Cucumber Mosaic Virus Is Restricted from Entering Minor Veins in Transgenic Tobacco Exhibiting Replicase-Mediated Resistance

Abstract

Transgenic tobacco plants expressing an altered form of the 2a replicase gene from cucumber mosaic virus (CMV) strain Fny exhibited a suppression of viral replication and restricted viral movement when inoculated mechanically or by insect vectors. Resistant plants could be infected, however, through a graft-union with an infected nontransformed plant. The infectious entity moved quickly through intergrafts of resistant tissue, indicating that it could move without replicating in the vascular system. Viral replication continued to be suppressed in systemically infected transgenic portions of grafted plants, as demonstrated by the synthesis of lower levels of viral RNA than in systemically infected nontransformed portions of the same grafted plants. Cell-to-cell spread within this tissue also occurred much more slowly than in nontransformed tobacco. Young inoculated leaves of transgenic-resistant plants exhibited limited cell-to-cell virus movement, revealed as chlorotic lesions, but no long-distance virus movement occurred. The results ofin situhybridization studies on these lesions indicated that CMV RNA does not traffic from bundle-sheath cells to vascular parenchyma or companion cells in chlorotic lesions on the inoculated leaves of transgenic-resistant tobacco plants. The inhibition of long-distance movement was a consequence of restricted entry of the infectious entity into the vascular system.