It has been suggested that the beet yellows closterovirus (BYV)-encoded p65 protein, a homologue of HSP70 cell chaperones, plays a role as a virus movement protein (MP). To test this hypothesis, we used two types of complementation experiments with plant viruses containing the triple gene block (TGB) of MP genes. In one, the BYV p65 gene was cloned into a 35S promoter plasmid and introduced into Nicotiana benthamiana plants by microprojectile bombardment along with the 35S promoter-driven GUS gene-tagged cDNA of a transport-deficient potexvirus mutant. Transient expression of p65 complemented the mutant as visualized by the significant increase in the number of cells expressing the GUS reporter gene in the infection foci. In the other test, the p65 gene was inserted into the infectious cDNA of the hordeivirus RNA beta component to replace either the 58 kDa MP gene or the whole TGB. Inoculation of Chenopodium quinoa and Chenopodium amaranticolor plants with the T7 transcripts of the chimeric RNA beta, together with the hordeivirus RNA alpha and RNA gamma, caused symptomless infection in inoculated leaves detected by hybridization of the total leaf RNA with a specific cDNA probe. The ability of BYV p65 to substitute for the potexvirus or hordeivirus MPs provides direct evidence for its involvement in the cell-to-cell movement of closterovirus infection.


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