Gene Duplication and Mutation in the Emergence of a Novel Aggressive Allele of the <i>AVR-Pik</i> Effector in the Rice Blast Fungus

Abstract

Higher yield potential and greater yield stability are common targets for crop breeding programs, including those in rice. Despite these efforts, biotic and abiotic stresses continue to impact rice production. Rice blast disease, caused by <i>Magnaporthe oryzae</i>, is the most devastating disease affecting rice worldwide. In the field, resistant varieties are unstable and can become susceptible to disease within a few years of release due to the adaptive potential of the blast fungus, specifically in the effector (avirulence [<i>AVR</i>]) gene pool. Here, we analyzed genetic variation of the effector gene <i>AVR-Pik</i> in 58 rice blast isolates from Thailand and examined the interaction between <i>AVR-Pik</i> and the cognate rice resistance gene <i>Pik</i>. Our results reveal that Thai rice blast isolates are very diverse. We observe four <i>AVR-Pik</i> variants in the population, including three previously identified variants, <i>AVR-PikA</i>, <i>AVR-PikD</i>, and <i>AVR-PikE</i>, and one novel variant, which we named <i>AVR-PikF</i>. Interestingly, 28 of the isolates contained two copies of <i>AVR-Pik</i>, always in the combination of <i>AVR</i>-<i>PikD</i> and <i>AVR</i>-<i>PikF</i>. Blast isolates expressing only AVR<i>-</i>PikF show high virulence to rice cultivars encoding allelic <i>Pik</i> resistance genes, and the AVR-PikF protein does not interact with the integrated heavy metal-associated domain of the Pik resistance protein in vitro, suggesting a mechanism for immune evasion.