Novel Variation and Evolution of AvrPiz-t of Magnaporthe oryzae in Field Isolates

Abstract

The product of the avirulence (Avr) gene of Magnaporthe oryzae can be detected by the product of the corresponding resistance (R) gene of rice and activates immunity to rice mediated by the R gene. The high degree of variability of M. oryzae isolates in pathogenicity makes the control of rice blast difficult. That resistance of the R gene in rice has been lost has been ascribed to the instability of the Avr gene in M. oryzae. Further study on the variation of the Avr genes in M. oryze field isolates may yield valuable information on the durable and effective deployment of R genes in rice production areas. AvrPiz-t and Piz-t are a pair of valuable genes in the Rice-Magnaporthe pathosystem. AvrPiz-t is detectable by Piz-t and determines the effectiveness of Piz-t. To effectively deploy the R gene Piz-t, the distribution, variation, and evolution of the corresponding Avr gene AvrPiz-t were found among 312 M. oryzae isolates collected from Yunnan rice production areas of China. PCR amplification and pathogenicity assays of AvrPiz-t showed that 202 isolates (64.7%) held AvrPiz-t alleles and were avirulent to IRBLzt-T (holding Piz-t). There were 42.3-83.3% avirulent isolates containing AvrPiz-t among seven regions in Yunnan Province. Meanwhile, 11 haplotypes of AvrPiz-t encoding three novel AvrPiz-t variants were identified among 100 isolates. A 198 bps insertion homologous to solo-LTR of the retrotransposon inago2 in the promoter region of AvrPizt in one isolate and a frameshift mutation of CDS in another isolate were identified among 100 isolates, and those two isolates had evolved to virulent from avirulent. Synonymous mutation and non-AUG-initiated N-terminal extensions keeps the AvrPizt gene avirulence function in M. oryzae field isolates in Yunnan. A haplotype network showed that H3 was an ancestral haplotype. Structure variance for absence (28.2%) or partial fragment loss (71.8%) of AvrPiz-t was found among 39 virulent isolates and may cause the AvrPiz-t avirulence function to be lost. Overall, AvrPiz-t evolved to virulent from avirulent forms via point mutation, retrotransposon, shift mutation, and structure variance under field conditions.

Loss and Natural Variations of Blast Fungal Avirulence Genes Breakdown Rice Resistance Genes in the Sichuan Basin of China

“…Previously, insertions were reported in the 5′-UTR of AVR-Pizt , resulting in the function loss of AVR-Pizt and breakdown of the Pizt resistance (Wang et al, 2020 ). The fragment was amplified via PCR with primers AvrPiztF and AvrPiztR ( Figure 5A ).…”

Association analysis of rice resistance genes and blast fungal avirulence genes for effective breeding resistance cultivars

“…Additionally, previous studies revealed that AVR-Pita alleles are highly diversified, and transposon insertions, point mutations, frame-shift mutations, partial or complete deletions, and sequence variations in the coding or promoter region caused a change from avirulence to virulence of AVR genes ( Xing et al, 2013 ; Chen et al, 2014 ). Similarly, the loss of function of AVR-Piz-t was also observed in 312 isolates collected from the rice-producing areas of Yunnan, and a 198-bp insertion homologous to solo-LTR of the retrotransposon inago2 in the promoter region of AVR-Piz-t in one isolate was found to have evolved from avirulence to virulence ( Wang et al, 2020 ). Meantime, the changes of virulence frequencies of M. oryzae for Pi-ta and Piz-t happened in different years, suggesting that the disease reaction of rice cultivars were changing overtime ( Shen, 2018 ).…”

“…Particularly, three AVR genes, namely AVR-Pita , AVR-Pik, and AVR-Piz-t , have a relatively large number of polymorphisms, including point mutations, transposon insertions, and partial or total gene deletions, and hence usually escape recognition by the corresponding R genes in rice ( Dean et al, 2005 ; Zhou et al, 2007 ; Chuma et al, 2011 ; Huang et al, 2014 ; Zhang et al, 2015 ). For example, Wang et al (2020) identified 11 haplotypes encoding three new AVR-Piz-t variants in 100 isolates among 312 isolates of M. oryzae in Yunnan, China, including a 39-bp deletion and a 192-bp solo-LTR (long terminal repeat) homolog of the retrotransposon inago2 that had inserted into the promoter region and created a frameshift in the coding sequence. Each of these genetic alterations pushed AVR-Piz-t to a virulent form.…”

The link between changing in host carbon allocation and resistance to Magnaporthe oryzae : a possible tactic for mitigating the rice blast fungus

“…Moreover, It is well known that M. oryzae adapts quickly. 102 , 103 Therefore, research must continue to stay ahead of adaptive responses, as there is a chance that the pathogen will develop mechanisms to evade carbon allocation strategies that are manipulated. Additionally, adjusting the allocation of carbon might have consequences for other vital physiological functions.…”