Strain Identification Of Soybean Mosaic Virus And Inheritance And Gene Mapping Of Its Resistance In Soybeans In Southern China

Soybean mosaic virus （SMV） disease is one of the most common and destructive viral diseases in soybean production worldwide. It declines the soybean yield and degrades the soybean commodity. A lot of studies on identification of SMV strains, screening for resistant germplasm, inheritance and gene mapping for SMV resistance in soybeans had been conducted. There is no a uniform SMV strain identification system worldwide so far, and now, the same set of soybean differentials for SMV strain identification was used In USA and Korea while another set of differentials was used in Japan, however in China, so many SMV strain differential systems and strains were reported. Too many differential systems did not facilitate the establishment of breeding programs for resistance to SMV, but made at certain extent confusion for soybean breeders in identification of resistant sources, exchange of resistance information and utilization of the nationwide germplasm resources. Therefore, soybean breeders expect to establish a uniform differential system in identifying SMV strains so that scientists can exchange information and resistance materials based on a common understanding. The uniform SMV strains differential system in China had been set up by Wang et al.（2003）, Yang （2002）, Zhan et al.（2006）.17strains （SC1-SC17） had been identified and reported in Nothern China and Huang-Huai Valleys under this set of soybean differentials system. Soybean production in Southern China, one of the three major soybean producing regions, the soybeans in this region are growing in various seasons from spring to winter under different cropping systems, which causes a continuous growing and infection of the SMV disease and its vector host soybean aphids and causes much more serious damage to the soybean in this region than in the other two regions. However, no knowledge about SMV strains in Southern China was available as yet, and therefore the objective of the present study was to make a thorough study on SMV strains in13provinces in Southern China based on the work in other areas by Wang et al.（2003）, Yang（2002）, Zhan et al.（2006）, Guo et al.（2005）, and Wang et al.（2005）, including to collect SMV samples thoroughly from13provinces in Southern China, to obtain the isolates from the specimens through single lesion isolation purification, serological determination, and to identify the SMV strains from the isolates under the same set of differentials, to reveal the constitution and distribution of SMV strains in Southern China, furtherly using software NTSYS-pc to cluster the21strains founded in China, linked with various soybean ecological regions and reveal the major widespread strains in various soybean ecological regions and the trend of virulence of SMV strains in China from North to South; and to make a study on inheritance and gene mapping of resistance for new found SMV strains in Southern China. The major results were as follows:（1） Identification and distribution of SMV strains in soybean production in Soutern China:In the present study,201SMV isolates were obtained after biological purification with single lesion isolation and serological determination from1417leaf specimens with SMV-like symptoms collected from1500field plots of117counties of13provinces（south of Shaanxi, Sichuan, Chongqing, Jiangsu, Shanghai, Zhejiang, Jiangxi, Hunan, Fujian, Guangxi, Guangdong, Guizhou and Yunnan） in Southern China during2004-2006. Based on the disease reactions on10soybean differentials （Nannong1138-2, Youbian30,8101, Tiefeng25, Davis, Buffalo, Zaoshu18, Kwanggyo, Qihuang No.1and Kefeng No.1）set up by Wang-Yang-Zhan （WYZ）as a representative from various reported differential systems, the201isolates were grouped into12strains according to their responses to the10soybean differentials. Of these12strains, eight were the same as the previously reported ones from other regions while the other four were new ones and designated as SC18, SC19, SC20, and SC21according to the nomination of SMV strains set by Wang et al.（2003）. The strains SC15and SC18were predominant and widespread strains in the Southern China, accounted for32.3%and26.4%of the total isolates and distributed in nine and ten provinces, respectively; In addition, SC7, SC9and SC10, detected in7,6and9provinces, respectively, were also the widespread and virulent strains in Southern China. The results of distribution of SMV strains showed that the strains SC15and SC20were predominant in Hunan, SC15and SC18were predominant in Fujian and Guangxi, SC15was predominant in Guangdong, SC10and SC18were predominant in Zhejiang.（2） Cluster of21SMV strains and the trend of virulence of SMV strains in China:So far, under the uniform differential system （set by WYZ）, total21strains （SC1-SC21） have been identified in all the soybean producing regions in China. They were grouped into five clusters under coefficient of similarity0.6, including Cluster Ⅰ with nine less virulent strains, Cluster Ⅱ with nine moderate virulent strains, and Cluster Ⅲ, Ⅳ, Ⅴ with only one virulent strain in each cluster, respectively. To link the results of virulence clustering analysis of21SMV strains in China with various soybean ecological regions, SC11in cluster I was the major widespread strain in Northeast China; SC3, SC11and SC7in cluster I and II, respectively, were the major widespread strains in Huang-Huai valley; while SCI8, SC7, SC10, SC9and SC15in cluster I, II, III, and V, respectively, were the major widespread strains in Southern China. Although the presence of less virulent and virulent strains overlapped among the three major soybean producing regions, less virulent strains in Cluster I were predominant in Northeast China, less and moderate virulent strains in Cluster I and II predominant in Huang-Huai valley, but moderate and most virulent strains in Cluster II, III and V predominant in Southern China. Therefore the trend of virulence of SMV strains in China appeared a gradually increase from the North to the South. The most virulent strain SC15, was widespread strain and, attention should be paid to its prevalence in Southern China.（3） Results of inheritance and gene mapping of its resistance to the widespread strain SC18in soybeans:The six RxS crosses （Kefeng No.lxNannong1138-2, Qihuang22xNannong1138-2, Zhongzuo00-683xNannong1138-2,8101×Bindou95-20,8101×Dongda No.2and Zhongpin661×Nannong1138-2） all F1plants of were resistant to SC18, the F2population showed a segregation of3R:1S, and all the F23lines showed a segregation of1R:2Seg:1S. And also184Kefeng No.1×Nannong1138-2RIL lines showed segregation with a ratio1R:1S. All of the above results showed that the six R parents carry a single dominant gene condition the resistance to SC18, respectively.Segregation in the F2population from RxR cross’Kefeng No.l xQihuang22’was consistent with a ratio of15R:1S. It was inferred that the resistance gene between Kefeng No.1and Qihuang22, were not at the same locus, and the two parents carry a single gene for resistant to SC18, respectively, the following text （Results of gene mapping for resistance to SC18in Kefeng No.1and Qihuang22） futher validated the conclusion in this study. It was primarily referred that the two dominant genes condition the resistance for SMV strain SC18in soybeans, and the two genes inherit independently. The gene symbol Rsc18A for resistance to SC18was assigned to Kefeng No.1, and inferred the genotype of Kefeng No.1is Rsc18ARsc18Arsc18B rsc18B, and another gene symbol Rsc18B was assigned to Qihuang No.22, and its corresponding genotype is rsc18A rsc18A Rsc18B Rsc18B.The genotype of Nannong1138-2referred was rsc18Arsc18A rsc18Brsc18B. Combined the results of responses of184RIL lines to SMV strains SCI8with the new integrated molecular marker map of Kefeng No.1×Nannong1138-2（Wang et al.2008, unpublished data）, Rsc18A was mapped on linkage group D1b using software Joinmap3.0and MapChart2.1. The distance and sequence between R gene and markers were Sat₃51（12.3cM）-Rsc18A-GNB095（2.8cM）. Meanwhile,220F2plants of Qihuang22×Nannong1138-2were used for the study of gene mapping for resistance to SC18, the bulked segregant analysis （BSA） of SSR markers was conducted to tag the Rsc18B gene. The three SSR markers Satt114, Soyhsp176and Satt334showed good polymorphism not only between Qihuang22（PI） and Nannong1138-2（P2）, but also between BR（DNA bulks of Resistant plants） and Bs （DNA bulks of Susceptible plants）. Therefore, they were further used combined with220F2plants of Qihuang22×Nannong1138-2to detect the relationship of linkage among markers and R gene. The results showed that all three markers Satt114, Soyhsp176and Satt334were closely linked to Rsc18B, with genetic distance of5.6cM,6.7cM and5.0cM, respectively. According to the integrated molecular marker map by Song et al.（2004）, Rsc18B was mapped on linkage group F.（4） Results of inheritance and gene mapping for its resistance to SC19, SC20and SC21in soybeans:Kefeng No.1, Nannong1138-2, and F,,184RIL lines was inoculated to SC19, SC20and SC21separately, the results showed that Kefeng No.1is resistant to the three strains, while Nannong1138-2is susceptible to them. F1plants showed susceptible to SC19, but resistant to SC20and SC21. According to the results of x test, responses of184lines showed1R:3S to SC19, and showed1R:1S to SC20and SC21. It was referred that two recessive genes control the resistance to SC19（this results is to be further validated by using more segregation populations） and a single dominant gene controlling the resistance to SC20and SC21in soybean Kefeng No.1. The gene symbols Rsc20and Rsc21were assigned to Kefeng No.1, respectively.As the genotype in soybean Kefeng No.1for resistance to SC19was unknown in the present study, and therefore, in the present study, gene mapping for resistance to SC20and SC21in soybean were only conducted using Kefeng No.1×Nannong1138-2RIL lines. Combined the results of responses of184RIL lines to SMV strains SC20and SC21with the new integrated molecular marker map of Kefeng No.1×Nannong1138-2（Wang et al.2008, unpublished data）,Rsc20and Rsc21were mapped on linkage group Dlb using software Joinmap3.0and MapChart2.1. Closely linked molecular markers with each R gene were detected in both flanks of each three R gene, especially the two BAC-SSR markers GNB026and GNB053flanks of Rsc20, with a genetic distance1.5cM and1.1cM to Rsc20, the both markers were closely linked to it. The results of the present study lay a certain foundation that breeding for resistance by MAS, fine mapping and map-based cloning of R gene.