| The rice blast caused by Magnaporthe oryzae is one of the most serious fungal diseases in rice production,characterized by a wide incidence area and strong infectivity,which seriously affects the yield and quality of rice.As a major province in rice production in China,Jiangsu’s rice yield is a key factor that directly affects food security.In recent years,with the changes in climate conditions and farming methods,rice blast has become increasingly prevalent in the rice areas of Jiangsu.Epidemic years of rice blast generally result in a 10%-20%reduction in rice production,causing huge losses to agricultural production in our province.At present,breeding disease resistant rice varieties is still the most economical and effective strategy to control rice blast,and rice and Magnaporthe oryzae are model organisms of interaction between plants and pathogens.In this study,to provide theoretical supports for rice blast monitoring and warning,varieties layout,and disease resistance breeding,we collected and isolated 660 single spore isolates of rice blast fungus from various regions in Jiangsu province,and detected these isolates using specific molecular markers of 10 avirulence genes of blast fungus.Based on this,we analyzed the distribution and variation patterns of each avirulence gene in different regions and years.Meanwhile,some representative strains were screened for overall analysis of the pathogenicity of rice blast fungus in Jiangsu.The main research findings are summarized as follows:1)The results of 10 avirulence genes detection on 660 single spore isolates showed that the avirulence genes with higher distribution frequency were Avr-Pizt and Avr-Pib,both close to 90%,followed by Avr-Pi9(80%).The lower distribution frequencies are PWL1,AvrCo39,Avr-Pii,Avr-Pita,all below 30%,with PWL1 having the lowest distribution frequency(1.66%).Statistics on the number of avirulence genes carried by each isolates showed that 88.2%of the isolates carried 3-7 avirulence genes,while a few isolates(5.6%)carried more than 7 avirulence genes.From the perspective of distribution years,the distribution frequencies of most avirulence genes were relatively similar over the past three years,while there were significant differences in Avr-Pi9,Avr-Pizt,Avr-Pik,and ACE1.The distribution frequency in 2020 was significantly lower than the other two statistical years.From the perspective of distribution regions,the frequencies of most avirulence genes were relatively similar among the three regions,with only Avr-Pik and Avr-Pia showing significant differences in distribution frequencies among different regions.2)Based on the results of avirulence genes detection,cluster analysis was conducted on 660 single spore isolates,and it was found that these isolates can be mainly divided into 7 groups(group Ⅰ to group Ⅲ).In 2019,the main epidemic groups were Ⅱ,Ⅴ,Ⅶ,and Ⅷ,in 2020 were Ⅱ and Ⅶ,and in 2021 were Ⅴ and Ⅶ.The Ⅰ and Ⅱ groups showed a downward trend in the past three years.The main groups prevalent in the northern region of Jiangsu were Ⅵ,Ⅶ,and the groups prevalent in central and southern regions of Jiangsu were relatively similar,with the Ⅱ,Ⅴ,Ⅵ,and Ⅶ as the mainstream groups.3)Based on the avirulence genes detection results,distribution regions,and pathogenic phenotypes of each single spore isolates,a total of 108 isolates were selected as representative isolates of rice blast disease in Jiangsu and were divided into 6 groups(group A to group F).Using these six groups to identify seedling blast resistance of 104 japonica rice varieties/lines in Jiangsu,we found that the pathogenicity of each single spore isolate was between 0%-57.47%,with most isolates having a pathogenicity rate of less than 25%(80%),and only two isolates with a pathogenicity rate of over 50%.There were significant differences in the pathogenicity rates among different groups.Group C had the highest average pathogenicity rate of 25%,followed by Group B and Group D,both of which were 20%.The average pathogenicity rates of the remaining groups were all around 16.5%.From the perspective of collection times and regions,there were significant differences in the pathogenicity of isolates collected in different years,while the average pathogenicity of strains collected in different regions was relatively similar.In addition,we also found there was no significant correlation between the number of avirulence genes carried by each isolate and their pathogenicity through correlation analysis and linear regression analysis,and the presence or absence of 10 avirulence genes did not significantly affect the pathogenicity of each isolate.4)20 single spore isolates carrying target avirulence gene with as few other avirulence genes as possible were selected to identify 24 rice blast single gene lines under the background of LijiangxintuanhHeigu(LTH).The results showed that the resistance spectrum of LTH was only 10%,and each resistance gene could indeed improve rice blast resistance.The highest resistance spectrum was increased from 10%to 75%for Pizt and Pi7,while the lowest resistance spectrum was only 30%for IRBLk-ka(pik).Through comparing the combination resistance spectrums of rice resistance genes and the corresponding avirulence genes carried by the isolates,we found the resistance spectrums of 7 combinations with corresponding avirulence genes were not consistent.The above results do not conform to the"gene to gene" hypothesis,and the relevant reasons have been discussed.5)Among the 95 rice varieties/lines with complete identification results,we found that the seedling blast resistance of Jiangsu japonica rice varieties was generally good,,with 31 varieties having a resistance spectrum of over 90%,accounting for 32.63%of the tested varieties.There were 77 varieties with a resistance spectrum of over 75%,accounting for 81.05%of the tested varieties,and only 6 varieties(6.3%)had a resistance spectrum of less than 50%.We further analyzed the contribution level of each R gene to seedling blast resistance through logistic regression,and found that the R genes conferring good seedling blast resistance on Jiangsu japonica rice were mainly Pia,Pita,Pi3,and Pid2.Among them,Pita was the core resistance gene,and the combinations of other resistance genes with Pita significantly improved the seedling blast resistance of the variety.Among them,the combination of Pita+Pi3 had the highest resistance spectrum of 94.14%,followed by the combination of Pita+Pia with a resistance spectrum of 91.7%.The above results elucidated the geographical distribution,avirulence genes distribution,pathogenicity and other related characteristics of physiological races of Magnaporthe oryzae in Jiangsu,and screened out excellent genes/gene combinations for rice blast resistance in Jiangsu,providing a theoretical basis for the rational selection of genes for rice blast resistance breeding in Jiangsu,which has important practical significance. |
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