Identification Of Pathogenicity And Avirulence Genes Of Magnaporthe Oryzae In The Sichuan Basin

Rice is the staple food for nearly half the world’s population.Rice blast is one of the most devastating fungal diseases in rice and the najor threat to rice production worldwide.So far,resistance breeding is the most effective and environmentfriendly measure to control rice blast.The blast population can overcome the R genemediated resistance in the field with the monocropping of a single resistant cultivar,due to the rapid evolution of rice blast fungus.Therefore,monitoring the distribution and variations of avirulence genes can provide important guidance for the rational use of resistant genes.In this study,isolates were collected from the rice blast nurseries in the Sichuan Basin from 2018 to 2020 using a single-spore isolation method.We analyzed the distribution of avirulence genes in rice blast population and the pathogenicity against24 IRBLs.The main results are as follows:1.Pathogenicity of isolates from the Sichuan Basin during 2018 to 2020 aginst 24 IRBLsMore than 70% isolates from 2018 were virulent to the IRBL lines carrying Pi12(t),Pish,Pi11,Pib,Pia,Pita,Pit and Pi19(t),implying that these resistance genes had been broken down by the blast populations in the Sichuan Basin.The frequency of virulent isolates was below 20% toward the IRBL lines for Pi-z5 and Pi-kh,implying that Pi-z5 and Pi-kh were still effective.Besides,the pathogenic frequencies of more than 52%isolates were between 20% and 50%,indicating that the diversity of the rice blast population in 2018 is relatively rich.The virulent frequencies of isolates from 2019 were more than 70% to the IRBL lines carrying Pik,Pikp,Pi12(t),Pib,Pit,Pi11,Pi19(t),Pikm,Pish,Pi7(t),Pita and Piz5,suggesting these genes were invalidated in 2019.Instead,Piz was demonstrated to be still effective.Meanwhile,the pathogenic frequencies of almost 50% isolates were over 70%,indicating that abundant variations may occur in the corresponding avirulence genes.The virulent frequencies of isolates from 2020 were more than 70% to the IRBL lines carrying Pib,Pit,Pi12(t),Pi3,Piks,Pia and Pii,implying that these resistance genes had lost function.The frequency of virulent isolates was below 20% toward the IRBL lines for Pi20,Pita2,Pi9(t),Pikh,Piz5 and Piz,implying these R genes were effective in 2020.The pathogenic frequencies of more than 62.5% isolates were between 20% and 50%,and isolates virulent to over 70% or below 20% different varieties were no found almost.2.The presence frequency and variations of avirulence genes in isolates from 2018We analyzed the presence and nucleotide sequences polymorphisms of eight R gene-paired AVR genes,namely,AVR-Pita1,AVR-Pii,AVR-Pia,AVR-Pik,AVR-Pizt,AVR-Pi9,AVR-Pib,and AVR-Pi54 in a total of 383 isolates.While AVR-Pik and AVRPi54 existed in all isolates,AVR-Pizt,AVR-Pi9,and AVR-Pib existed in over 96% of the isolates.The frequency of AVR-Pita1,AVR-Pii,and AVR-Pia was less than 50%,indicating gradual loss of these AVRs.Moreover,we detected the insertions of Pot3,Mg-SINE,and deletion in the CDS region of AVR-Pib,solo-LTR of Inago2 in AVR-Pizt,and gene duplications in AVR-Pik,which might contribute to the invalidation of the blast resistance genes in the Sichuan Basin.3.Presence frequency of avirulence genes in isolates from 2018 to 2020 through re-sequencingWe checked the presence frequency of avirulence genes in 744 isolates from 2018 to 2020.The results showed that the frequency of AVR-Pi54,AVR-Pi9,AVR-Pib and AVR-Pizt was over 90%.However,AVR1-CO39 was not found in all isolates.The frequency of AVR-Pia and AVR-Pii varied significantly from 2018 to 2020,and the frequency of AVR-Pik and AVR-Pita1 changed slightly.This study systematically analyzed the frequency of avirulence genes in rice blast population in the Sichuan Basin and the pathogenicity against 24 IRBLs,providing guidance for the utilization of resistance genes and rational deployment of varieties in the Sichuan Basin.The results showed that Pia,Pib,Pit,Pish,Pi12(t),Pi19(t)and Pi11 had almost lost resistance,and rice varieties harboring these genes should not be planted widly in the Sichuan Basin.However,Pi54,Pikh and Piz were still effective in the Sichuan Basin,and these genes can be used in the resistance breeding.Piz5,Pizt,Pi1,Pi5(t),Pi9(t)and Pikm can be used depending on the results of resistance assessment in the field,and these genes can also be used with other resistance genes.