Soybean mosaic virus resistance in transgenic soybean caused by posttranscriptional gene silencing

Somatic embryos of Glycine max (L.) Merrill cultivar 'Jack' were co-transformed with the coat protein (CP) gene of attenuated isolates of Soybean mosaic virus (SMV) and hygromycin phosphotransferase gene by microprojectile bombardment. CP gene was detected in eleven transgenic plants, and three independent lines highly resistant to SMV were obtained in a previous study. One of these lines, line No. 55, which was assumed to have acquired RNA-mediated resistance, was selected for further gene expression analysis of T-4 and T-5 plants in relation to their viral resistance. The resistant plants contained a lower level of transgene-derived RNA than the susceptible ones. On the other hand, based on RNA analysis after mechanical inoculation, SMV-specific RNAs were detected faintly in the resistant plants, while large amounts of RNAs were found in the susceptible ones. During the development of the resistant lines, SMV CP sequence-specific small interfering RNAs (siRNAs) appeared initially in the leaves of the first leaf stage but not in those of the primary stage, and were detected thereafter in the leaves of later stages constantly. The presence of the siRNAs before SMV inoculation was strongly correlated with the resistant phenotype of the lines tested.