A rapid survey of avirulence genes in field isolates of Magnaporthe oryzae

Authors: ZHANG, ZHEN - Zhejiang Academy Of Agricultural Sciences; Jia, Yulin; WANG, YANLI - Zhejiang Academy Of Agricultural Sciences; SUN, GUOCHANG - Zhejiang Academy Of Agricultural Sciences

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2019
Publication Date: 1/13/2020
Citation: Zhang, Z., Jia, Y., Wang, Y., Sun, G. 2020. A rapid survey of avirulence genes in field isolates of Magnaporthe oryzae. Plant Disease. https://doi.org/10.1094/PDIS-08-19-1688-RE.
DOI: https://doi.org/10.1094/PDIS-08-19-1688-RE

Interpretive Summary: Rice blast disease is one of the lethal diseases to rice worldwide. Major blast resistance (R) genes are effective only when the fungal pathogen Magnaporthe oryzae isolates contain the matched avirulence (AVR) genes. In this study, we developed 4 AVR gene specific polymerase chain reaction (PCR) markers, AVR-Pik, AVR-Pizt, AVR-Pia, and AVR-Pii and along with a known AVR-Pita1 marker to investigate the existence of these five AVR genes in 258 blast isolates collected from southern US commercial rice fields from 1975-2009. PCR products of AVR genes were obtained from 232 isolates and were classified into 10 haplotype groups. The absence of PCR products in remaining 26 isolates suggest that these isolates do not contain the tested AVR genes. The results showed that 174 isolates of M. oryzae carried AVR-Pita1, 225 isolates carried AVR-Pizt, 44 isolates carried AVR-Pik, 3 isolates carried AVR-Pia, and 1 isolate carried AVR-Pii. After sequence analysis, we found 40 AVR-Pita1 variants and 11 AVR-Pik variants in avirulent isolates suggesting that both AVR-Pita1 and AVR-Pik are unstable. We only found one variant for AVR-Pizt, AVR-Pia and AVR-Pii suggesting that they are relatively stable. The presence of five AVR genes, AVR-Pita1, AVR-Pik, AVR-Pizt, AVR-Pia, and AVR-Pii in M. oryzae strains suggests that their corresponding resistance genes Pi-ta, Pi-k, Pi-zt, Pia and Pii can be deployed in preventing blast disease in the Southern US. This study demonstrates that monitoring AVR genes in pathogen populations is critical for ensuring the effectiveness and stability of deployed blast R genes in commercial rice fields.

Technical Abstract: Magnaporthe oryzae is the causal agent for the devastating blast disease of rice. The avirulence (AVR) genes in Magnaporthe oryzae are required to initiate robust disease resistance mediated by the corresponding resistance (R) genes in rice. Therefore, monitoring pathogen AVR genes is important to predict the stability of R gene-mediated blast resistance. In the present study, we analyzed the DNA sequence dynamics of five AVR genes, namely, AVR-Pita1, AVR-Pik, AVR-Pizt, AVR-Pia, and AVR-Pii, in field isolates of M. oryzae in order to understand the effectiveness of the R genes, Pi-ta, Pi-k, Pi-zt, Pia and Pii in the Southern United States. Genomic DNA of 258 blast isolates collected from commercial fields of the Southern U.S. during 1975-2009 were subjected to PCR amplification with AVR gene-specific PCR markers. PCR products were obtained from 232 isolates. The absence of PCR products in remaining 26 isolates suggest that these isolates do not contain the tested AVR genes. Amplified PCR products were subsequently gel purified and sequenced. Based on the presence or absence of the five AVR genes, 232 field isolates were classified into ten haplotype groups. The results revealed that 174 isolates of M. oryzae carried AVR-Pita1, 225 isolates carried AVR-Pizt, 44 isolates carried AVR-Pik, 3 isolates carried AVR-Pia, and one isolate carried AVR-Pii. AVR-Pita1 was highly unstable, and 40 AVR-Pita1 haplotypes were identified in avirulent isolates. AVR-Pik had four nucleotide sequence site changes resulting in amino acid substitutions whereas three other AVR genes, AVR-Pizt, AVR-Pia and AVR-Pii, were relatively stable. In conclusion, four AVR genes (AVR-Pik, AVR-Pizt, AVR-Pia, and AVR-Pii ) were found to exist in the field isolates, which suggested that their corresponding R genes Pi-k, Pi-zt, Pia and Pii can be deployed in preventing blast disease in the Southern U.S. in addition to Pi-ta. This study demonstrates that continued AVR monitoring in pathogen population is critical for ensuring the effectiveness of deployed blast R genes in commercial rice fields.