Transgenic soybean (Glycine max) plants expressing Soybean mosaic virus (SMV) helper component-protease (HC-Pro) showed altered vegetative and reproductive phenotypes and responses to SMV infection. When inoculated with SMV, transgenic plants expressing the lowest level of HC-Pro mRNA and those transformed with the vector alone initially showed mild SMV symptoms. Plants that accumulated the highest level of SMV HC-Pro mRNA showed very severe SMV symptoms initially, but after 2 weeks symptoms disappeared, and SMV titers were greatly reduced. Analysis of SMV RNA abundance over time with region-specific probes showed that the HC-Pro region of the SMV genome was degraded before the coat protein region. Transgenic soybean plants that expressed SMV HC-Pro showed dose-dependent alterations in unifoliate leaf morphologies and seed production where plants expressing the highest levels of HC-Pro had the most deformed leaves and the lowest seed production. Accumulation of microRNAs (miRNAs) and mRNAs putatively targeted by miRNAs was analyzed in leaves and flowers of healthy, HC-Pro-transgenic, and SMV-infected plants. Neither expression of SMV HC-Pro nor SMV infection produced greater than twofold changes in accumulation of six miRNAs. In contrast, SMV infection was associated with twofold or greater increases in the accumulation of four of seven miRNA-targeted mRNAs tested.

Soybean mosaic virus (SMV) helper component protease (HC-Pro), a suppressor of post-transcriptional gene silencing, was evaluated for its ability to enhance production of soybean hygromycin-resistant somatic embryos (HR-SEs), and stabilize transgene expression. Immature soybean cotyledonary explants were co-cultured with Agrobacterium tumefaciens strain KYRT1 harboring either pCAMBIA1302, carrying a hygromycin phosphotransferase gene (hpt) and a gene encoding green fluorescent protein; pCAMBIA1305.1, carrying hpt and β-glucuronidase (uidA) genes; pG2-HC-Pro, a derivative of pCAMBIA1305.1 containing SMV G2 HC-Pro; or pG5-HC-Pro, a derivative of pCAMBIA1305.1 containing SMV G5 HC-Pro, but lacking uidA. Significantly (ρ < 0.02) higher numbers of HR-SEs were obtained from explants transformed with Agrobacterium harboring either pG2-HC-Pro or pG5-HC-Pro than with either of the vector controls (pCAMBIA1302 or pCAMBIA1305.1). β-glucuronidase (GUS) expression was significantly (P < 0.003) higher in 50-day-old transgenic plants expressing GUS along with SMV-HC-Pro and in SMV-infected GUS transgenic plants than in transgenic plants expressing GUS alone. Together, these data suggest that SMV-HC-Pro enhanced recovery of HR-SEs by suppressing silencing of the hygromycin phosphotransferase gene.

Fifty-two North American (NA) ancestral soybean [Glycine max (L.) Merr.] lines were screened for resistance to Bean pod mottle virus (BPMV), Soybean mosaic virus (SMV) strains G1 and G5, Tobacco ringspot virus (TRSV), and Tobacco streak virus (TSV). Seven ancestors, ‘CNS’, ‘Haberlandt’, ‘Ogden’, ‘Peking’, PI 71506, PI 88788, and ‘Tokyo’, were resistant to SMV-G1. Sixteen entries, ‘A.K. (Harrow)’, ‘Capital’, CNS, FC 33243, Haberlandt, ‘Illini’, ‘Improved Pelican’, ‘Laredo’, ‘Lincoln’, ‘Mandarin’, ‘Mandarin (Ottawa)’, Ogden, ‘Palmetto’, Peking, PI 88788, and Tokyo were resistant to SMV-G5. All ancestral lines tested were susceptible to BPMV and TRSV. Only one ancestor, ‘Tanner’, was resistant to TSV. On the basis of cultivar registration articles through 2002, there were 15 public soybean cultivars with reported resistance to SMV. The possible donors of resistance for each were identified. Two soybean ancestors, CNS and Ogden, were the most important possible sources of SMV resistance genes in U.S. commercial soybean cultivars, as the pedigree of 75 and 56% of the reported resistant cultivars contained CNS and Ogden, respectively. In most of the cultivar registration articles, reactions to SMV were not reported. With the relatively high frequency of SMV resistance in major ancestral lines, SMV resistance in U.S. cultivars may be more common than expected.

Soybean mosaic virus (SMV) is an aphid- and seed-transmitted virus that infects soybean (Glycine max) plants and causes significant yield losses. Seed-borne infections are the primary sources of inoculum for SMV infections. The strain specificity of SMV transmission through seed and SMV-induced seed-coat mottling were investigated in field experiments. Six soybean plant introductions (PIs) were inoculated with eight SMV strains and isolates. Transmission of SMV through seed ranged from 0 to 43%, and isolate-by-soybean line interactions occurred in both transmission rates and percentages of mottled seeds. For example, SMV 746 was transmitted through 43% of seed in PI 229324, but was not transmitted through seed of PIs 68522, 68671, or 86449. In contrast, SMV 413 was transmitted through seed from all PIs. SMVs that were transmitted poorly by the Asian soybean aphid, Aphis glycines, also were transmitted poorly through seed. No predicted amino acid sequences within the helper-component protease or coat protein coding regions differentiated the two groups of SMV strains. The loss of aphid and seed transmissibility by repeated mechanical transmission suggests that constant selection pressure is needed to maintain the regions of the SMV genome controlling the two phenotypes from genetic drift and loss of function.