Suppression of wheat blast resistance by an effector ofPyricularia oryzaeis counteracted by a host specificity resistance gene in wheat

Wheat blast caused by theTriticumpathotype ofPyricularia oryzaeposes a serious threat to wheat production in South America and Asia and is now becoming a pandemic disease. Here, we show thatRmg8, a promising wheat gene for resistance breeding, is suppressed byPWT4, an effector gene ofP. oryzae, and in turn that the suppression is counteracted byRwt4, a wheat gene recognizingPWT4. WhenPWT4was introduced into a wheat blast isolate carryingAVR-Rmg8(an avirulence gene corresponding toRmg8),PWT4suppressed wheat resistance conferred byRmg8.PWT4did not alter the expression ofAVR-Rmg8, but higher expression ofPWT4led to more efficient suppression. This suppression was observed inrwt4carriers, but not inRwt4carriers, indicating that it is counteracted byRwt4. PWT4was assumed to have been horizontally transferred from a weed-associated cryptic species,P. pennisetigena, to anAvenaisolate ofP. oryzaein Brazil. This implies a potential risk of the acquisition ofPWT4by the wheat blast fungus and the 'breakdown' ofRmg8. We suggest thatRmg8should be introduced together withRwt4into a wheat cultivar when it is used for resistance breeding.