Tissue and cell tropism of Indian cassava mosaic virus (ICMV) and its AV2 (precoat) gene product.

In order to establish defined viruses for challenging plants in resistance breeding programmes, Indian cassava mosaic virus (ICMV; family Geminiviridae) DNA clones were modified to monitor viral spread in plants by replacing the coat protein gene with the green fluorescent protein (GFP) reporter gene. Comparative in situ hybridization experiments showed that ICMV was restricted to the phloem in cassava and tobacco. GFP-tagged virus spread similarly, resulting in homogeneous fluorescence within nuclei and cytoplasm of infected cells. To analyze viral intercellular transport in further detail, GFP was fused to AV2, a protein that has been implicated in viral movement. Expressed from replicating viruses or from plasmids, AV2:GFP became associated with the cell periphery in punctate spots, formed cytoplasmic as well as nuclear inclusion bodies, the latter as conspicuous paired globules. Upon particle bombardment of expression plasmids, AV2:GFP was transported into neighboring cells of epidermal tissues showing that the intercellular transport of the AV2 protein is not restricted to the phloem. The results are consistent with a redundant function of ICMV AV2 acting as a movement protein, presumably as an evolutionary relic of a monopartite geminivirus that may still increase virus fitness but is no longer necessary in a bipartite genome. The fusion of ICMV ORF AV2 to the GFP gene is the first example of a reporter construct that follows the whole track of viral DNA from inside the nucleus to the cell periphery and to the next cell.