A plant virus-encoded protein facilitates long-distance movement of heterologous viral RNA

Abstract

Transport of plant viruses from cell to cell typically involves one or more viral proteins that supply specific cell-to-cell movement functions. Long-distance transport of viruses through the vascular system is a less well understood process with requirements different from those of cell-to-cell movement. Usually viral coat protein (CP) is required for long-distance movement, but groundnut rosette umbravirus (GRV) does not code for a CP. However, this virus moves efficiently from cell to cell and long distance. We demonstrate here that the protein encoded by ORF3 of GRV can functionally replace the CP of tobacco mosaic virus (TMV) for long-distance movement. In spite of low levels of virus RNA accumulation in infected cells, chimeric TMV with a replacement of the CP gene by GRV ORF3 was able to move rapidly through the phloem. Moreover, this chimeric virus complemented long-distance movement of another CP-deficient TMV derivative expressing the gene encoding the green fluorescent protein. Thus, the GRV ORF3-encoded protein represents a class of trans-acting long-distance movement factors that can facilitate trafficking of an unrelated viral RNA.

Figure 1

Schematic representation of TMV-based vector TMV(30B) and its derivatives expressing GRV ORF3, CMV 2b gene, PVY^O HC-Pro gene, or GFP gene with and without deletion of the CP gene. Boxes represent ORFs, lines represent untranslated sequences. MP, TMV movement protein; CP, TMV coat protein; ●, subgenomic promoters. Deleted sequences are indicated.

Figure 2

Representative Northern blot analysis of viral RNAs isolated from inoculated (i) and uninoculated (u) leaves of N. benthamiana plants infected with TMV(30B), TMV(ΔCP), and TMV(ORF3) (9 DPI), as indicated. Exposure time for autoradiography is indicated, and the position of TMV genomic RNA is marked.

Figure 3

N. benthamiana plants photographed under long-wavelength UV light 8 days (A and B) and 12 days (C–E) after infection with TMV(30B)-GFP (A and C), TMV(ΔCP)-GFP (B), or TMV(ΔCP)-GFP + TMV(ORF3) (D and E). Inoculated (I) and systemically infected (S) leaves are indicated.