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United States Department of Agriculture

Agricultural Research Service

Research Project: RESPONSE OF DIVERSE RICE GERMPLASM TO BIOTIC AND ABIOTIC STRESSES

Location: Dale Bumpers National Rice Research Center

Title: Diversification and evolution of the avirulence gene AVR-Pita1 in field isolates of Magnaporthe oryzae

Authors
item Dai, Yuntao - UNIV. OF AR
item Jia, Yulin
item Correll, James - UNIV. OF AR
item Wang, Xueyan - ZHEJIANG UNIV, CHINA
item Wang, Yanli - ZHEJIANG ACAD. AG.SCI.

Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 6, 2010
Publication Date: September 6, 2010
Citation: Dai, Y., Jia, Y., Correll, J.C., Wang, X., Wang, Y. 2010. Diversification and evolution of the avirulence gene AVR-Pita1 in field isolates of Magnaporthe oryzae. Fungal Genetics and Biology. 47:973-980.

Interpretive Summary: The ascomycete fungus Magnaporthe oryzae is known to produce the AVR-Pita1 pathogenicity factor to promote blast disease. The Pi-ta gene in rice has evolved to recognize the AVR-Pita1 allele in triggering resistance responses. Pi-ta has been effectively deployed in commercial rice cultivars in the southern United States since 1990s. However, the Pi-ta gene occasionally failed to prevent blast disease because of the emergence of more virulent races of M. oryzae. In the present study, 166 isolates of M. oryzae were collected from the US, China, Colombia, Egypt, India, and the Philippines during the past 40 years. Among them, 151 were found to be avirulent to Pi-ta containing rice cultivars while 15 were found to be virulent to Pi-ta containing cultivars based on pathogenicity assays. AVR-Pita1 was amplified from all 151 avirulent isolates as well as 5 virulent isolates collected from China. These 156 isolates were sequenced to study diversity present at the AVR-Pita1 allele. Sequence analysis revealed 39 AVR-Pita alleles with 27 predicted proteins highly similar to the AVR-Pita1 metalloprotease. These AVR-Pita1 alleles were grouped as seven major clades with minor DNA sequence variation. Most DNA sequence variation was identified in the exon regions, and most of these DNA variations resulted in amino acid substitutions. The ratio of Ka/Ks is 3.88 suggesting the evolution of AVR-Pita1 allele is under intense selective pressure. In the five virulent isolates from which AVR-Pita1 was amplified, two additional nucleotides were found in the first exon of the coding region of AVR-Pita1. The insertion of two nucleotides led a frame-shift that would produce a predicted truncated nonfunctional AVR-Pita1 metalloprotease. In 10 other virulent isolates, deletion of a portion of the AVR-Pita1 allele from the M. oryzae genome was predicted by Southern blot analysis. These findings suggest that the frame-shift and partial deletion of AVR-Pita1 are two mechanisms the fungus evolved to overcome Pi-ta mediated resistance, and AVR-Pita1 is under diversified selection. This study provides important knowledge for the identification of critical pathogenicity factors responsible for future rice blast epidemics and thus will benefit the development of strategies to control the blast disease.

Technical Abstract: The study of the evolution of the AVR-Pita1 genes should benefit the deployment of the resistance gene Pi-ta for protecting rice production. The AVR-Pita1 avirulence gene in races of Magnaporthe oryzae triggers an effective resistance response when M. oryzae infects rice plants that contain the Pi-ta resistance gene. In the present study, 166 isolates of M. oryzae were collected from the US, China, Colombia, Egypt, India, and the Philippines during the past 40 years. Among them, 151 were found to be avirulent to the Pi-ta containing rice cultivars while 15 were found to be virulent to Pi-ta containing cultivars based on pathogenicity assays. AVR-Pita1 was amplified from all 151 avirulent isolates as well as 5 virulent isolates collected from China. These 156 isolates were sequenced to study diversity present at the AVR-Pita1 allele. Sequence analysis revealed 39 AVR-Pita alleles with 27 predicted proteins highly similar to the AVR-Pita1 metalloprotease. These AVR-Pita1 alleles were grouped as seven major clades with minor DNA sequence variation. Most DNA sequence variation was identified in the exon regions, and most of these DNA variation resulted in amino acid substitutions. The ratio of Ka/Ks is 3.88 suggesting the evolution of AVR-Pita1 allele is under intense selective pressures. In the five virulent isolates from which AVR-Pita1 was amplified, two additional nucleotides were found in the first exon of the coding region of AVR-Pita1. The insertion of two nucleotides led to a frame-shift that would produce a predicted truncated nonfunctional AVR-Pita1 metalloprotease. In 10 other virulent isolates, deletion of a portion of the AVR-Pita1 allele from the M. oryzae genome was predicted by Southern blot analysis. These findings suggest that the frame-shift and partial deletion of AVR-Pita1 are the two mechanisms the fungus has evolved to overcome Pi-ta mediated resistance, and AVR-Pita1 is under diversified selection.

Last Modified: 9/2/2014
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