Altered local and systemic spread of movement deficient virus in transgenic tobacco plants expressing the cucumber mosaic virus 3a protein

The 3a protein encoded by RNA 3 of cucumber mosaic virus has been identifie d as the viral cell-to-cell movement protein. The constitutive expression i n transgenic tobacco plants of 3a protein from a subgroup I strain was able to complement in trans the short distance movement of a 3a defective CMV m utant belonging to a different taxonomic subgroup. This ability was depende nt upon the accumulation levels of the 3a protein in transgenic tobacco pla nts. However, an initial delay in viral accumulation and spread of the defe ctive virus as compared to the wild type virus was determined in complement ation tests. Furthermore, a reduction in disease symptoms as well as a diff erent pattern of systemic viral distribution from those of the wild type vi rus was detected. These results show that the early events in viral infecti on affect the long distance spread of the virus. Finally, the wild type vir us moved faster in the 3a protein-expressing plants than in control plants, thus indicating that the constitutive expression of the 3a protein favours long-distance viral spread.