Molecular basis of resistance to soybean mosaic virus: reverse transcription (RT)-PCR and strain complementation analysis in soybeans
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Soybean (Glycine max(L.) Merr.) is grown in both developed and developing countries primarily for vegetable oil and protein products. It is the third most widely planted crop in the United States. Soybean mosaic virus (SMV) can affect soybean production by causing reduction in grain yield and quality. The virus is a member of the large and economically important family Potyviridae and several strains have been identified. Soybean lines exist that contain single genes (Rsv) for resistance to SMV. Therefore, SMV and soybean have been used as a model to understand plant resistance to virus disease. One facet of this study has been development of an accurate reverse transcription-polymerase chain reaction (RT-PCR)-based assay for differentiation of SMV strains. This technique was then used to study evidence for strain complementation in soybeans;Because SMV strains can occur as mixtures in infected soybean (cultivar Williams 82), an RT-PCR was developed to detect and discriminate among strains at the RNA level. The assay was applied to virus strains G2 and G7 and utilized oligonucleotides complementary to unique regions in the cylindrical inclusion protein cistron. Total RNAs from plants infected with either strain or a mixture of strains were amplified with G2 and G7 specific primers. Specific fragments were obtained. The assay allowed discrimination of strains in a mixed infection of a single soybean host plant;Previous evidence for putative complementation of the SMV strain G2, that does not infect soybean lines containing the Rsv gene, was reexamined. The model suggested that cell-to-cell spread of G2 could occur in plants systemicaly preinfected with the related SMV G7 or G7a strains in soybean lines PI 96983, L78-379, and Davis (immune to G2, but susceptible to G7 and G7a). Time course assay revealed differences in infectivity profiles of tissues from inoculated lines PI 96983 and L78-379. The optimum time for analysis of virus strains in the infected plants of the two genotypes was 20 to 25 days after inoculation. Analysis provided no evidence that G2 movement could be complemented by G7 or G7a in lines PI 96983, L78-379 and Davis.