A Variant of Cucumber mosaic virus Is Restricted to Local Lesions in Inoculated Tobacco Leaves with a Hypersensitive Response

A variant of Cucumber mosaic virus, CMV(Y/GM2), was isolated from a tobacco plant with mild green mosaic symptoms that was regenerated in vitro from a yellow strain of CMV [CMV(Y)]-infected tobacco leaves by tissue culture. CMV(Y/GM2) has two amino acid substitutions at 36 and 111 positions in the coat protein encoded on RNA3. CMV, assembled by mixing in vitro transcribed CMV(Y) RNA1 and RNA2 plus infectious RNA3 transcribed in vitro from cDNA to RNA3 of CMV(Y/GM2), was prepared and designated as CMV(Y/GM2)tr. When tobacco (Nicotiana tabacum cv. Xanthi nc) plants were inoculated with CMV(Y/GM2)tr, large necrotic local lesions in which the virus was localized, developed on the inoculated leaves. This host response unique to CMV(Y/GM2)tr was similar to the hypersensitive response (HR), which is a common resistance response to avirulent pathogens and was observed in five cultivars of Nicotiana tabacum and eight Nicotiana species. The revertant virus, however, accumulated to quite different levels in the various hosts. CMV(Y/GM2)tr induced pathogenesis-related 1 (PR-1) protein accumulation and systemic acquired resistance (SAR) which were generally observed in the HR. However, when tobaccos were inoculated with CMV(S36P)tr and CMV(V111I)tr, which have an amino acid substitution at either the 36 or 111 position in the coat protein of CMV(Y), respectively, CMV(S36P)tr was restricted to the primary infection site without necrotic local lesion formation and PR-1 protein and SAR induction. CMV(V111I)tr, however, systemically spread and induced mild green mosaic symptoms, while the host had the HR to CMV(Y/GM2)tr. The localization of CMV(Y/GM2)tr at the primary infection site may not only be caused by the HR, but also by the restriction of virus systemic movement resulting from the amino acid substitution at position 36 in the coat protein of CMV(Y). Received 15 December 1999/ Accepted in revised form 18 April 2000     

Generation and Molecular Mapping of a Sequence Characterized Amplified Region Marker Linked with the Bct Gene for Resistance to Beet curly top virus in Common Bean

ABSTRACT A random amplified polymorphic DNA (RAPD) marker directly linked (0.0 cM) with a resistance gene was identified in a snap bean recombinant inbred population (Moncayo x Primo) consisting of 94 F(5:7) recombinant inbred lines that had uniform segregation for disease reaction to Beet curly top virus (BCTV) across three field locations. Resistance was conditioned by a single dominant allele tentatively designated Bct. Seven hundred and fifty decamer primers were screened to obtain the linked RAPD marker that was then converted to a sequence characterized amplified region (SCAR) marker SAS8.1550. The SCAR mapped within a cluster of resistance genes on linkage group B7 of the core map. A survey of 103 BCTV-resistant and -susceptible snap and dry bean genotypes was conducted using SAS8.1550. Results showed that the SCAR would be highly useful for marker-assisted selection of Bct in snap and dry bean originating from the Andean gene pool. Marker-assisted selection for Bct will expedite the development of BCTV-resistant cultivars and minimize the need for cumbersome pathogen tests.