kobe University) in collaboration with the Iwate Biotechnology Research Center, Kyoto University, Okayama University, Ohio State University, Kansas University and Kentucky University. The findings were published on July 7 in Science.
Wheat blast is caused by a subgroup of a filamentous fungus known as Pyricularia oryzae or Magnaporthe oryzae. It has emerged through host jumps from a weed pathogen, and become a potential threat to human food production. After appearing in Brazil in 1985, it spread to neighboring countries such as Bolivia, Paraguay and Argentina, and caused extensive damage to wheat production in South America. The distribution of this fungus was restricted to South America for 30 years, but in 2016 it suddenly appeared on the Eurasian continent. After hitting Bangladesh, it caused devastating yield losses in India (the world's second largest wheat producer in 2017), showing signs of becoming a pandemic. It has become urgent to understand the mechanisms of its evolution and life cycle.
The research team found that a close relative of the wheat blast fungus - ryegrass pathogen - cannot parasitize wheat due to recognition of the "avirulence" gene PWT3 in the ryegrass pathogen by the "resistance" gene Rwt3 in wheat. When PWT3 was disrupted, the ryegrass pathogen became pathogenic to wheat, indicating that the host jump from ryegrass to wheat is caused by the loss of this single "avirulence" gene. The team also found various mutations occurring in PWT3, such as the insertion of transposon and base substitutions, in the wheat blast fungus.
However, some strains of the wheat blast fungus collected in the early period of wheat blast outbreak in Brazil carried a functional PWT3 gene. Why were such strains isolated from wheat? To answer this question, the research team surveyed global distribution of the Rwt3 resistance gene. The team discovered that, although most wheat cultivars possessed Rwt3, a wheat cultivar lacking Rwt3 was introduced to Brazil, recommended to farmers, and widely cultivated in the early 1980s. They concluded that the ryegrass pathogen successfully parasitized this cultivar, increased its population size, and then mutated into strains that lost the PWT3 function. The new strains were pathogenic to Rwt3 carriers, and were finally established as the wheat blast fungus that is pathogenic to the entire common wheat population. This model implies that the wheat cultivar lacking Rwt3 served as a springboard for the host jump. In other words, an agricultural practice by mankind - the cultivation of the Rwt3- noncarrier(s) - accelerated the emergence of the wheat blast fungus.