A modified form of the alfalfa mosaic virus movement protein induces stressed phenotypes in transgenic tobacco

A modified form of the movement protein (P3) of alfalfa mosaic virus, lacking amino acids 21 to 34, was transgenically expressed in Nicotiana tabacum cv. Xanthi (genotype nn) and cv. Xanthi nc (genotype NN). The modified protein (designated P3Δ[21–34]) was expressed more strongly than the full-length protein. The localization of P3Δ[21–34] was investigated by subcellular fractionation and immunocytochemistry. Immunolabelling was most frequent in vascular parenchymal cells, mainly in the cytoplasm (endoplasmic reticulum, Golgi, plasma membrane vesicles) but also in the cell wall. In contrast, full-length P3 accumulated almost exclusively in a cell-wall enriched fraction. Transgenically expressed P3Δ[21–34] increased the plasmodesmal gating capacity of epidermal cells, as did transgenically expressed P3. Thus, the plasmodesma-gating domain of P3 does not include amino acids 21 to 34. Plants expressing P3Δ[21–34] at a high level exhibited stressed phenotypes. Phenotype 1, only observed in 'Xanthi' NN lines, was characterized by stunting, small, thick, and hairy leaves, locally high starch accumulation, and occasional necrotic cells, mainly in the bundle sheath and vascular tissue. Phenotype 2, observed in both 'Xanthi' NN and nn lines, was characterized by short internodes, numerous small green leaves, sterile flowers, regular starch accumulation, and absence of necrotic cells. The stress-inducing activity of P3Δ[21–34] may be due to either its molecular conformation or its low efficiency of export toward the cell wall. Keywords: cell to cell movement, stress reaction, plasmodesmata, transgenic plant, ultrastructure, immunocytochemistry.