BRIEF REPORT A single amino acid change in HC-Pro of soybean mosaic virus alters symptom expression in a soybean cultivar carrying Rsv1 and Rsv3 Jang-Kyun Seo • Seong-Han Sohn • Kook-Hyung Kim Received: 30 June 2010 / Accepted: 29 September 2010 / Published online: 12 October 2010 Ó Springer-Verlag 2010 Abstract It is generally believed that infidelity of RNA virus replication combined with R-gene-driven selection is one of the major evolutionary forces in overcoming host resistance. In this study, we utilized an avirulent soybean mosaic virus (SMV) mutant to examine the possibility of emergence of mutant viruses capable of overcoming R-gene-mediated resistance during serial passages. Inter- estingly, we found that the emerged progeny virus induced severe rugosity and local necrotic lesions in Jinpumkong-2 (Rsv1 ? Rsv3) plants, while SMV-G7H provoked a lethal systemic hypersensitive response. Genome sequence anal- ysis of the emerged progeny virus revealed that the muta- tion in CI that had caused SMV-G7H to lose its virulence was restored to the original sequence, and a single amino acid was newly introduced into HC-Pro, which means that the symptom alteration was due to this single amino acid mutation in HC-Pro. Our results suggest that SMV HC-Pro functions as a symptom determinant in the SMV-soybean pathosystem. soybean mosaic virus (SMV), one of the most common viruses in soybean, is a single-stranded positive-sense RNA virus belonging to the genus Potyvirus [19]. As in all potyviruses, the SMV genome encodes a large polyprotein of about 350 kDa, which is cleaved to yield at least 10 mature proteins: P1, HC-Pro, P3, 6K1, cylindrical inclusion (CI), 6K2, VPg, NIa-Pro, NIb, and coat protein (CP) [21]. Recently, an additional small open reading frame embed- ded in the P3 cistron of potyviruses, termed pipo, has been discovered [3, 29]. SMV can cause very different disease symptoms, such as mosaic, mottling, stunting, necrosis, and/or rugosity in leaves, depending on the strain-cultivar specificity. While three distinct resistance (R) genes, Rsv1 (several alleles at the Rsv1 locus), Rsv3, and Rsv4, con- ferring resistance to different strains of SMV have been characterized [2, 10, 15], accumulating evidence has sug- gested that a variety of disease reactions caused by SMV are the result of interactions between strain-specific path- ogenic determinants and different Rsv genes [7, 13, 24]. Recently, strain-specific P3 and HC-Pro proteins of SMV have been identified as pathogenic determinants of Rsv1- mediated disease reactions [7, 13], and SMV CI protein has been characterized as a pathogenic determinant of Rsv3- mediated disease reactions [24]. RNA viruses are known to exist as quasispecies because of the lack of proofreading ability of the viral RNA- dependent RNA polymerase (RdRp) [5]. It is believed that this genetic heterogeneity of the virus population is J.-K. Seo Á K.-H. Kim (&) Department of Agricultural Biotechnology and Plant Genomics and Breeding Institute, College of Agriculture and Life Sciences, Seoul National University, Seoul 151-921, Korea e-mail: kookkim@snu.ac.kr S.-H. Sohn National Academy of Agricultural Science, Rural Development Administration, Suwon 441-707, Republic of Korea K.-H. Kim Research Institute for Agriculture and Life Sciences, Seoul National University, Seoul 151-921, Republic of Korea Present Address: J.-K. Seo Department of Plant Pathology and Microbiology, University of California, Riverside, CA 92521, USA 123 Arch Virol (2011) 156:135–141 DOI 10.1007/s00705-010-0829-3