Abstract
BackgroundRice blast disease is one of the most destructive fungal disease of rice worldwide. The avirulence (AVR) genes of Magnaporthe oryzae are recognized by the cognate resistance (R) genes of rice and trigger race-specific resistance. The variation in AVR is one of the major drivers of new races. Detecting the variation in the AVR gene in isolates from a population of Magnaporthe oryzae collected from rice production fields will aid in evaluating the effectiveness of R genes in rice production areas. The Pik gene contains 5 R alleles (Pik, Pikh, Pikp, Pikm and Piks) corresponding to the AVR alleles (AVR-Pik/kh/kp/km/ks) of M. oryzae. The Pik gene specifically recognizes and prevents infections by isolates of M. oryzae that contain AVR-Pik. The molecular variation in AVR-Pik alleles of M. oryzae and Pik alleles of rice remains unclear.ResultsWe studied the possible evolutionary pathways of AVR-Pik alleles by analyzing their DNA sequence variation and assaying their avirulence to the cognate Pik alleles of resistance genes under field conditions in China. The results of PCR products from genomic DNA showed that 278 of the 366 isolates of M. oryzae collected from Yunnan Province, China, carried AVR-Pik alleles. Among the isolates from six regions of Yunnan, 66.7–90.3% carried AVR-Pik alleles. Moreover, 10 AVR-Pik haplotypes encoding five novel AVR-Pik variants were identified among 201 isolates. The AVR-Pik alleles evolved to virulent from avirulent forms via stepwise base substitution. These findings demonstrate that AVR-Pik alleles are under positive selection and that mutations are responsible for defeating race-specific resistant Pik alleles in nature.ConclusionsWe demonstrated the polymorphism and distribution of AVR-Pik alleles in Yunnan Province, China. By pathogenicity assays used to detect the function of the different haplotypes of AVR-Pik, for the first time, we showed the avoidance and stepwise evolution of AVR-Pik alleles in rice production areas of Yunnan. The functional AVR-Pik possesses diversified sequence structures and is under positive selection in nature.
Highlights
Rice blast disease is one of the most destructive fungal disease of rice worldwide
Efficacy of Pik genes and detection frequency of AVR-Pik alleles Based on the disease reactions, the efficacy of the Pik genes Pik, Pikm, Pikp, Pikh and Piks were examined
The frequency of avirulence to Pik, Pikm, Pikp, Pikh and Piks was 60.9, 69.9, 42.1, 75.4 and 22.7%, respectively, while the remaining 143, 110, 212, 90 and 283 isolates were virulent to the corresponding R gene (Table 1)
Summary
The avirulence (AVR) genes of Magnaporthe oryzae are recognized by the cognate resistance (R) genes of rice and trigger racespecific resistance. The Pik gene contains 5 R alleles (Pik, Pikh, Pikp, Pikm and Piks) corresponding to the AVR alleles (AVR-Pik/kh/kp/km/ks) of M. oryzae. The Pik gene recognizes and prevents infections by isolates of M. oryzae that contain AVR-Pik. The molecular variation in AVR-Pik alleles of M. oryzae and Pik alleles of rice remains unclear. Two hypotheses have been proposed regarding these dynamics: arms race and trench warfare evolution between host resistance genes (R) and pathogen avirulence genes (AVR) [1]. The arms race hypothesis is considered the principal hypothesis, in which both host R genes and pathogen AVR genes are under directional selection and the alleles are derived by mutation. To date, ≤26 R genes in rice have been cloned: Pb1, Pia, Pib, Pid, Pid, Pik, Pikh/Pi54, Pikm, Pikp, Pish, Pit, Pita, Pizt, Pi1, Pi2, Pi5, Pi9, pi, Pi25, Pi36, Pi37, Li et al BMC Plant Biology (2019) 19:204
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