Localization of genetic determinants for pathogenicity of Maize dwarf mosaic virus and Bermudagrass southern mosaic virus

Maize dwarf mosaic virus (MDMV) and Bermuda grass southern mosaic virus (BgSMV) are economically important potyviruses of cereals. BgSMV is very similar in genomic sequence to MDMV, but cannot infect Johnsongrass and is not transmitted by Rhopalosiphum maidis. Comparison of their genomes showed an additional stretch of 90 nucleotides in BgSMV coat protein but not in MDMV. Since the 90-nucleotide region is located in the N-terminal of BgSMV coat protein, it seems to have a role in biological properties such as vector transmission and pathogenicity. Recombinant virus constructs were made with and without the 90 nucleotides using SOEing PCR (MDMV (+90) and BgSMV (-90). Johnsongrass plants inoculated with the wild-type MDMV and recombinant BgSMV (-90) showed mosaic symptoms after 16 and 23 days, respectively, whereas plants inoculated with the wild-type BgSMV and recombinant MDMV (+90) didnt show any symptoms until three months after inoculation. The qRT-PCR results detected significantly higher levels of BgSMV (-90) and MDMV compared to BgSMV and MDMV (+90), respectively. Also, R. maidis was able to transfer only the wild type MDMV and BgSMV (-90) from infected to healthy plants. These results confirmed that the insertion of the 90-nt region into the coat protein of MDMV affects the pathogenicity of the virus.