A Monopartite and a Bipartite BEGOMOVIRUS differentially respond to mixed infection with RNA viruses and to silencing suppression

Background: Multiple infections and synergistic interactions among viruses are common in fields and can alter infection parameters and tissue distribution, relying on the ability of a virus to provide (ancillary) functions to unrelated viruses or modify plant defence. Methods: We investigated symptoms, virus titres and tropism in Nicotiana benthamiana plants doubly infected by phloem-limited Begomoviruses, either the bipartite Abutilon mosaic virus (AbMV) or the monopartite Tomato yellow leaf curl Sardinia virus (TYLCSV) and RNA viruses, either Cowpea aphid-borne mosaic virus (CABMV) or Artichoke mottled crinkle virus (AMCV). We also studied the response of the two Begomoviruses in transgenic plants expressing the silencing suppressor proteins of CABMV (HC-Pro) or AMCV (p19). Results: Plants co-infected with AbMV and either CABMV or AMCV displayed symptom aggravation and supported higher levels of AbMV, which also invaded non-phloem cells. Conversely, in spite of symptom worsening in co-infection by TYLCSV and CABMV and to a lesser extent AMCV, TYLCSV titres did not change and the virus remained phloem-limited. HC-Pro or p19 transgenic plants supported a limited increase in AbMV titre, but not phloem escape. Conclusions: HC-Pro and p19 contribute to increase AbMV titres, but the stronger effects observed in double infections might indicate that further CABMV or AMCV proteins are involved; besides, phloem escape does not simply depend on the silencing suppression pathways triggered by HC-Pro or p19. Since TYLCSV infection parameters were unchanged, this monopartite begomovirus must replicate and move independently of CABMV and AMCV and must have evolved its own anti-silencing strategies.