Genetic analysis traces how pathogen evolved to evade defenses
A wheat strain that let its guard down may have paved the way for a crop-destroying fungus to infect the species.
About 1980, Brazilian farmers began growing a strain of wheat called Anahuac, which is suited to the country’s nonacidic soils. And that, researchers report July 7 in Science, may be when wheat started to lose an arms race with blast fungus (Pyricularia oryzae, also known as Magnaporthe oryzae). Using genetic analysis, the researchers traced how the fungus – which before 1985 was known to infect just rice and some other grains – evolved to infect wheat, too.
Anahuac lacks a functional copy of the Rwt3 gene, which wheat uses to defend itself against the fungus, Yoshihiro Inoue of Kobe University in Japan and colleagues discovered. That gene’s protein recognizes a corresponding protein called PWT3 made by the pathogen. Wheat plants lacking the protective gene don’t see the fungus as a threat. Similarly, blast fungus that doesn’t make PWT3 is essentially invisible to the wheat’s defense mechanism.
Inoue and colleagues propose that blast fungus may have started to grow on the Anahuac strain in the early 1980s in Brazil. Other strains of wheat grown nearby still had the defense mechanism. So that put evolutionary pressure on the fungus to change. After a while, blast fungus that doesn’t make PWT3 emerged, allowing it to infect all types of wheat. Blast fungus spread from Brazil to neighboring countries and also popped up in the United States in 2011. The fungus invaded Bangladesh in 2016, where it caused the loss of up to 90 percent of the wheat crop in some areas.
Y. Inoue et al. Evolution of the wheat blast fungus through functional losses in a host specificity determinant. Science. Vol. 357, July 7, 2017, p. 80. doi: 10.1126/science.aam9654.