Soybean mosaic virus HC-Pro enhances viral symptoms, bean pod mottle virus and transgene RNA stability,siRNA accumulation, and reduces soybean seed production

The interactions between plant viruses and their hosts are constantly evolving. When plants produce a defense system against viral infection, viruses develop means to overcome this host response. Soybean [Glycine max (L.) Merr.] plants infected with Bean pod mottle virus (BPMV) alone develop acute symptoms that nearly disappear as plants grow. If BPMV infection is accompanied with Soybean mosaic virus (SMV), then severe symptoms develop. Plant recovery from a single infection by BPMV has been linked to degradation of viral RNA by post-transcriptional gene silencing (PTGS), which is characterized by the accumulation of small interfering RNAs (siRNAs). In soybean plants infected with BPMV alone, siRNA accumulation was correlated with accumulation of BPMV genomic RNAs, but siRNA were not detected in plants that recovered from symptom. Soybean plants infected with BPMV and SMV accumulated high levels of BPMV RNA and BPMV siRNAs. Similarly, BPMV-infected transgenic plants expressing SMV helper component protease (HC-Pro) at high levels had severe symptoms and enhanced accumulation of BPMV RNA and siRNAs. Transgenic lines expressing high levels of SMV HC-Pro mRNA had altered unifoliate leaf morphologies and reduced seed production. When inoculated with three SMV strains, the severity of initial symptoms was directly related to the levels of HC-Pro mRNA accumulation in the transgenic plants. i.e, plants expressing the lowest level of HC-Pro mRNA showed mild initial symptoms, and plants accumulating the highest level of SMV HC-Pro initially showed very severe symptoms. Later, symptoms disappeared and SMV titers were greatly reduced. The accumulations of SMV-specific siRNAs were related to the levels of viral genomic RNA accumulation, and siRNA diminished as symptoms disappeared in SMV infected transgenic lines. Tobacco ring spot virus (TRSV) induced a hypersensitive reaction when inoculated on transgenic plants that expressed G2 HC-Pro, but not in transgenic plants expressing G5 HC-Pro. SMV HC-Pro enhanced the recovery of hygromycin-resistant somatic embryos from immature soybean cotyledonary explants co-cultured with Agrobacterium . Expression of SMV HC-Pro in transgenic plants partially stabilized the expression of a beta-glucuronidase (GUS) transgene that was susceptible to PTGS. SMV infection stabilized GUS gene expression, enzyme activity, and enhanced GUS siRNA accumulation.