Inoculation of Vigna unguiculata (cowpea) with transcripts synthesized in vitro from a genome-length cDNA clone of the cowpea strain of southern bean mosaic virus (SBMV-C) resulted in a systemic SBMV-C infection of this host. Capped RNA was about five times more infectious than uncapped RNA as determined by a local lesion assay. The SBMV-C cDNA clone was also used for mutagenesis of the four SBMV-C open reading frames (ORFs). ORF1, ORF3, and coat protein (CP) mutants were not infectious in cowpea. Electroporation of cowpea protoplasts with mutant transcripts demonstrated that the ORF1, ORF3, and CP gene products were not required for SBMV-C RNA synthesis, and the ORF1 and ORF3 gene products were not required for SBMV-C assembly. From these results, it was concluded that the ORF1 and ORF3 proteins and the CP are required for SBMV-C cell-to-cell movement. One of the ORF3 mutants pSBMV2-UAA1833 contained a nonsense codon between the predicted -1 ribosomal frameshift site (SBMV-C nucleotides 1796-1802) and a potential ORF3 translation initiation codon at SBMV-C nucleotide 1895. The lack of infectivity of this mutant suggested that ORF3 was expressed by a -1 ribosomal frameshift in ORF2 rather than by initiation of translation at nucleotide 1895.
Vigna unguiculata (cowpea) and Phaseolus vulgaris (common bean) are permissive hosts for southern cowpea mosaic virus (SCPMV) and southern bean mosaic virus (SBMV), respectively. Neither of these two sobemoviruses systemically infects the permissive host of the other. Although bean cells are permissive for SCPMV RNA synthesis, they do not support the assembly of this virus. Thus, the host range restriction of SCPMV in bean may occur at the level of movement and may involve the inability of SCPMV to assemble in this host. In this study, it was demonstrated that SCPMV accumulates in an encapsidated form in the inoculated and systemic leaves of bean plants following coinoculation with SBMV. No evidence was observed that the SCPMV that accumulated in coinoculated bean plants had an altered host range relative to wild-type SCPMV. These results suggested that SBMV complemented the host range restriction of SCPMV in bean. Additional experiments demonstrated that cowpea protoplasts are permissive for SBMV RNA synthesis and assembly. It was concluded from these results that the host range restriction of SBMV in cowpea occurs at the level of movement. In mixed infections of cowpea with SCPMV and SBMV, the latter was recovered from the inoculated but not the systemic leaves. Its recovery from the inoculated leaves, however, was not dependent on the presence of SCPMV in the inoculum. From these results, it was concluded that SCPMV did not complement the host range restriction of SBMV in cowpea.
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