Genetic study of a lethal necrosis to soybean mosaic virus in PI 507389 soybean.

PI 507389 soybean [Glycine max (L.) Merr.], a large-seeded line from Japan, exhibits a rapid, lethal, necrotic response to strains G1, G2, G5, and G6 of soybean mosaic virus (SMV). Unlike the hypersensitive necrotic reaction, this stem-tip necrosis can be a serious threat to soybean production. To investigate the genetic basis of lethal necrosis (LN), PI 507389 was crossed with the susceptible (S) cv. Lee 68 and with resistant (R) lines PI 96983, cv. York, and cv. Marshall, which carry single dominant genes for SMV resistance at the Rsv1 locus. F(1) plants, F(2) populations, and F(2:3) lines were inoculated with G1 and G6 in the greenhouse or in the field. Results indicated that LN is controlled by a single gene allelic to Rsv1, and this allele in PI 507389 is recessive to R alleles in PI 96983, York, and Marshall. The LN allele is codominant with the allele for S, for the heterozygotes showed a mixed phenotype of both necrosis (N) and mosaic (M) symptoms (NM). The LN allele becomes recessive to the S allele as the mixed NM shifts to S at a later stage in response to more virulent strains. The gene symbol Rsv1-n is assigned for the allele conferring LN in PI 507389. Rsv1-n is the only allele at the Rsv1 locus conditioning N to G1 and no R to any other SMV strains, and thus a unique genotype for SMV strain differentiation. The phenotypic expression of heterozygotes and the dominance relationships among R, N, and S depend on the virulence of SMV strains, source of alleles, and developmental stage.