Molecular Studies on Soybean Mosaic Virus-Soybean Interations
In the U.S., soybean mosaic virus (SMV) is classified into seven strain groups, designated G1 to G7, based on their different responses on resistant soybean [Glycine max (L.) Merr.] cultivars. These responses are: symptomless or resistant (R), necrotic (N), and mosaic or susceptible (S). The gene-for-gene model has been proposed for SMV-soybean interactions. In the majority of cultivars, a single dominant gene, Rsv1, confers both the R and N responses. In the first part of this study, the coat protein (CP) genes of two SMV strains, G1 and G6 were isolated, cloned, and sequenced. Gene isolation was done by reverse transcription-polymerase chain reaction (RT-PCR) on partially purified virus preparation without prior RNA extraction. Amplified products were blunt-end ligated into pNoTA/T7 vector and transformed into competent cells. Sequencing was performed in both directions on heat-denatured double-stranded plasmids. The predicted 265 amino acid sequence of the CP of G1 and G6 strains were 98.9% identical, with only two amino acid differences. Correlating the CP sequences of G1, G2, G6, and G7, with their virulence on resistant soybean cultivars indicated that the CP is not likely to be the R- and/or N-determinant in the SMV-soybean system. The second part of the study involved studying the pathogenesis of G1, G6, and G7 strains on inoculated leaves of R, N, and S soybean cultivars by leaf imprint immunoassay. Results indicated four types of reactions: i) susceptible, showing unrestricted replication and spread; ii) immune, where no virus was detected; iii) systemic spread, showing unrestricted replication but limited spread along the veins; and iv) restricted replication and spread, where infection was restricted to few foci along the veins. Results of this study indicated that Rsv1-mediated resistance is a multicomponent type of resistance that involves both inhibition of virus replication as well as cell-to-cell movement. The third part of the study aimed at investigating Rsv1-mediated resistance at the cellular level. For this purpose, an SMV-soybean protoplast system was developed. Protoplast isolation was based on a combined cellulase-pectolyase Y-23 digestion and metrizamide-sorbitol gradient purification protocol. Virus inoculation of protoplasts was facilitated by either polyethelene glycol (PEG) or poly-L-ornithine (PLO), and method of detection was by Western blotting using antiserum to whole virus. Inoculation by PEG was successful, but results were irreproducible because of the adverse effect of PEG on protoplast viability. Inoculation by PLO was inconclusive because of the high background from residual inoculum. Additional research is needed before a protoplast system can be used to study the mechanism of Rsv1 resistance to SMV at the cellular level.