The hypersensitive response to cucumber mosaic virus in Chenopodium amaranticolor requires virus movement outside the initially infected cell.

Cucumber mosaic virus (CMV) expressing the green fluorescent protein (GFP), and lacking either the 3a movement protein or the coat protein (CP), failed to induce a hypersensitive response producing local lesions in inoculated leaves of Chenopodium amaranticolor. Cytological analysis showed that both viral-encoded proteins are required for cell-to-cell movement of the virus and the simultaneous appearance of cellular necrosis. In the absence of either or both proteins, infection was confined to single, non-necrotized, epidermal cells. CMV with a mutation in the 3a protein (M8 CMV) could infect tobacco systemically but did not induce necrotic lesions in C. amaranticolor. In this host, the mutated 3a protein was unable to promote viral movement out of the initially infected epidermal cell. Movement-deficient CMV expressing wild-type (WT) 3a protein as a fusion to the GFP, as well as WT CP, also failed to induce necrosis. Finally, single epidermal cells infected with a movement-deficient CMV expressing WT 3a protein, WT CP, and free GFP did not show necrosis. These data indicate that viral movement out of the initially infected epidermal cell, and not the simultaneous expression in this cell of the 3a protein and the CP, is required for the induction of cell death.