A monopartite begomovirus-associated DNA beta satellite substitutes for the DNA B of a bipartite begomovirus to permit systemic infection.

DNA beta is a circular single-stranded satellite DNA which co-infects with certain monopartite helper begomoviruses to cause economically important diseases, such as cotton leaf curl disease (CLCuD). DNA beta encodes a single protein, betaC1. Tomato leaf curl New Delhi virus (ToLCNDV) is a bipartite begomovirus in which both DNA A and DNA B are required for systemic infection. Inoculation of tomato plants with ToLCNDV DNA A alone induced local but not systemic infection, whereas co-inoculation with DNA A and the DNA beta associated with CLCuD resulted in systemic infection. DNA beta containing a disrupted betaC1 open reading frame (ORF) did not mobilize DNA A systemically. Co-inoculation of plants with DNA A and a construct of the betaC1 ORF, under the control of the cauliflower mosaic virus 35S promoter, resulted in the systemic movement of DNA A. In inoculated tobacco and onion epidermal cells, betaC1 fused to GFP was localized at the cell periphery in association with punctate bodies, around and within the cell nucleus and with the endoplasmic reticulum. It is concluded that heterologous betaC1 protein can replace the movement function of the DNA B of a bipartite begomovirus. Evidence is also provided that tomato leaf curl virus-encoded C4 protein confers the same movement function to ToLCNDV DNA A. The intracellular distribution of betaC1 is consistent with the hypothesis that it has a role in transporting the DNA A from the nuclear site of replication to the plasmodesmatal exit sites of the infected cell.