Gain of Virulence on Rsv1-Genotype Soybean by an Avirulent Soybean mosaic virus Requires Concurrent Mutations in Both P3 and HC-Pro

In soybean, Rsv1, a single dominant resistance gene, invokes extreme resistance (ER) against most Soybean mosaic virus (SMV) strains, including SMV-N, but not SMV-G7, which provokes a virulent lethal systemic hypersensitive response (LSHR). The elicitor functions of the two viruses provoking Rsv1-mediated ER and LSHR have been mapped to the N-terminal 271 amino acids of P3 from SMV-N and SMV-G7, respectively, which differ by nine residues between the two strains. To identify amino acids of P3 from SMV-N provoking Rsv1-mediated ER, the unique residues of SMV-G7 were substituted with those of SMV-N. Of the mutants tested on Rsv1-genotype soybean, only SMV-G7(I788R) and SMV-G7(T948A) lost virulence. However, substitution of amino acids of SMV-N, individually or in combination, with the reciprocal residues from SMV-G7 at these two positions failed to confer virulence to SMV-N. In the search for additional virulence determinants, a series of SMV-N chimeras was generated in which fragments within a region from near the middle of the helper-component proteinase (HC-Pro) cistron to the 5' end of the cytoplasmic inclusion cistron, nucleotides 1,605 to 3,787, were replaced with those of SMV-G7. Only SMV-N-derived chimeras harboring the 3' region of HC-Pro, at least from nucleotide 2,013, and the entire 5' end of P3 (nucleotides 2,430 to 3,237) from SMV-G7 were virulent whereas reciprocal exchanges resulted in loss of SMV-G7 virulence. This region of HC-Pro differs by three amino acids between SMV-N and SMV-G7. Analyses of SMV-G7-derived HC-Pro site-directed mutants showed that only SMV-G7(M683R) lost virulence on Rsv1-genotype soybean; however, SMV-N(R682M) failed to gain virulence. Nevertheless, an SMV-N derived mutant with three concurrent substitutions, R682M+R787I+A947T, gained virulence. The data indicate that both P3 and HC-Pro are involved in virulence of SMV on Rsv1-genotype soybean.