Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2017
Publication Date: 7/5/2017
Citation: Wang, X., Jia, Y., Wamishe, Y., Jia, M.H., Valent, B. 2017. Dynamic changes in the rice blast population in the USA over six decades. Molecular Plant-Microbe Interactions. doi:10.1094/MPMI-04-17-0101-R. Interpretive Summary: Rice blast disease caused by the fungus Magnaporthe oryzae is one of the major constraints for rice production in the southern USA and worldwide. M. oryzae has been known to be highly unstable and can rapidly overcome from host resistance. In this study, we analyzed genetic profiles of 1022 US blast isolates collected from 1959 to 2015 with simple sequence repeat genetic markers and/or pathogenicity assays to guide resistance gene deployment. We showed that there are 6 genetic clusters in US blast isolates, and the virulence of pathogenic isolates has been increasing over time through asexual reproduction. The observed gene clusters were not associated with geographic origin of the isolates but associated with collection time periods. These findings suggest that host genotypes are a driving force for rapid adaptation of the rice blast fungus; and the resulting knowledge will benefit the identification of rice resistance genes for developing blast resistant rice varieties.
Technical Abstract: Rice blast disease caused by Magnaporthe oryzae is one of the most destructive diseases of rice. Field isolates of M. oryzae rapidly adapt to the hosts and climate. Tracking the genetic and pathogenic variability of the field isolates is essential to understand how M. oryzae interacts with hosts and environments. In this study, a total of 1022 US field isolates from 1959 to 2015 were analyzed for pathogenicity toward 8 international rice differentials (IRD). The sub-set consisting of 457 isolates was genotyped with 10 polymorphic Simple Sequence Repeat (SSR) markers. The average Polymorphism information content (PIC) value of markers was 0.55 suggesting that the SSR markers were highly informative to capture the population variances. Six genetic clusters were identified by both structure and Discriminant Analysis of Principal Components (DAPC) methods. Overall, Nei’s diversity of M. oryzae in the USA was 0.53 which is higher than previously reported in a world rice blast collection (0.19). The observed subdivision was not associated with geographic origin of the isolates but associated with collection time periods. The race such as IE1 has been identified across almost all collection periods, and all clusters; another race such as IA1 has a much high frequency in certain period and clusters. Both genomic and pathogenicity changes of US blast isolates were associated with collection year suggesting that hosts are a driving force for the genomic instability of rice blast fungus.