Soybean [Glycine max (L.) Merr.] PI486355 is resistant to all the identified strains of soybean mosaic virus (SMV) and possesses two independently inherited resistance genes. To characterize the two genes, PI486355 was crossed with the susceptible cultivars 'Lee 68' and 'Essex' and with cultivars 'Ogden' and 'Marshall', which are resistant to SMV-G1 but systemically necrotic to SMV-G7. The F2 populations and F2∶3 progenies from these crosses were inoculated with SMV-G7 in the greenhouse. The two resistance genes were separated in two F3∶4 lines, 'LR1' and 'LR2', derived from Essex x PI486355. F1 individuals from the crosses of LR1 and LR2 with Lee 68, Ogden, and 'York' were tested with SMV-G7 in the greenhouse; the F2 populations were tested with SMV-G1 and G7. The results revealed that expression of the gene in LR1 is gene-dosage dependent, with the homozygotes conferring resistance but the heterozygotes showing systemic necrosis to SMV-G7. This gene was shown to be an allele of the Rsv1 locus and was designated as Rsv1-s. It is the only allele identified so far at the Rsv1 locus which confers resistance to SMV-G7. Rsv1-s also confers resistance to SMV-G1 through G4, but results in systemic necrosis with SMV-G5 and G6. The gene in LR2 confers resistance to strains SMV-G1 through G7 and exhibits complete dominance. It appears to be epistatic to genes at the Rsv1 locus, inhibiting the expression of the systemic necrosis conditioned by the Rsv1 alleles. SMV-G7 induced a pin-point necrotic reaction on the inoculated primary leaves in LR1 but not in LR2. The unique genetic features of the two resistance genes from PI486355 will facilitate their proper use and identification in breeding and contribute to a better understanding of the interaction of SMV strains with soybean resistance genes.

No abstract

The gene symbol Rsv2 was previously assigned to the gene in the soybean [Glycine max (L.) Merr.] line OX670 for resistance to soybean mosaic virus (SMV). The Rsv2 gene was reported to be derived from the Raiden soybean (PI 360844) and to be independent of Rsv1. Accumulated data from our genetic experiments were in disagreement with this conclusion. In this study, Raiden and L88-8431, a Williams BC5 isoline with SMV resistance derived from Raiden, were crossed with two SMV-susceptible cultivars to investigate the mode of inheritance of SMV resistance in Raiden. They were also crossed with five resistant cultivars to examine the allelomorphic relationships of the Raiden gene with other reported genes at the Rsv1 locus. F1 plants, F2 populations, and F2-derived F3 (F2:3) lines were tested with SMV strains G1 or G7 in the greenhouse or in the field. The individual plant reactions were classified as resistant (R, symptomless), necrotic (N, systemic necrosis), or susceptible (S, mosaic). The F2 populations from R x S crosses segregated in a ratio of 3 (R + N):1 S and the F2:3 lines from Lee 68 (S) x Raiden (R) exhibited a segregation pattern of 1 (all R):2 segregating:1 (all S). The F2 populations and F2:3 progenies from all R x R crosses did not show any segregation for susceptibility. These results demonstrate that the resistance to SMV in Raiden and L88-8431 is controlled by a single dominant gene and the gene is allelic to Rsv1. The heterozygous plants from R x S and R x N crosses exhibited systemic necrosis when inoculated with SMV G7, indicating a partial dominance nature of the resistance gene. Raiden and L88-8431 are both resistant to SMV G1-G4 and G7, but necrotic to G5, G6, and G7A. Since the resistance gene in Raiden is clearly an allele at the Rsv1 locus and it exhibits a unique reaction to the SMV strain groups, assignment of a new gene symbol, Rsv1-r, to replace Rsv2 would seem appropriate. Further research is ongoing to investigate the possible existence of the Rsv2 locus in OX670 and its relatives.

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.

OX670. Since Raiden was the only parent in the pedigree with known SMV resistance, it was assumed to be Soybean mosaic virus (SMV) resistance in the soybean [Glycine the source of resistance in OX670 (Fig. 1) (Buzzell and max (L.) Merr.] breeding line OX670 previously was postulated to . The Rsv3 gene derived from 'Columbia' conbe controlled by a gene derived from the cultivar Raiden and designated as Rsv2. Subsequently, it was shown that Raiden has a single ditions systemic necrosis to SMV-G1 (Buzzell and Tu, resistance gene at the Rsv1 locus, suggesting that the resistance gene 1989). Another Rsv3 allele from 'Hardee' confers susin OX670 was not from Raiden. Most of the remaining ancestry of ceptibility to SMV strains G1 through G4, but resistance OX670 is derived from 'Harosoy'. The objectives of this study were to strains G5 through G7 (Buss et al., 1999). to determine (i) the reaction of Harosoy to SMV-G1 through G7 As previously mentioned PI486355 possesses two instrains; (ii) the inheritance of SMV resistance in Harosoy and OX670; dependent resistance genes, one of which is at the Rsv1 and (iii) the allelomorphic relationship of resistance genes in these locus. The other resistance gene (non-Rsv1) was isocultivars with previously described resistance genes. OX670 and Harolated in the soybean line LR2 and found to be indepensoy were crossed with the SMV susceptible cultivar Lee 68 to study dent of Rsv1 and Rsv3 (Ma et al., 1995). LR2 was a the inheritance of resistance. OX670 and Harosoy were also crossed selection from PI486355 ϫ 'Essex'. V94-5152 is a resewith the resistant lines L78-379, L88-8431, PI96983, L29, and V94-5152 to elucidate the allelomorphic relationships between the genes in lection from LR2 that was registered as resistant germ-OX670, Harosoy, and previously reported genes. Our results indicated plasm (Buss et al., 1997). Due to the lack of an allelism that Harosoy, which is resistant to SMV-G5 through G7 and susceptitest with an Rsv2 source, a gene symbol has not been ble to SMV-G1 through G4, possesses a single partially dominant assigned to the non-Rsv1 gene in V94-5152. However, SMV resistance gene at the Rsv3 locus. Inheritance studies indicatedit will be referred to as Rsv4 for reference purposes in that OX670, which is resistant to SMV-G1 through G7, possesses two this paper.independent dominant genes for SMV resistance. One is allelic to the Buzzell and Tu's (1984) conclusion that OX670 con-Rsv1 locus and derived from Raiden, while the other is allelic to the tains Rsv2 was based on the segregation pattern ob- Rsv3 locus and derived from Harosoy. Presence of both Rsv1 andtained in an F 2 population of OX670 (R) ϫ L78-379 Rsv3 in OX670 confers resistance to SMV-G1 through G7. Therefore, (S). Although, they described L78-379 as necrotic to the previously proposed Rsv2 locus does not appear to exist in OX670 or its ancestors.