Evolution of Soybean mosaic virus-G7 molecularly cloned genome in Rsv1-genotype soybean results in emergence of a mutant capable of evading Rsv1-mediated recognition.

Plant resistance (R) genes direct recognition of pathogens harboring matching avirluent signals leading to activation of defense responses. It has long been hypothesized that under selection pressure the infidelity of RNA virus replication together with large population size and short generation times results in emergence of mutants capable of evading R-mediated recognition. In this study, the Rsv1/Soybean mosaic virus (SMV) pathosystem was used to investigate this hypothesis. In soybean line PI 96983 (Rsv1), the progeny of molecularly cloned SMV strain G7 (pSMV-G7) provokes a lethal systemic hypersensitive response (LSHR) with up regulation of a defense-associated gene transcript (PR-1). Serial passages of a large population of the progeny in PI 96983 resulted in emergence of a mutant population (vSMV-G7d), incapable of provoking either Rsv1-mediated LSHR or PR-1 protein gene transcript up regulation. An infectious clone of the mutant (pSMV-G7d) was synthesized whose sequences were very similar but not identical to the vSMV-G7d population; however, it displayed a similar phenotype. The genome of pSMV-G7d differs from parental pSMV-G7 by 17 substitutions, of which 10 are translationally silent. The seven amino acid substitutions in deduced sequences of pSMV-G7d differ from that of pSMV-G7 by one each in P1 proteinase, helper component-proteinase, and coat protein, respectively, and by four in P3. To the best of our knowledge, this is the first demonstration in which experimental evolution of a molecularly cloned plant RNA virus resulted in emergence of a mutant capable of evading an R-mediated recognition.