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

Abstract

DNA β 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 β encodes a single protein, β C1. 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 β associated with CLCuD resulted in systemic infection. DNA β containing a disrupted β C1 open reading frame (ORF) did not mobilize DNA A systemically. Co-inoculation of plants with DNA A and a construct of the β C1 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, β C1 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 β C1 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 β C1 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.