Role of cucumovirus capsid protein in long-distance movement within the infected plant.

Direct evidence is presented for a host-specific role of the cucumovirus capsid protein in long-distance movement within infected plants. Cucumber (Cucumis sativus L.) is a systemic host for cucumber mosaic cucumovirus (CMV). Tomato aspermy cucumovirus, strain 1 (1-TAV), multiplied to the levels of CMV (i.e., replicated, moved from cell to cell, and formed infectious particles) in the inoculated leaves of cucumbers but was completely unable to spread systemically. The defective long-distance systemic movement of 1-TAV was complemented by CMV in mixed infections. Coinfection of cucumbers with 1-TAV RNA with various combinations of transcripts from full-length cDNA clones of CMV genomic RNA 1, RNA2, and RNA3 showed that CMV RNA3 alone complemented 1-TAV long-distance movement. We obtained mutants containing mutations in the two open reading frames in CMV RNA3 encoding the 3a protein and the capsid protein (CP), both of which are necessary for cell-to-cell movement of CMV. Complementation experiments with mutant CMV RNA3 showed that only 3a protein mutants, i.e., those with an intact CP, complemented the long-distance movement of 1-TAV in cucumbers. Since CMV and TAV have common systemic host plants, the results presented here are strong evidence for an active, host-specific function of the CPs of these two cucumoviruses for long-distance spread in the phloem. The results also suggest that the plasmodesmata in the vascular system and/or at the boundary between the mesophyll and the vascular system, involved in long-distance movement through the phloem, and those in the mesophyll, involved in cell-to-cell movement, differ functionally.