Separate Regions on the Tomato Bushy Stunt Virus p22 Protein Mediate Cell-to-Cell Movement versus Elicitation of Effective Resistance Responses

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Abstract
The objective of the present study on tomato bushy stunt virus (TBSV) was to determine the relationship between TBSV p22 protein domains that control its viral cell-to-cell movement function versus those that regulate the p22-mediated elicitation of necrotic local lesions on selected resistant plants. For this purpose, eight site-specific p22 mutants were generated that targeted clustered or individual charged amino acids that were presumed likely to be exposed on the protein surface and therefore candidates for interaction with viral RNA or host factors. Three cell-to-cell movement impaired mutants were identified, but expression of two such mutant p22 genes from a potato virus X gene vector showed that their encoded proteins retained the ability to elicit the formation of necrotic local lesions on Nicotiana edwardsonii. Conversely, inoculation of this normally resistant plant with two movement-positive p22 mutants resulted in chlorotic rather than necrotic lesions on inoculated leaves and infectious virus penetrated into the upper parts of young plants. These results suggest that p22 protein residues essential for cell-to-cell movement are separable from those that are crucial for the elicitation of effective resistance responses.