December 30, 2004
Source: British Soc. Plant Pathol., New Disease Reports, Vol. 10 [edited]

First report of an Rsv resistance-breaking isolate of Soybean mosaic virus in Korea
J.M. Koo, Department of Biology and Medicinal Science, Pai Chai University, Daejeon, Korea 302-735; B.K. Choi (as for Koo); H.J. Ahn (as for Koo); H.J. Yum (as for Koo); C.W. Choi [(as for Koo) and BioMedicinal Research Center, Pai Chai University, Daejeon, Korea 302-735]. Accepted for publication 17 Dec 2004.

Soybean mosaic virus (SMV), a member of the genus Potyvirus, is the most prevalent pathogen of soybean (_Glycine max_) in Korea. Since a single dominant gene, Rsv1 and its alleles, were identified in resistant soybean lines or cultivars (Chen et al., 2002), additional resistance genes Rsv3 and Rsv4 for SMV have been characterised
(Gunduz et al., 2002). In Korea, SMV-G5H in the 1980's (Cho & Chung, 1986) and SMV-G7H in the 1990's (Kim et al., 2003) were regarded as dominant resistance-breaking (RB) strains on soybean cultivars carrying Rsv1 alleles alone.

SMV isolate SMV-CN18 was collected from naturally-infected soybean, cv. Taeankong, at Chungnam Agricultural Research & Extension Services, Daejeon, Korea in 2000. It was identified serologically by positive reaction to SMV-G1 antiserum (ATCC PVAS-94) in an agarose gel double diffusion test, subjected to successive isolations from single lesions on bean (_Phaseolus vulgaris_) cv. Topcrop and maintained in soybean Lee 68 by mechanical inoculation.

To determine the biological traits of the isolate, differential soybean cultivars and lines, Lee68 (rsv), PI96983 (Rsv1), York (Rsv1-y), Marshall (Rsv1-m), Kwanggyo (Rsv1-k), Ogden (Rsv1-t), Suweon97 (Rsv1-h), L29 (Rsv3) and V94-5152 (Rsv4), were mechanically inoculated with the isolate and symptom development was observed for 2-3 weeks in a temperature-controlled greenhouse.

All soybean plants developed systemic mild mosaic symptoms on the upper uninoculated leaves, without causing local necrotic lesions on the lower leaves. To verify the systemic infection, upper leaves were collected from each cultivar, tested for the presence of virus by RT-PCR analysis and the positive results were confirmed.

The gene-specific primers to amplify coding regions of SMV-CN18 were designed based on the conserved nucleotide sequences of SMV-G2, -G7 and -N. The optimal conditions for RT-PCR consistently yielded amplified products of P1 (927 bp), HC-Pro (1371 bp), P3 (1197 bp), CI (1902 bp including 6K), NIa (1298 bp including VPg), NIb (1551 bp) and CP (795 bp) (Fig. 2).

The amplified DNA fragments encoding SMV genes were introduced into pGEM-T Easy Vector (Promega, USA), their nucleotide sequences were determined in both directions. In addition, the 5' and 3' non-coding regions were determined by 5'- and 3'-RACE kit (Takara, Japan). The complete sequence was submitted to the EMBL database under accession number AJ619757.

The identities of nucleotide sequences for the full length genome of SMV-CN18 were between 93 and 94 percent with known SMV strains (G2, G5K, G7, G7d, G7H, Aa, Huanghuai 5 (HH5), Severe (HZ), N, and Aa15-M2), and the identities of amino acid sequences for the coding regions were between 95 and 96 percent, confirming that CN18 is a strain of SMV.

This is the 1st report on the emergence of a SMV RB isolate overcoming the resistance genes, Rsv1, Rsv3 and Rsv4 in the soybean cultivars and lines.

References

Chen P, Buss GR, Tolin SA, Gunduz I, Cicek M, 2002. A valuable gene in Suweon 97 soybean for resistance to soybean mosaic virus. Crop Science 42, 333-7.

Cho EK, Chung KW, 1986. Strains of soybean mosaic virus causing soybean necrotic disease in Korea. Korean Journal of Breeding 18, 150-153.

Gunduz I, Buss GR, Chen P, Tolin SA, 2002. Characterization of SMV resistance genes in Tousan 140 and Hourei soybean. Crop Science 42, 90-5.

Kim YH, Kim OS, Lee BC, Moon JK, Lee SC, Lee JY, 2003. G7H, a new Soybean mosaic virus strain: its virulence and nucleotide sequence of CI gene. Plant Disease 87, 1372-1375.

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[The emergence of resistance-breaking strains of SMV is a blow to plant breeders and pathologists. SMV is one of the most widespread viruses in soybean, and new resistance-breaking strains continue to emerge. An example is the cultivar Hutcheson, developed in Virginia, that is resistant to the common strains of SMV, but new resistance-breaking (RB) isolates of SMV have emerged in natural infections to break the resistance of Hutcheson containing the Rsv1 allele. Resistance to SMV is controlled by single dominant genes at 3 distinct loci, Rsv1, Rsv3, and Rsv4. The Rsv3 gene induces extreme resistance, hypersensitive response, or restriction to virus replication and movement, which are strain-specific. The Rsv4 gene functions in a non-strain specific and non-necrotic manner,
restricting both cell-to-cell and long-distance movement of SMV. The Rsv1, Rsv3, and Rsv4 resistance genes exhibit a continuum of SMV-soybean interactions that include complete susceptibility, local and systemic necrosis, restriction of virus movement (both cell-to-cell and long distance), reduction in virus accumulation, and extreme
resistance with no detectable virus. Cultivars containing 2 genes for resistance (Rsv1 and Rsv3 or Rsv1 and Rsv4) were resistant to multiple strains of SMV tested and show great potential for gene pyramiding efforts to ensure a wider and more durable resistance to SMV in soybeans.

Links:
<http://www.bspp.org.uk/ndr/jan2005/2004-83.asp>
<http://scholar.lib.vt.edu/theses/available/etd-12182003-093547/unrestricted/FrontMatter.pdf>
- Mod.DH]